Earth’s Interior - Taunton Public Schools · way it did millions of years ago. People wonder, “What’s inside Earth?” The extreme conditions in Earth’s interior prevent exploration

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Earth’s InteriorGuide for Reading■ How have geologists learned about Earth’s inner structure?

■ What are the characteristics of Earth’s crust, mantle, and core?

Earth’s surface is constantly changing. Earth looks different today from the way it did millions of years ago. People wonder, “What’s inside Earth?” The extreme conditions in Earth’s interior prevent exploration far below the surface. Geologists have used two main types of evidence to learn about Earth’s interior: direct evidence from rock samples and indirect evidence from seismic waves.

Rocks from inside Earth give geologists clues about Earth’s structure. Geologists can make inferences about conditions deep inside Earth where these rocks formed. Using data from seismic waves produced by earthquakes, geologists have learned that Earth’s interior is made up of several layers.

The three main layers of Earth are the crust, the mantle, and the core. These layers vary greatly in size, composition, temperature, and pressure. Beneath the surface, the temperature decreases for about 20 meters, then increases until the center of Earth is reached. Pressure results from a force pressing on an area. Pressure inside Earth increases as you go deeper.

The crust is the layer of rock that forms Earth’s outer skin. The crust is a layer of solid rock that includes both dry land and the ocean floor. Oceanic crust consists mostly of rocks such as basalt, dark rock with a fine texture. Continental crust, the crust that forms the continents, consists mainly of rocks such as granite. Granite is a rock that usually is a light color and has a coarse texture.

Below a boundary 40 kilometers beneath the surface is the solid material of the mantle, a layer of hot rock. Earth’s mantle is made up of rock that is very hot, but solid. Scientists divide the mantle into layers based on the physical characteristics of those layers. The uppermost part of the mantle and the crust together form a rigid layer called the lithosphere. Below the lithosphere is a soft layer called the asthenosphere. Beneath the asthenosphere, the mantle is solid. This solid material, called the lower mantle, extends all the way to Earth’s core.

The core is made mostly of the metals iron and nickel. It consists of two parts—a liquid outer core and a solid inner core. The outer core is a layer of molten metal that surrounds the inner core. The inner core is a dense ball of solid metal.

Scientists think that movements in the liquid outer core create Earth’s magnetic field. Because Earth has a magnetic field, the planet acts like a giant bar magnet.

Plate Tectonics ■ Section Summary


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Convection and the MantleGuide for Reading■ How is heat transferred?

■ What causes convection currents?

■ What causes convection currents in Earth’s mantle?

The movement of energy from a warmer object to a cooler object is called heat transfer. Heat is always transferred from a warmer substance to a cooler substance. There are three types of heat transfer: radiation, conduction, and convection.

The transfer of energy through empty space is called radiation. Heat transfer by radiation takes place with no direct contact between a heat source and an object. For example, radiation enables sunlight to warm Earth’s surface.

Heat transfer by direct contact of particles of matter is called conduction. In conduction, the heated particles of a substance transfer heat to other particles through direct contact. An example is when a spoon heats up in a hot pot of soup.

The transfer of heat by the movement of a heated fluid is called convection. Fluids include liquids and gases. During convection, heated particles of a fluid begin to flow, transferring heat energy from one part of the fluid to another.

Heat transfer by convection is caused by differences in temperature and density within a fluid. Density is a measure of how much mass there is in a volume of a substance. When a liquid or gas is heated, the particles move faster. As they move faster, they spread apart. Because the particles of the heated fluid are farther apart, they occupy more space. The fluid’s density decreases. But when a fluid cools, the particles move closer together and density increases.

An example of convection occurs in heating a pot of soup on a stove. As soup at the bottom of the pot gets hot, it expands and becomes less dense. The warm, less dense soup moves upward, floating over cooler, denser soup. At the surface, the warm soup spreads out and cools, becoming denser. Then gravity pulls this cooler, denser soup down to the bottom, where it is heated again and begins to rise. This flow that transfers heat within a fluid is called a convection current. The heating and cooling of the fluid, changes in the fluid’s density, and the force of gravity combine to set convection currents in motion. Convection currents continue as long as heat is added to the fluid.

Convection currents flow in the asthenosphere. The heat source for these currents is heat from Earth’s core and from the mantle itself. Hot columns of mantle material rise slowly. At the top of the asthenosphere, the hot material spreads out and pushes the cooler material out of the way. This cooler material sinks back into the asthenosphere. Convection currents like these have been moving inside Earth for more than four billion years!



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Drifting ContinentsGuide for Reading■ What was Alfred Wegener’s hypothesis about the continents?

■ What evidence supported Wegener’s hypothesis?

■ Why was Alfred Wegener’s theory rejected by most scientists of his day?

In 1910, a young German scientist named Alfred Wegener became curious about why the coasts of several continents matched so well, like the pieces of a jigsaw puzzle. He formed a hypothesis that Earth’s continents had moved! Wegener’s hypothesis was that all the continents had once been joined together in a single landmass and have since drifted apart. He named this supercontinent Pangaea, meaning “all lands.” According to Wegener, Pangaea existed about 300 million years ago. Over tens of millions of years, Pangaea began to break apart. The pieces of Pangaea slowly moved toward their present-day locations, becoming the continents of today. The idea that the continents slowly moved over Earth’s surface became known as continental drift. In a book called The Origin of Continents and Oceans, Wegener presented his evidence. Wegener gathered evidence from different scientific fields to support his ideas about continental drift. He studied land features, fossils, and evidence of climate change.

Mountain ranges and other landforms provided evidence for continental drift. For example, Wegener noticed that when he pieced together maps of Africa and South America, a mountain range running from east to west in South Africa lines up with a range in Argentina. Also, European coal fields match up with coal fields in North America.

Fossils also provided evidence to support Wegener’s theory. A fossil is any trace of an ancient organism preserved in rock. The fossils of the reptiles Mesosaurus and Lystrosaurus and a fernlike plant called Glossopteris have been found on widely separated landmasses. This convinced Wegener that the continents had once been united.

Wegener used evidence from climate change to further support his theory. For example, an island in the Arctic Ocean contains fossils of tropical plants. According to Wegener, the island once must have been located close to the equator. Wegener also pointed to scratches on rocks made by glaciers. These scratches show that places with mild climates today once had climates cold enough for glaciers to form. According to Wegener’s theory, Earth’s climate has not changed. Instead, the positions of the continents have changed.

Wegener also attempted to explain how the drift of continents took place. Unfortunately, Wegener could not provide a satisfactory explanation for the force that pushes or pulls the continents. Because he could not identify the cause of continental drift, most geologists rejected his theory. For nearly half a century, from the 1920s to the 1960s, most scientists paid little attention to the idea of continental drift. Then new evidence about Earth’s structure led scientists to reconsider Wegener’s bold theory.



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Sea-Floor SpreadingGuide for Reading■ What is the process of sea-floor spreading?

■ What is the evidence for sea-floor spreading?

■ What happens at deep-ocean trenches?

The longest chain of mountains in the world is the system of mid-ocean ridges. In the mid-1900s, scientists mapped the mid-ocean ridges using sonar. Sonar is a device that bounces sound waves off underwater objects and then records the echoes of these sound waves. The mid-ocean ridges curve along the sea floor, extending into all of Earth’s oceans. Most of the mountains in the mid-ocean ridges lie hidden under hundreds of meters of water. A steep-sided valley splits the top of some mid-ocean ridges.

Earth’s ocean floors move like conveyor belts, carrying the continents along with them. This movement begins at a mid-ocean ridge. A ridge forms along a crack in the oceanic crust. At a mid-ocean ridge, molten material rises from the mantle and erupts. The molten material then spreads out, pushing older rock to both sides of the ridge. As the molten material cools, it forms a strip of solid rock in the center of the ridge. Then more molten material splits apart the strip of solid rock that formed before, pushing it aside. This process, called sea-floor spreading, continually adds new material to the ocean floor.

Scientists have found strange rocks shaped like pillows in the central valley of mid-ocean ridges. Such rocks can form only if molten material hardens quickly after erupting under water. The presence of these rocks supports the theory of sea-floor spreading. More support came when scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized “stripes.” The pattern is the same on both sides of the ridge. These stripes hold a record of reversals in Earth’s magnetic field. The final proof of sea-floor spreading came from rock samples obtained by drilling into the ocean floor. Scientists found that the farther from a ridge the rocks were taken, the older they were.

The ocean floor does not just keep spreading. Instead, it sinks beneath deep underwater canyons called deep-ocean trenches. Where there are trenches, subduction takes place. Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle. At deep-ocean trenches, subduction allows part of the ocean floor to sink back into the mantle, over tens of millions of years.

The processes of subduction and sea-floor spreading can change the size and shape of the oceans. Because of these processes, the ocean floor is renewed about every 200 million years. The Pacific Ocean is shrinking. Its many trenches are swallowing more ocean crust than the mid-ocean ridge is producing. The Atlantic Ocean is expanding. In most places, the oceanic crust of the Atlantic Ocean is attached to continental crust. As the Atlantic’s floor spreads, the continents along its edges also move.



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The Theory of Plate TectonicsGuide for Reading■ What is the theory of plate tectonics?

■ What are the three types of plate boundaries?

Earth’s lithosphere is broken into separate sections called plates. The plates fit closely together along cracks in the crust. They carry the continents, or parts of the ocean floor, or both. Plate tectonics is the geological theory that states that pieces of Earth’s lithosphere are in constant, slow motion, driven by convection currents in the mantle. A scientific theory is a well-tested concept that explains a wide range of observations. The theory of plate tectonics explains the formation, movement, and subduction of Earth’s plates.

The plates float on top of the asthenosphere. Convection currents rise in the asthenosphere and spread out beneath the lithosphere, causing the movement of Earth’s plates. As the plates move, they produce changes in Earth’s surface, including volcanoes, mountain ranges, and deep-ocean trenches. The edges of different pieces of the lithosphere meet at lines called plate boundaries. Faults—breaks in Earth’s crust where rocks have slipped past each other—form along these boundaries.

There are three types of plate boundaries: transform boundaries, divergent boundaries, and convergent boundaries. The plates move at amazingly slow rates, from about 1 to 24 centimeters per year. They have been moving for tens of millions of years. A transform boundary is a place where two plates slip past each other, moving in opposite directions. Earthquakes occur frequently along these boundaries. The place where two plates move apart, or diverge, is called a divergent boundary. Most divergent boundaries occur at the mid-ocean ridge. When a divergent boundary develops on land, two slabs of Earth’s crust slide apart. A deep valley called a rift valley forms along the divergent boundary. The place where two plates come together, or converge, is a convergent boundary. When two plates converge, the result is called a collision. When two plates collide, the density of the plates determines which one comes out on top. Oceanic crust is more dense than continental crust.

When two plates carrying oceanic crust meet at a trench, the plate that is less dense dives under the other plate and returns to the mantle. This is the process of subduction. When a plate carrying oceanic crust collides with a plate carrying continental crust, the more dense oceanic plate plunges beneath the continental plate through the process of subduction. When two plates carrying continental crust collide, subduction does not take place because both plates are mostly low-density granite rock. Instead, the plates crash head-on. The collision squeezes the crust into mighty mountain ranges.

About 260 million years ago, the continents were joined together in the supercontinent Pangaea. About 225 million years ago, Pangaea began to break apart. Since then, the continents have moved to their present locations.



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Forces in Earth’s CrustGuide for Reading■ How does stress in the crust change Earth’s surface?

■ Where are faults usually found, and why do they form?

■ What land features result from the forces of plate movement?

The movement of Earth’s plates creates enormous forces that squeeze or pull the rock in the crust. A force that acts on rock to change its shape or volume is stress. Stress adds energy to the rock. The energy is stored in the rock until it changes shape or breaks.

Three different kinds of stress can occur in the crust—tension, compression, and shearing. Tension, compression, and shearing work over millions of years to change the shape and volume of rock. Tension pulls on the crust, stretching rock so that it becomes thinner in the middle. Compression squeezes rock until it folds or breaks. Shearing pushes a mass of rock in two opposite directions.

When enough stress builds up in rock, the rock breaks, creating a fault. A fault is a break in the rock of the crust where rock surfaces slip past each other. Most faults occur along plate boundaries, where the forces of plate motion push or pull the crust so much that the crust breaks. There are three main types of faults: normal faults, reverse faults, and strike-slip faults.

Tension causes a normal fault. In a normal fault, the fault is at an angle, and one block of rock lies above the fault while the other block lies below the fault. The block of rock that lies above is called the hanging wall. The rock that lies below is called the footwall. Compression causes reverse faults. A reverse fault has the same structure as a normal fault, but the blocks move in the opposite direction. Shearing creates strike-slip faults. In a strike-slip fault, the rocks on either side of the fault slip past each sideways, with little up or down motion.

Over millions of years, the forces of plate movement can change a flat plain into landforms such as anticlines and synclines, folded mountains, fault-block mountains, and plateaus. A fold in rock that bends upward into an arch is an anticline. A fold in rock that bends downward to form a valley is a syncline. Anticlines and synclines are found on many parts of the Earth’s surface where compression forces have folded the crust. The collision of two plates can cause compression and folding of the crust over a wide area. Where two normal faults cut through a block of rock, fault movements may push up a fault-block mountain. The forces that raise mountains can also uplift, or raise plateaus. A plateau is a large area of flat land elevated high above sea level.

Earthquakes ■ Section Summary


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Models of EarthGuide for Reading■ How do maps and globes represent Earth’s surface?

■ What reference lines are used to locate points on Earth?

■ What are three common map projections?

A map is a flat model of all or part of Earth’s surface as seen from above. A globe is a sphere that represents Earth’s entire surface. Maps and globes are drawn to scale and use symbols to represent topography and other features on Earth’s surface. A map’s scale relates distance on a map to a distance on Earth’s surface. Mapmakers use shapes and pictures called symbols to stand for features on Earth’s surface. A map’s key, or legend, is a list of all the symbols used on the map with an explanation of their meaning.

Most maps and globes show a grid. Because Earth is a sphere, the grid curves to cover the entire planet. Two of the lines that make up the grid, the equator and prime meridian, are the baselines for measuring distances on Earth’s surface.

To locate positions on Earth’s surface, scientists use units called degrees. A degree (°) is 1

360 of the way around a circle. On Earth’s surface, each degree is a measure of an angle formed by lines drawn from the center of Earth to points on the surface.

Halfway between the North and South Poles, the equator forms an imaginary line that circles Earth. The equator divides Earth into the Northern and Southern Hemispheres. A hemisphere is one half of the sphere that makes up Earth’s surface. Another imaginary line, called the prime meridian, makes a half circle from the North Pole to the South Pole through Greenwich, England. Places east of the prime meridian are in the Eastern Hemisphere. Places west of the prime meridian are in the Western Hemisphere.

The lines of latitude and longitude form a grid that can be used to find locations anywhere on Earth. The equator is the starting line for measuring latitude, or distance in degrees north or south of the equator. The distance in degrees east or west of the prime meridian is called longitude.

To show Earth’s curved surface on a flat map, mapmakers use map projections. A map projection is a framework of lines that helps in transferring points on Earth’s three-dimensional surface onto a flat map. Three common map projections are the Mercator projection, the equal-area projection, and the conic projection. In a Mercator projection, all the lines of latitude and longitude appear as straight, parallel lines that form a rectangle. Shapes and sizes of landmasses near the poles are distorted. An equal-area projection shows areas correctly, but distorts some shapes around the edges of the map. In a conic projection, lines of longitude appear as straight lines while lines of latitude are curved.

Mapping Earth’s Surface ■ Section Summary


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Maps and ComputersGuide for Reading■ How does computer mapping differ from earlier ways of mapmaking?

■ What sources of data are used in making computer maps?

For centuries, mapmakers drew maps by hand. Explorers made maps by sketching coastlines as seen from their ships. More accurate maps were made by locating points on Earth’s surface in a process called surveying. In surveying, mapmakers determine distances and elevations using instruments and the principles of geometry. During the twentieth century, people learned to make highly accurate maps using photographs taken from airplanes.

Since the 1970s, computers have revolutionized mapmaking. With computers, mapmakers can store, process, and display map data electronically. All of the data used in computer mapping must be written in numbers. The process by which mapmakers convert the location of map points to numbers is called digitizing. The digitized data can be easily displayed on a computer screen, modified, and printed out in map form.

Computers can automatically make maps that might take a person hours to draw by hand. Computers produce maps using data from many sources, including satellites and the Global Positioning System. Much of the data used in computer mapping is gathered by satellites. Mapping satellites use electronic devices to collect computer data about the land surface. Pictures of the surface based on these data are called satellite images.

A satellite image is made up of thousands of tiny dots called pixels. Each pixel in a satellite image contains information on the color and brightness of a small part of Earth’s surface. When the satellite image is printed, the computer translates these digitized data into colors.

Today, mapmakers can collect data for maps using the Global Positioning System, or GPS. The Global Positioning System is a method of finding latitude, longitude, and elevation of points on Earth’s surface by using a network of satellites.



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Topographic MapsGuide for ReadingA topographic map is a map showing the surface features of an area. Topographic maps use symbols to portray the land as if you were looking down on it from above. Topographic maps provide highly accurate information on the elevation, relief, and slope of the ground surface.

Mapmakers use contour lines to represent elevation, relief, and slope on topographic maps. On topographic maps a contour line connects points of equal elevation. The change in elevation from contour line to contour line is called the contour interval. The contour interval for a given map is always the same. Usually, every fifth contour line, known as an index contour, is darker and heavier than the others. Index contours are labeled with the elevation above sea level in round units, such as 2,000 feet above sea level.

To read a topographic map, you must familiarize yourself with the map’s scale and symbols and interpret the map’s contour lines. Topographic maps usually are large-scale maps. A large-scale map is one that shows a close-up view of part of Earth’s surface. In the United States, most topographic maps are at a scale of 1:24,000, or 1 centimeter equals 0.24 kilometers. At this scale, a map can show the details of elevation and features such as rivers and coastlines. Large buildings, airports, and major highways appear as outlines at the correct scale. Symbols are used to show houses and other small features.

On a topographic map, closely spaced contour lines indicate steep slopes. Widely spaced contour lines indicate gentle slopes. A contour line that forms a closed loop with no other contour lines inside it indicates a hilltop. A closed loop with dashes inside indicates a depression. V-shaped contour lines pointing downhill indicate a ridge line. V-shaped contour lines pointing uphill indicate a valley.

Topographic maps have many uses in science and engineering, business, government, and everyday life. Businesses use them, and so do cities and towns. Topographic maps have recreational uses as well.



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Rocks and WeatheringGuide for Reading■ How do weathering and erosion affect Earth’s surface?

■ What are the causes of mechanical weathering and chemical weathering?

■ What determines how fast weathering occurs?

Weathering is the process that breaks down rock and other substances of Earth’s surface. Erosion is the removal of rock particles by wind, water, ice, or gravity. Weathering and erosion work together continuously to wear down and carry away the rocks at Earth’s surface. The weathering and erosion that geologists observe today also shaped Earth’s surface millions of years ago. How do geologists know this? Geologists make inferences based on the principle of uniformitarianism.This principle states that the same processes that operate today operated in the past.

There are two kinds of weathering: mechanical weathering and chemical weathering. Both types of weathering act slowly, but over time they break down even the biggest, hardest rocks. The type of weathering in which rock is physically broken into smaller pieces is called mechanical weathering. The causes of mechanical weathering include freezing and thawing, release of pressure, plant growth, actions of animals, and abrasion. The term abrasion refers to the grinding away of rock by rock particles carried by water, ice, wind, or gravity.

In cool climates, water expands when it freezes and acts as a wedge. This process is called ice wedging. With repeated freezing and thawing, cracks slowly expand until pieces of rock break off.

Another type of weathering that attacks rocks is chemical weathering, a process that breaks down rock through chemical changes. The causes of chemical weathering include action of water, oxygen, carbon dioxide, living organisms, and acid rain. Chemical weathering can produce new minerals as it breaks down rock. Chemical and mechanical weathering often work together. As mechanical weathering breaks rocks into pieces, more surface area becomes exposed to chemical weathering.

Water is the most important cause of chemical weathering. Water weathers rock by dissolving it. The oxygen in air is an important cause of chemical weathering. Iron combines with oxygen in the presence of water in a process called oxidation. The product of oxidation is rust.

The most important factors that determine the rate at which weathering occurs are the type of rock and the climate. Some types of rock weather more rapidly than others. For example, some rock weathers easily because it is permeable, which means that it is full of air spaces that allow water to seep through it.

Weathering and Soil Formation ■ Section Summary


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How Soil FormsGuide for Reading■ What is soil made of, and how does soil form?

■ How do scientists classify soils?

■ What is the role of plants and animals in soil formation?

Soil is the loose, weathered material on Earth’s surface in which plants can grow. Bedrock is the solid layer of rock beneath the soil.

Soil is a mixture of rock particles, minerals, decayed organic material, air, and water. The decayed organic material in soil is humus, a dark-colored substance that forms as plant and animal remains decay. Humus helps create spaces in soil for air and water that plants must have. The fertility of soil is a measure of how well the soil supports plant growth.

Soil texture depends on the size of individual particles. The largest soil particles are gravel. Next in size are sand particles, followed by silt particles. Clay particles are the smallest. Texture is important for plant growth. Plants can “drown” for lack of air in clay soil, and they may die from lack of water in sandy soil. The best soil for growing most plants is loam, which is soil that is made up of about equal parts of clay, sand, and silt.

Soil forms as rock is broken down by weathering and mixes with other materials on the surface. It is constantly formed wherever bedrock is exposed. Soil formation continues over a long period, and gradually soil develops layers called horizons. A soil horizon is a layer of soil that differs in color and texture from the layers above or below it. The top layer, the A horizon, is made up of topsoil, a crumbly, dark brown soil that is a mixture of humus, clay, and other minerals. The next layer, the B horizon, often called subsoil, usually consists of clay and other particles washed down from the A horizon, but little humus. Below that layer is the C horizon, which contains only partly weathered rock.

Scientists classify different types of soil into major groups based on climate, plants, and soil composition. The most common plants found in a region are also used to help classify the soil. Major soil types in North America include forest, prairie, desert, mountain, tundra, and tropical soils.

Soil teems with living things. Some soil organisms make humus, the material that makes soil fertile. Other soil organisms mix the soil and make spaces in it for air and water. Plants contribute most of the organic remains that form humus. The leaves that plants shed form a loose layer on the ground called litter. Humus forms in a process called decomposition, in which organisms that live in the soil turn dead organic material into humus. The organisms that break the remains of dead organisms into smaller pieces and digest them with chemicals are called decomposers. Fungi, bacteria, worms, and other organisms are the main soil decomposers. Earthworms do most of the work of mixing humus with other materials in soil. Earthworms and burrowing animals also help aerate, or mix air into, the soil.

Weathering and Soil Formation ■ Section Summary


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Changing Earth’s SurfaceGuide for Reading■ What processes wear down and build up Earth’s surface?

■ What causes the different types of mass movement?

Erosion is the process by which natural forces move weathered rock and soil from one place to another. Gravity, running water, glaciers, waves, and wind all cause erosion. The material moved by erosion is sediment. When the agents of erosion lay down sediment, deposition occurs. Deposition changes the shape of the land. Weathering, erosion, and deposition act together in a cycle that wears down and builds up Earth’s surface. Erosion and deposition are at work everywhere on Earth. Erosion and deposition are never-ending.

Gravity pulls everything toward the center of Earth. Gravity is the force that moves rock and other materials downhill. Gravity causes mass movement, any one of several processes that move sediment downhill. The different types of mass movement include landslides, mudslides, slump, and creep. Mass movement can be rapid or slow.

The most destructive type of mass movement is a landslide, which occurs when rock and soil slide quickly down a steep slope. Some landslides contain huge masses of rock, while others may contain only a small amount of rock and soil.

A mudflow is the rapid downhill movement of a mixture of water, rock, and soil. The amount of water in a mudflow can be as high as 60 percent. Mudflows often occur after heavy rains in a normally dry area. In clay soils with a high water content, mudflows may occur even on very gentle slopes. An earthquake can trigger both mudflows and landslides.

In the type of mass movement known as a slump, a mass of rock and soil suddenly slips down in one large mass. It looks as if someone pulled the bottom out from under part of the slope. A slump often occurs when water soaks the base of a mass of soil that is rich in clay.

Creep is the very slow downhill movement of rock and soil. It occurs most often on gentle slopes. Creep often results from the freezing and thawing of water in cracked layers of rock beneath the soil. Creep is so slow that you can barely notice it, but you can see its effects in objects such as telephone poles, gravestones, and fenceposts. Creep may tilt these objects at spooky angles.

Erosion and Deposition ■ Section Summary


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The Force of Moving WaterGuide for Reading■ What enables water to do work?

■ How does sediment enter rivers and streams?

■ What factors affect a river’s ability to erode and carry sediment?

A river’s water has energy. Energy is the ability to do work or cause change. There are two kinds of energy. Potential energy is energy that is stored and waiting to be used later. Kinetic energy is the energy an object has due to its motion. As gravity pulls water down a slope, the water ’s potential energy changes to kinetic energy that can do work. All along a river, moving water causes changes. A river is always moving sediment from place to place. At the same time, a river is also eroding its banks and its valley.

In the process of water erosion, water picks up and moves sediment. Sediment can enter rivers in a number of ways. Most sediment washes or falls into a river as a result of mass movement and runoff. Other sediment erodes from the bottom or sides of the river. Wind may also drop sediment into the water. Abrasion is another process by which a river obtains sediment. Abrasion is the wearing away of rock by a grinding action.

The amount of sediment that a river carries is its load. Gravity and the force of moving water cause sediment to move downstream.

A river’s slope, its volume of flow, and the shape of its streambed all affect how fast the river flows and how much sediment it can erode. A fast-flowing river carries more and larger particles of sediment. When a river slows down, it deposits some of its sediment load. Generally, as a river’s slope increases, its speed also increases. A river’s slope is the amount the river drops toward sea level over a given distance. If a river ’s speed increases, its sediment load and power to erode may increase. A river’s flow is the volume of water that moves past a point on the river at any given time. As more water flows through a river, its speed increases.

Friction is the force that opposes the motion of one surface as it moves across another surface. Friction affects a river’s speed. Where a river is deep, less water comes in contact with the streambed. This reduces friction and allows the river to flow faster. In a shallow river, there is more friction, which reduces the river’s speed. A streambed is often full of boulders and other obstacles. This roughness increases friction and reduces a river’s speed. The water moves every which way in a type of movement called turbulence. Turbulence slows a stream’s flow, but a turbulent stream has great power to erode.

The shape of a river affects the way it deposits sediment. Deposition occurs along the sides of a river, where the water moves more slowly. If a river curves, the water moves fastest on the outside of the curve. There, the river erodes. On the inside of the curve, where the speed is slowest, the river deposits sediment.



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GlaciersGuide for Reading■ What are the two kinds of glaciers?

■ How does a valley glacier form and move?

■ How do glaciers cause erosion and deposition?

A glacier is any large mass of ice that moves slowly over land. There are two kinds of glaciers—continental glaciers and valley glaciers. A continental glacier is a glacier that covers much of a continent or large island. Today, continental glaciers cover about 10 percent of Earth’s land, including Antarctica and most of Greenland. Continental glaciers can flow in all directions.

Many times in the past, continental glaciers have covered large parts of Earth’s surface. These times are known as ice ages. For example, about 2.5 million years ago, continental glaciers covered about a third of Earth’s land. The glaciers advanced and retreated, or melted back, several times. They retreated for the last time about 10,000 years ago. A valley glacier is a long, narrow glacier that forms when snow and ice build up high in a mountain valley. Valley glaciers are found on high mountains.

Glaciers can form only in an area where more snow falls than melts. When the depth of snow and ice reaches more than 30 to 40 meters, gravity begins to pull the glacier downhill. Valley glaciers move, or flow, at a rate of a few centimeters to a few meters per day. Sometimes a valley glacier slides down more quickly in what is called a surge.

The movement of a glacier changes the land beneath it. Although glaciers work slowly, they are a major force of erosion. The two processes by which glaciers erode the land are plucking and abrasion. As a glacier flows over the land, it picks up rocks in a process called plucking. Plucking can move even huge boulders. Many rocks remain on the bottom of a glacier, and the glacier drags them across the land. This process, called abrasion, gouges and scratches the bedrock.

A glacier gathers huge amounts of rock and soil as it moves. When a glacier melts, it deposits the sediment it eroded from the land, creating various landforms. The mixture of sediments that a glacier deposits directly on the surface is called till. The till deposited at the edges of a glacier forms a ridge called a moraine. A terminal moraine is the ridge of till at the farthest point reached by a glacier. Retreating glaciers also create features called kettles. A kettle is a small depression that forms when a chunk of ice is left in glacial till. When the ice melts, the kettle remains. A kettle that is filled with water is called a kettle lake.



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FossilsGuide for Reading■ How do fossils form?

■ What are the different kinds of fossils?

■ What does the fossil record tell about organisms and environments of the past?

Fossils are the preserved remains or traces of living things. Fossils provide evidence of how life has changed over time. Most fossils form when living things die and are buried by sediments. The sediments slowly harden into rock and preserve the shapes of the organisms. Fossils are usually found in sedimentary rock, the type of rock that is made of hardened sediment.

Most fossils form from animals or plants that once lived in or near quiet water such as swamps, lakes, or shallow seas. When an organism dies, generally only its hard parts leave fossils. Fossils found in rock include molds and casts, petrified fossils, carbon films, and trace fossils. Other fossils form when the remains of organisms are preserved in substances such as tar, amber, or ice.

The most common fossils are molds and casts. A mold is a hollow area in sediment in the shape of an organism or part of an organism. A mold forms when the hard part of an organism, such as a shell, is buried in sediment. Later, water carrying dissolved minerals may seep into the empty space of a mold. If the water deposits the minerals there, the result is a cast, a solid copy of the shape of an organism. Petrified fossils are fossils in which minerals replace all or part of an organism. Another type of fossil is a carbon film, an extremely thin coating of carbon on rock. Trace fossils provide evidence of the activities of ancient organisms. Fossil footprints, trails, and burrows are examples of trace fossils. Some processes preserve the remains of organisms with little or no change. Organisms can be preserved in tar, amber, or ice.

Scientists who study fossils are called paleontologists. Paleontologists collect and classify fossils. Together, all the information that paleontologists have gathered about past life is called the fossil record. The fossil record provides evidence about the history of life on Earth. The fossil record also shows that groups of organisms have changed over time. It also reveals that fossils occur in a particular order, showing that life on Earth has evolved, or changed. Thus, the fossil record provides evidence to support the theory of evolution. A scientific theory is a well-tested concept that explains a wide range of observations. Evolution is the gradual change in living things over long periods of time. The fossil record shows that millions of types of organisms have evolved. Some have become extinct. A type of organism is extinct if it no longer exists and will never again live on Earth.

Fossils provide evidence of Earth’s climate in the past. Paleontologists also use fossils to learn about past environments and changes in Earth’s surface.

A Trip Through Geologic Time ■ Section Summary


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The Relative Age of RocksGuide for Reading■ What is the law of superposition?

■ How do geologists determine the relative age of rocks?

■ How are index fossils useful to geologists?

The relative age of a rock is its age compared with the ages of other rocks. The absolute age of a rock is the number of years since the rock formed. The sediment that forms sedimentary rocks is deposited in flat layers. Over years, the sediment hardens and changes into sedimentary rock. These rock layers provide a record of Earth’s geologic history.

It can be difficult to determine the absolute age of a rock. Geologists use the law of superposition to determine the relative ages of sedimentary rock layers. According to the law of superposition, in horizontal sedimentary rock layers the oldest layer is at the bottom. Each higher layer is younger than the layer below it.

There are other clues to the relative ages of rocks. To determine relative age, geologists also study extrusions and intrusions of igneous rock, faults, and gaps in the geologic record. Igneous rock forms when magma or lava hardens. Lava that hardens on the surface is called an extrusion. The rock layers below an extrusion are always older than the extrusion. Beneath the surface, magma may push into bodies of rock. There, the magma cools and hardens into a mass of igneous rock called an intrusion. An intrusion is always younger than the rock layers around and beneath it.

More clues come from the study of faults. A fault is a break in Earth’s crust. A fault is always younger than the rock it cuts through. The surface where new rock layers meet a much older rock surface beneath them is called an unconformity. An unconformity is a gap in the geologic record. An unconformity shows where some rock layers have been lost because of erosion.

To date rock layers, geologists first give a relative age to a layer of rock at one location and then give the same age to matching layers at other locations. Certain fossils, called index fossils, help geologists match rock layers. To be useful as an index fossil, a fossil must be widely distributed and represent a type of organism that existed only briefly. Index fossils are useful because they tell the relative ages of the rock layers in which they occur. Geologists use particular types of organisms, such as ammonites, as index fossils. Ammonites were a group of hard-shelled animals that evolved in shallow seas more than 500 million years ago. They later became extinct. Ammonite fossils have been found in many different places.



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Early EarthGuide for Reading■ When did Earth form?

■ How did Earth’s physical features develop during Precambrian Time?

■ What were early Precambrian organisms like?

Scientists hypothesize that Earth formed at the same time as the other planets and the sun, roughly 4.6 billion years ago. According to this hypothesis, Earth collided with a large object. The collision threw a large amount of material from both bodies into orbit around Earth. This material combined to form the moon. Scientists think that Earth began as a ball of dust, rock, and ice in space. Gravity pulled this mass together. As Earth grew larger, its gravity increased, pulling in nearby dust, rock, and ice. As the growing Earth traveled around the sun, its gravity also captured gases such as hydrogen and helium. However, this first atmosphere was lost when the sun released a strong burst of particles.

During the first several hundred million years of Precambrian Time, an atmosphere, oceans, and continents began to form. After Earth lost its first atmosphere, a second atmosphere formed. The new atmosphere was made up mostly of carbon dioxide, nitrogen, and water vapor. Volcanic eruptions released carbon dioxide, water vapor, and other gases from Earth’s interior. Collisions with comets added other gases to the atmosphere. A comet is a ball of dust and ice that orbits the sun.

At first, Earth’s surface was too hot for water to remain as a liquid. All water evaporated into water vapor. However, as Earth’s surface cooled, the water vapor began to condense to form rain. Gradually, rainwater began to accumulate to form an ocean. Over time, the oceans affected the composition of the atmosphere by absorbing much of the carbon dioxide.

Within 500 million years of Earth’s formation, continents formed.Scientists have found that the continents move very slowly over Earth’s

surface because of forces inside Earth. This process is called continental drift. The movement is slow—only a few centimeters per year. Over billions of years, Earth’s landmasses have repeatedly formed, broken apart, and then crashed together again, forming new continents.

Scientists cannot pinpoint when or where life began on Earth. But scientists have found fossils of single-celled organisms in rocks that formed about 3.5 billion years ago. These earliest life forms were probably similar to present-day bacteria. Scientists hypothesize that all other forms of life on Earth arose from these simple organisms.



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Earth in SpaceGuide for Reading■ How does Earth move in space?

■ What causes the cycle of seasons on Earth?

The study of the moon, stars, and other objects in space is called astronomy. Ancient astronomers studied the movements of the sun and moon. They thought Earth was standing still and the sun and moon were moving. The sun and moon seem to move mainly because Earth is rotating on its axis, the imaginary line that passes through Earth’s center and the North and South poles. The spinning of Earth on its axis is called its rotation. Earth’s rotation on its axis causes day and night. It takes Earth about 24 hours to rotate once on its axis.

The movement of one object around another object is called revolution. Earth completes one revolution around the sun once every year. Earth’s path as it revolves around the sun is called its orbit. Earth’s orbit is a slightly elongated circle, or ellipse.

Many cultures have tried to make a workable calendar. A calendar is a system of organizing time that defines the beginning, length, and divisions of a year. This is not easy because Earth takes about 365 1/4 days to circle the sun, and 12 moon cycles make up fewer days than a calendar year.

Sunlight hits Earth’s surface most directly at the equator. Closer to the poles, sunlight hits Earth’s surface at an angle. That is why it is generally warmer near the equator than near the poles.

Earth has seasons because its axis is tilted as it moves around the sun. Earth’s axis is tilted at an angle of 23.5° from vertical. As Earth revolves around the sun, its axis is tilted away from the sun for part of the year and toward the sun for part of the year. When the north end of Earth’s axis is tilted toward the sun, the Northern Hemisphere has summer. At the same time, the south end of Earth’s axis is tilted away from the sun. As a result, the Southern Hemisphere has winter. The hemisphere tilted toward the sun has more daylight hours than the hemisphere tilted away from the sun. The combination of direct rays and more hours of sunlight heats the surface more than at any other time of the year.

On two days each year, the sun reaches its greatest distance north or south of the equator. Each of these days is known as a solstice. Halfway between the solstices, neither hemisphere is tilted toward the sun. On those two days, the noon sun is directly overhead at the equator. Each of these days is known as an equinox, meaning “equal night.” During an equinox, the length of nighttime and daytime are about the same.

Earth, Moon, and Sun ■ Section Summary


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Gravity and MotionGuide for Reading■ What determines the strength of the force of gravity between two objects?

■ What two factors combine to keep the moon and Earth in orbit?

Earth revolves around the sun in a nearly circular orbit. The moon orbits Earth in the same way. But what keeps Earth and the moon in orbit? Why don’t they just fly off into space? The first person to answer these questions was the English scientist Isaac Newton. Newton told a story about how watching an apple fall from a tree in 1666 had made him think about the moon’s orbit. Newton realized that there must be a force acting between Earth and the moon that kept the moon in orbit. A force is a push or a pull.

Newton hypothesized that the force that pulls an apple to the ground also pulls the moon toward Earth, keeping it in orbit. This force, called gravity, attracts all objects toward each other. In Newton’s day, most scientists thought that forces on Earth were different from those elsewhere in the universe. Although Newton did not discover gravity, he was the first to realize that gravity occurs everywhere. Newton’s law of universal gravitation states that every object in the universe attracts every other object.

The strength of gravity is measured in units called newtons, named after Isaac Newton. The strength of the force of gravity between two objects depends on two factors: the masses of the objects and the distance between them. According to the law of universal gravitation, all of the objects around you are pulling on you. Why don’t you notice this pull? Because the strength of gravity depends, in part, on the masses of the objects. Mass is the amount of matter in an object.

Because Earth is so massive, it exerts a much greater force on you than a book does. Similarly, Earth exerts a gravitational pull on the moon, large enough to keep the moon in orbit. The force of gravity on an object is known as its weight. An object’s weight can change depending on its location. For example, on the moon, you would weigh about one-sixth of your weight on Earth. This is because the moon is much less massive than Earth, so the pull of its gravity on you would be much less.

The tendency of an object to resist a change in motion is inertia. Isaac Newton stated his ideas about inertia as a scientific law. Newton’s first law of motion says that an object at rest will stay at rest and an object in motion will stay in motion with a constant speed and direction unless acted on by a force.

Why do Earth and the moon remain in their orbits? Newton concluded that two factors—inertia and gravity—combine to keep Earth in orbit around the sun and the moon in orbit around Earth. Earth’s gravity keeps pulling the moon toward it, preventing the moon from moving in a straight line. At the same time, the moon keeps moving ahead because of its inertia. If not for Earth’s gravity, inertia would cause the moon to move off through space in a straight line. In the same way, Earth revolves around the sun because the sun’s gravity pulls on it while Earth’s inertia keeps it moving ahead.



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Phases, Eclipses, and TidesGuide for Reading■ What causes the phases of the moon?

■ What are solar and lunar eclipses?

■ What causes the tides?

As the moon moves, the positions of the moon, Earth, and the sun change in relation to each other. The changing relative positions of the moon, Earth, and the sun cause the phases of the moon, eclipses, and tides.

The moon revolves around Earth about once every 27.3 days. It also rotates on its own axis about once every 27.3 days. The same side of the moon always faces Earth. The different shapes of the moon you see from Earth are called phases. The phase of the moon you see depends on how much of the sunlit side of the moon faces Earth.

When the moon’s shadow hits Earth or Earth’s shadow hits the moon, an eclipse occurs. An eclipse occurs when an object in space comes between the sun and a third object, and casts a shadow on that object. There are two types of eclipses: solar and lunar.

A solar eclipse occurs when the moon passes between Earth and the sun, blocking the sunlight from reaching Earth. The moon’s shadow then hits Earth. So a solar eclipse occurs when a new moon blocks your view of the sun. The darkest part of the moon’s shadow is called the umbra. From any part of the umbra, the moon completely blocks light from the sun. Only people in the umbra see a total solar eclipse. Another part of the shadow is less dark and larger than the umbra. It is called the penumbra. From within the penumbra, people see a partial eclipse because part of the sun is still visible.

A lunar eclipse occurs at a full moon when Earth is directly between the moon and the sun. During a lunar eclipse, Earth’s shadow falls on the moon. Earth’s shadow also has an umbra and a penumbra. When the moon is completely within Earth’s umbra, you see a total lunar eclipse. A partial lunar eclipse happens when the moon moves partly into Earth’s umbra.

Tides are the rise and fall of the ocean’s water every 12.5 hours or so. The force of gravity pulls the moon and Earth toward each other. Tides are caused mainly by differences in how much the moon pulls on different parts of Earth. As Earth rotates, the moon’s gravity pulls water toward the point on Earth’s surface closest to the moon. The moon pulls least on the side of Earth farthest away. Two tides occur each day because of this difference in the pull of the moon’s gravity.

Twice a month, the moon, Earth, and the sun are in a straight line. The combined forces of the gravity of the sun and moon produce an especially high tide—called a spring tide—and an especially low tide. Also twice a month, the pull of gravity of the sun and moon are at right angles to each other. At those times the high tide is lower than usual, and is called a neap tide. The low tides then are higher than usual.



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Observing the Solar SystemGuide for Reading■ What are the geocentric and heliocentric systems?

■ How did Copernicus, Galileo, and Kepler contribute to our knowledge of the solar system?

■ What objects make up the solar system?

Observers in ancient Greece noticed that although the stars seemed to move, they stayed in the same position relative to one another. These patterns of stars, called constellations, kept the same shapes from night to night and from year to year.

The Greeks thought that Earth was inside a rotating dome called a celestial sphere. Since the word geo is the Greek word for Earth, an Earth-centered explanation is known as a geocentric system. In a geocentric system, Earth is at the center of the revolving planets and stars. About A.D. 140, the Greek astronomer Ptolemy further developed the geocentric model. Like the earlier Greeks, Ptolemy thought Earth was at the center of a system of planets and stars. In Ptolemy’s model, however, the planets moved on small circles that moved on bigger circles. Copernicus was able to work out the arrangement of the known planets and how they move around the sun.

A Greek scientist developed the heliocentric system. In a heliocentric system, Earth and the other planets revolve around the sun.

In the early 1500s, the Polish astronomer Nicolas Copernicus developed a new model for the motions of the planets. His sun-centered system is also called heliocentric. Helios is Greek for “sun.” Copernicus was about to work out the arrangement of the known planets and how they move around the sun. Later, Galileo used the newly invented telescope to make discoveries that supported the heliocentric model.

Copernicus thought that the planets’ orbits were circles. He based his conclusions on observations made by the ancient Greeks. In the late 1500s, Tycho Brahe made more accurate observations of the planets’ orbits. Johannes Kepler analyzed Brahe’s data. Kepler found that the orbit of each planet is an ellipse. An ellipse is an oval shape, which may be elongated or nearly circular. Kepler used the new scientific evidence gathered by Brahe to disprove the long-held belief that the planets moved in perfect circles.

Since Galileo’s time, our knowledge of the solar system has increased dramatically. Today we know that the solar system consists of the sun, nine planets and their moons, and several kinds of smaller objects that revolve around the sun.

The Solar System ■ Section Summary


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The SunGuide for Reading■ What are the three layers of the sun’s interior?

■ What are the three layers of the sun’s atmosphere?

■ What features form on or above the sun’s surface?

The sun’s mass is 99.8 percent of all the mass in the solar system. Because the sun is so large, its gravity is strong enough to hold all of the planets and other distant objects in orbit.

Unlike Earth, the sun does not have a solid surface. Like Earth, the sun has an interior and an atmosphere. The sun’s interior consists of the core, radiation zone, and convection zone. Each layer has different properties.

The sun produces an enormous amount of energy in its core, or central region. The sun’s energy comes from nuclear fusion. In the process of nuclear fusion, hydrogen atoms in the sun join to form helium.

The light and heat produced by the sun’s core first pass through the middle layer of the sun’s interior, the radiation zone. The radiation zone is a region of very tightly packed gas where energy is transferred mainly in the form of electromagnetic radiation.

The convection zone is the outermost layer of the sun’s interior. Hot gases rise from the bottom of the convection zone and gradually cool as they approach the top. Cooler gases sink, forming loops of gas that move heat toward the sun’s surface.

The sun’s atmosphere consists of the photosphere, the chromosphere, and the corona. The inner layer of the sun’s atmosphere is called the photosphere. Photo means “light,” so the photosphere is the sphere that gives off visible light.

At the beginning and end of a solar eclipse, you can see a reddish glow around the photosphere. This glow comes from the middle layer of the sun’s atmosphere, the chromosphere. Chromo means “color,” so the chromosphere is the “color sphere.”

During a total solar eclipse, a fainter layer called the corona is visible. The corona sends out a stream of electrically charged particles called solar wind.

Features on or above the sun’s surface include sunspots, prominences, and solar flares. Sunspots are areas of gas on the sun that are cooler than the gas around them. Sunspots usually occur in groups. Reddish loops of gas called prominences link different parts of sunspot regions. Sometimes the loops in sunspot regions suddenly connect, releasing large amounts of energy. The energy heats gas on the sun to millions of degrees Celsius, causing the gas to explode into space. These explosions are known as solar flares. Solar flares can greatly increase the solar wind.



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The Inner PlanetsGuide for Reading■ What characteristics do the inner planets have in common?

■ What are the main characteristics that distinguish each of the inner planets?

Mercury, Venus, Earth, and Mars are more similar to one other than they are to the five outer planets. The four inner planets are small and dense and have rocky surfaces. These planets are often called the terrestrial planets, from the Latin word terra, or “earth.”

Earth is unique in our solar system in having liquid water at its surface. Earth has a suitable atmosphere and temperature range for water to exist as liquid, gas, or solid. Earth has an atmosphere that is rich in oxygen. Nearly all of the remaining atmosphere consists of nitrogen, along with small amounts of other gases such as argon and carbon dioxide. The atmosphere also includes water vapor.

Mercury is the smallest terrestrial planet and the planet closest to the sun. Mercury is smaller than Earth’s moon and has no moons of its own. The planet’s interior is probably made of iron, and its surface has many plains and craters. Because the planet is so close to the sun, the side facing the sun reaches temperatures of 430°C. However, the temperature drops to –170°C at night.

Venus is similar in size and mass to Earth. Venus’ density and internal structure are similar to Earth’s. But in other ways, Venus and Earth are very different. Venus rotates from east to west, the opposite direction from most other planets and moons. The pressure of Venus’s atmosphere is 90 times greater than the pressure of Earth’s atmosphere. The atmosphere is mostly carbon dioxide, with clouds partly made up of sulfuric acid. The carbon dioxide in the planet’s atmosphere traps the sun’s heat, causing the surface temperature of Venus to be about 460°C. This trapping of heat by the atmosphere is called the greenhouse effect. Venus is covered with rock, similar to many rocky areas on Earth. Venus also has many volcanoes and broad plains formed by lava flows.

Mars is called the “red planet.” Its surface is covered with red dust. The planet Mars has a very thin atmosphere that is mostly carbon dioxide. Temperatures on the surface range from –140ºC to 20ºC. Images of Mars show a variety of features that look as if they were made by ancient streams, lakes, or floods. Scientists think that a large amount of liquid water flowed on Mars’s surface in the distant past. At present, liquid water cannot exist for long on Mars’s surface. However, some water is frozen in the planet’s two polar ice caps. A large amount of water may be frozen underground. Like Earth, Mars is tilted on its axis, so its seasons change. Some regions of Mars have giant volcanoes. Mars has two very small moons, Phobos and Deimos.



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The Outer PlanetsGuide for Reading■ What characteristics do the gas giants have in common?

■ What are some characteristics that distinguish each of the outer planets?

The first four outer planets—Jupiter, Saturn, Uranus, and Neptune—are much larger and more massive than Earth, and they do not have solid surfaces. Because these four planets are all so large, they are often called the gas giants. The fifth outer planet, Pluto, is small and rocky, like the terrestrial planets.

Like the sun, the gas giants are composed mainly of hydrogen and helium. Because they are so massive, they exert a much stronger gravitational force than the terrestrial planets. This prevents their gases from escaping, so they have thick atmospheres. All of the giants have many moons and are surrounded by a set of rings. A ring is a thin disk of small particles of ice and rock.

Jupiter is the largest and most massive planet. Jupiter has a thick atmosphere made up mainly of hydrogen and helium. An interesting feature of Jupiter’s atmosphere is its Great Red Spot, a storm that is larger than Earth. Jupiter probably has a dense core of rock and iron at its center, surrounded by a thick mantle of liquid hydrogen and helium. Galileo discovered Jupiter’s four largest moons: Io, Europa, Ganymede, and Callisto.

Saturn is the second-largest planet in the solar system. Its average density is less than that of water. The rings around Saturn are made of chunks of ice and rock. Saturn has the most spectacular rings of any planet.

Uranus is about four times the diameter of Earth and is twice as far from the sun as Saturn. Uranus looks blue-green because of traces of methane in its atmosphere. Uranus’s axis of rotation is tilted at an angle of about 90 degrees from the vertical. It rotates from top to bottom instead of from side to side.

Neptune is a cold, blue planet. Its atmosphere contains visible clouds. Neptune was discovered as a result of a mathematical prediction. Astronomers have discovered at least 13 moons orbiting Neptune.

Pluto has a solid surface and is much smaller and denser than the other outer planets. Pluto has a single moon, Charon. Because Charon is more than half the size of Pluto, some astronomers consider them to be a double planet instead of a planet and a moon. Pluto revolves around the sun only once every 248 Earth years.



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Characteristics of StarsGuide for Reading■ How are stars classified?

■ How do astronomers measure distances to the stars?

■ What is an H-R diagram and how do astronomers use it?

When ancient observers around the world looked up at the night sky, they imagined that groups of stars formed pictures of people or animals. Today, we call these imaginary patterns of stars constellations.

Astronomers classify stars according to their physical characteristics. Characteristics used to classify stars include color, temperature, size, composition, and brightness. Stars vary in their chemical composition. Astronomers use spectrographs to determine the elements found in stars. A spectrograph is a device that breaks light into colors and produces an image of the resulting spectrum.

The brightness of a star depends upon both its size and its temperature. How bright a star looks from Earth depends on both its distance from Earth and how bright the star actually is. The brightness of a star can be described in two different ways: apparent brightness and absolute brightness. A star’s apparent brightness is its brightness as seen from Earth. Astronomers can measure apparent brightness fairly easily using electronic devices. A star’s absolute brightness is the brightness the star would have if it were at a standard distance from Earth.

Distances on Earth’s surface are often measured in kilometers. However, distances to the stars are so large that kilometers are not very practical units. Astronomers use a unit called the light-year to measure distances between the stars. A light-year is the distance that light travels in one year, about 9.5 million million kilometers.

Standing on Earth looking up at the sky, it may seem as if there is no way to tell how far away the stars are. However, astronomers have found ways to measure those distances. Astronomers often use parallax to measure distances to nearby stars. Parallax is the apparent change in position of an object when you look at it from different places.

Two important characteristics of stars are temperature and absolute brightness. Ejnar Hertzsprung and Henry Norris-Russell made a graph to find out whether these characteristics are related. The graph they made is called the Hertzsprung-Russell diagram, or H-R diagram. Astronomers use the H-R diagram to classify stars and to understand how stars change over time. Most of the stars in the H-R diagram form a diagonal line called the main sequence. More than 90 percent of all stars, including the sun, are main-sequence stars. In the main sequence, surface temperature increases as brightness increases.

Stars, Galaxies, and the Universe ■ Section Summary


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Star Systems and GalaxiesGuide for Reading■ What is a star system?

■ What are the major types of galaxies?

■ How do astronomers describe the scale of the universe?

Our solar system has only one star, the sun. Most stars are members of groups of two or more stars, called star systems. Star systems that have two stars are called double stars or binary stars. A system in which one star periodically blocks the light from another is called an eclipsing binary.

Astronomers have discovered more than 100 planets around other stars. Most of these new planets are very large. Some scientists think it is possible that life could be on planets in other solar systems. A few astronomers are using radio telescopes to search for signals that could not have come from natural sources.

Many stars belong to larger groups called star clusters. Open clusters have a loose, disorganized appearance and contain no more than a few thousand stars. Globular clusters are large groups of older stars. Some may contain more than a million stars.

A galaxy is a huge group of single stars, star systems, star clusters, dust, and gas bound together by gravity. Astronomers classify most galaxies into the following types: spiral, elliptical, and irregular. Galaxies that appear to have a bulge in the middle and arms that spiral outward, like pinwheels, are called spiral galaxies. Elliptical galaxies look like round or flattened balls. Galaxies that do not have regular shapes are known as irregular galaxies. Quasars are active young galaxies with giant black holes at their centers.

Our solar system is located in a spiral galaxy called the Milky Way. The Milky Way is usually thought of as a standard spiral galaxy. When you see the Milky Way at night during the summer, you are looking toward the center of our galaxy.

Astronomers define the universe as all of space and everything in it. Since the numbers astronomers use are often very large or very small, they frequently use scientific notation to describe sizes and distances in the universe. Scientific notation uses powers of ten to write very large or very small numbers in shorter form.

The structures in the universe vary greatly in scale. Beyond the solar system, the sizes of observable objects become much larger. Beyond our galaxy are billions of other galaxies, many which contain billions of stars. The Milky Way is a part of a cluster of 50 or so galaxies called the Local Group. The Local Group is part of the Virgo Supercluster, which contains hundreds of galaxies.



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The Expanding UniverseGuide for Reading■ What is the big bang theory?

■ How did the solar system form?

■ What do astronomers predict about the future of the universe?

Astronomers theorize that billions of years ago, the universe was no larger than the period at the end of this sentence. This tiny universe was incredibly hot and dense. The universe then exploded in what astronomers call the big bang. According to the big bang theory, the universe formed in an instant, billions of years ago, in an enormous explosion.

Edwin Hubble discovered that most of the galaxies are moving away from us and away from each other. Hubble also discovered that there is a relationship between the distance to a galaxy and its speed. Hubble’s law states that the farther away a galaxy is, the faster it is moving away from us. Hubble’s law provides strong support for the big bang theory.

In 1965, two physicists accidentally detected faint radiation on their radio telescope. This mysterious glow was coming from all directions in space. Scientists later concluded that this glow, now known as cosmic background radiation, is radiation left over from the big bang. Astronomers estimate that the universe is about 13.7 billion years old.

After the big bang, there was only cold, dark gas and dust where the solar system is now. About five billion years ago, a giant cloud of gas and dust collapsed to form our solar system. A large cloud of gas and dust such as the one that formed our solar system is called a solar nebula. Slowly, gravity began to pull the solar nebula together. As the solar nebula shrank, it spun faster and faster and eventually flatted into a rotating disk. Gravity pulled most of the gas into the center of the disk, where the gas eventually became hot and dense enough for nuclear fusion to begin. The sun was born.

Meanwhile, in the outer parts of the disk, gas and dust formed small asteroid-like bodies called planetesimals. These formed the building blocks of the planets. Planetesimals collided and grew larger by sticking together and eventually combining to form the planets.

New observations have led many astronomers to conclude that the universe will likely expand forever. Astronomers have discovered that the matter that astronomers can see, such as stars and nebulas, makes up as little as ten percent of the mass of galaxies. The remaining mass in galaxies exists in the form of dark matter. Dark matter is matter that does not give off electromagnetic radiation. Astronomers have observed that the expansion of the universe appears to be accelerating. They infer that a mysterious new force, which they call dark energy, is causing the expansion of the universe to accelerate. Most of the universe is thought to be made of dark matter and dark energy.



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Introduction to Matter ■ Section Summary

Changes in MatterGuide for Reading■ What is a physical change?

■ What is a chemical change?

■ How are changes in matter related to changes in energy?

Chemistry is the study of changes in matter. Matter can change in two ways. In a physical change, matter changes its appearance but does not change into a different substance. A substance that undergoes a physical change is still the same substance after the change. One example of a physical change is a change in state. Changing from a solid to a liquid or from a liquid to a gas is a change in state. Other kinds of physical changes are dissolving, bending, crushing, and filtering.

The other way that matter can change is a chemical change. In a chemical change, matter changes into one or more new substances. Unlike a physical change, a chemical change produces new substances with different properties from those of the original substances. Combustion, or burning, is one chemical change. When natural gas burns, it combines with oxygen in the air to produce carbon dioxide gas and water. Other examples of chemical change are electrolysis, oxidation, and tarnishing.

Although it may seem like matter disappears when it burns, that is not what is really happening. It has long been proven that mass is not lost or gained when matter changes. The law of conservation of mass states that matter is not created or destroyed in any chemical or physical reaction.

Any time that matter changes, energy is involved. Energy is the ability to do work or cause change.Every chemical or physical change in matter includes a change in energy.When ice melts, it absorbs energy from the surrounding matter.

One kind of energy is thermal energy. Thermal energy is the total energy of all the particles in an object. Thermal energy always moves from warm matter to cool matter. Thermal energy is different from temperature. Temperature does depend on the amount of thermal energy an object has. Temperature is a measure of the average energy of motion of the particles in an object.

Thermal energy is the most common form of energy released or absorbed when matter changes. When ice absorbs thermal energy from its surroundings, it melts. The melting of ice is an endothermic change. An endothermic change is a change in which energy is taken in, or absorbed. When wood burns, energy is given off in the form of heat and light. An exothermic change releases, or gives off, energy.


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Solids, Liquids, and Gases

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Section Summary

States of Matter

Guide for Reading

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What are the characteristics of a solid?

■ What are the characteristics of a liquid?

■ What are the characteristics of a gas?

Matter can be classified as solids, liquids, or gases. These three states of matter are defined mainly by the way they hold their volume and shape.

A solid has a definite volume and a definite shape. The particles that make up a solid are packed very closely together. Each particle is tightly fixed in one position. This fixed, closely packed arrangement of particles causes a solid to have a definite shape and volume. The particles in a solid are not completely motionless. The particles vibrate, or move back and forth slightly.

In many solids, the particles form a regular, repeating pattern. These patterns create crystals. Solids that are made up of crystals are called crystalline solids. Salt, sugar, and snow are examples of crystalline solids. When a crystalline solid is heated, it melts at a specific temperature.

In other solids, called amorphous solids, the particles are not arranged in a regular pattern. Amorphous solids include plastics, rubber, and glass. Unlike a crystalline solid, an amorphous solid does not melt at a distinct temperature. Instead, when it is heated it may become softer and softer or change into other substances.

A liquid has a definite volume but no shape of its own. A liquid takes on the shape of its container. Without a container, a liquid spreads into a wide, shallow puddle. The particles in a liquid are packed almost as closely as in a solid. However, the particles in a liquid move around one another freely. Because its particles are free to move, a liquid has no definite shape. However, it does have a definite volume.

A liquid can flow from place to place. For this reason, a liquid is also called a fluid, meaning “a substance that flows.”

One property of liquids, surface tension, is caused by the inward pull of the molecules making up a liquid. This pull brings the molecules on the surface closer together. This explains why water forms droplets and supports the weight of certain insects on its surface.

Another property of water, viscosity, is a liquid’s resistance to flowing. Viscosity depends on the size and shape of the particles of a liquid, and the attractions between particles. Liquids with high viscosity flow slowly. Liquids with low viscosity flow quickly.

Unlike solids and liquids, a gas can change volume very easily. The particles of a gas move at high speeds in all directions. As they move gas particles spread apart, filling all the space available. Thus, a gas has neither definite shape nor definite volume.


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Atoms and Bonding ■ Section Summary

Elements and AtomsGuide for Reading■ Why are elements sometimes called the building blocks of matter?

■ How did atomic theory develop and change?

Elements are the simplest pure substances. They cannot be broken down into any other substances. Iron and oxygen are elements. Elements are often called the building blocks of matter because all matter is composed of one element or a combination of two or more elements.

Elements usually exist with other elements in the form of compounds. A compound is a pure substance made of two or more elements that are combined chemically in a specific ratio. Table salt is an example of a compound. Elements can also mix with other elements without combining chemically. A mixture is two or more substances that are in the same place but are not chemically combined. Air is an example of a mixture. The smallest particle of an element is an atom.

Scientific theories about the atom began to develop in the 1600s. A scientific theory is a well-tested idea that explains and connects a wide range of observations. Theories often include models—physical or other representations of an idea to help people understand what they cannot observe directly. Atomic theory grew as a series of models that developed from experimental evidence. As more evidence was collected, the theory and models were revised.

John Dalton proposed one of the first models of the atom. Dalton thought that atoms were like smooth, hard balls that could not be broken into smaller pieces. In 1897, J. J. Thomson discovered that atoms contained negatively charged particles. He proposed a model of the atom in which negatively charged particles were scattered throughout a ball of positive charge. The negatively charged particles later became known as electrons.

In 1911, Ernest Rutherford did experiments that showed that an atom is mostly empty space, with electrons moving around a small, positively charged center. Rutherford called this small positive region in the center of the atom the nucleus. He determined that the nucleus contained positively charged particles, which he named protons. In 1913, Niels Bohr revised the atomic model again. Bohr showed that electrons move around the nucleus in certain orbits according to their energy. According to Bohr, the electrons were like planets orbiting the sun. In the 1920s, the atomic model changed again. Scientists determined that electrons could be anywhere in a cloudlike region around the nucleus. A region where electrons of the same energy are likely to be found is called an energy level. In 1932, James Chadwick discovered another particle in the nucleus of the atom. It was called a neutron because it is electrically neutral. Since the 1930s, the model of the atom has not changed much. Scientists conclude the atom consists of a small, positively charged nucleus, containing protons and neutrons, which is surrounded by a cloudlike region of negatively charged electrons.


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Chemical Reactions ■ Section Summary

Observing Chemical ChangeGuide for Reading■ How can matter and changes in matter be described?

■ How can you tell when a chemical reaction occurs?

Matter is anything that has mass and takes up space. The study of matter and how matter changes is called chemistry. Matter can be described in terms of two kinds of properties—physical properties and chemical properties. Changes in matter can be described in terms of physical changes and chemical changes.

A physical property is a characteristic of a substance that can be observed without changing the substance into another substance. The temperature at which a solid melts is a phyical property. Color, hardness, and texture are other physical properties.

A chemical property property is a characteristic of a substance that describes its ability to change into other substances. To observe the chemical properties of a substance, you must change it into another substance. For example, to observe the chemical reactivity of magnesium, you can let magnesium combine with oxygen to form a new substance called magnesium oxide.

A physical change is any change that alters the form or appearance of a substance but that does not make the substance into another substance. Examples of physical changes are bending and cutting.

A change in matter that produces one or more new substances is a chemical change, or chemical reaction. The burning of gasoline in a car’s engine is a chemical change. Chemical changes occur when bonds form between atoms, or when bonds break and new bonds form. As a result, new substances are produced.

One way to detect chemical reactions is to observe changes in the properties of the materials involved. Chemical reactions involve two main kinds of changes you can observe—formation of new substances and changes in energy. Changes in properties result when new substances form. A change in color may signal that a new substance has formed. Another indicator might be the formation of a solid when two solutions are mixed. A solid that forms from solution during a chemical reaction is called a precipitate. A third indicator is the formation of a gas when solids or liquids react. These and other kinds of observable changes in properties may indicate that a chemical reaction has occurred.

As matter changes in a chemical reaction, it can either absorb or release energy. One indication that energy has been absorbed or released is a change in temperature. An endothermic reaction is a reaction in which energy is absorbed. A reaction that releases energy in the form of heat is called an exothermic reaction.


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Describing Chemical ReactionsGuide for Reading■ What information does a chemical equation contain?

■ What does the principle of conservation of mass state?

■ What must a balanced chemical equation show?

■ What are three categories of chemical reactions?

A chemical equation is a short, easy way to show a chemical reaction. Chemical equations use chemical formulas and other symbols instead of words to summarize a reaction. All chemical equations have a common structure. A chemical equation tells you the substances you start with in a reaction and the substances you get at the end. The substances you have at the beginning are called the reactants. When the reaction is complete, you have new substances called the products. The formulas for the reactants are written on the left side of the equation, followed by an arrow (→). You read the arrow as “yields.” The formulas for the products are written on the right side of the equation. When there are two or more reactants or products, they are separated by plus signs.

The principle called conservation of mass was first demonstrated in the late 1700s. The principle of conservation of mass states that in a chemical reaction, the total mass of the reactants must equal the total mass of the products. In an open system, matter can enter from or escape to the surroundings. A match burning in the air is an example of an open system. You cannot measure the mass of all the reactants and products in an open system. A closed system is a system in which matter cannot enter from or escape to the surroundings. A sealed plastic bag is an example of a closed system. A closed system allows you to measure the mass of all reactants and products in a reaction.

To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation. An equation is balanced when it accurately represents conservation of mass. To balance a chemical equation, you may have to use coefficients. A coefficient is a number placed in front of a chemical formula in an equation. It tells you how many atoms or molecules of a reactant or a product take part in the reaction.

Many chemical reactions can be classified in one of three categories: synthesis, decomposition, or replacement. When two or more elements or compounds combine to make a more complex substance, the reaction is called a synthesis reaction. The reaction of hydrogen and oxygen to make water is a synthesis reaction. A reaction called a decomposition reaction breaks down compounds into simpler products. For example, hydrogen peroxide decomposes into water and oxygen gas. When one element replaces another in a compound, or when two elements in different compounds trade places, the reaction is called a replacement reaction.


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Controlling Chemical ReactionsGuide for Reading■ How is activation energy related to chemical reactions?

■ What factors affect the rate of a chemical reaction?

Activation energy is the minimum amount of energy needed to start a chemical reaction. All chemical reactions need a certain amount of activation energy to get them started. Even exothermic reactions need activation energy to get started. Once a few molecules react, the rest will quickly follow, because the first few reactions provide the activation energy for more molecules to react. Endothermic reactions not only need activation to get started. They also need additional energy from the environment to keep going.

Chemical reactions don’t all occur at the same rate. How fast a reaction happens depends on how often and with how much energy the particles of the reactants come together. Chemists can control rates of reactions by changing factors such as surface area, temperature, and concentration, and by using substances called catalysts and inhibitors.

A third way to increase the rate of a reaction is to increase the concentration of the reactants. The concentration is the amount of a substance in a given volume. Increasing the concentration of reactants makes more particles available to react.

When a solid reacts with a liquid or a gas, only the particles on the surface of the solid come in contact with the other reactant. To increase the rate of reaction, you can break the solid into smaller pieces that have more surface area. More material is exposed, so the reaction happens faster.

Another way to increase the rate of a reaction is to increase its temperature. When you heat something, its particles move faster. Faster-moving particles come into contact more often, which means there are more opportunities for a reaction to occur. Faster-moving particles also have more energy. This energy helps the reactants get over the activation energy “hump.”

Another way to control the rate of a reaction is to change the activation energy needed. If you decrease the activation energy, the reaction happens faster. A catalyst is a material that increases the rate of a reaction by lowering the activation energy. Catalysts affect the reaction rate, but they are not considered reactants. The cells in your body contain biological catalysts, called enzymes. Enzymes increase the reaction rates of chemical reactions necessary for life.

Sometimes a reaction is more useful when it can be slowed down rather than speeded up. A material used to decrease the rate of a reaction is called an inhibitor. Most inhibitors work by preventing reactants from coming together.


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Safety in the Science LaboratoryGuide for Reading■ Why is preparation important when carrying out scientific investigations

in the lab and in the field?

■ What should you do if an accident occurs?

Good preparation helps you stay safe when doing science activities in the laboratory. Preparing for a lab should begin the day before you will perform the lab. It is important to read through the procedure carefully and make sure you understand all the directions. Also, review the general safety guidelines in Appendix A. The most important safety rule is simple: Always follow your teacher’s instructions and the textbook directions exactly. Labs and activities in this textbook include safety symbols. These symbols alert you to possible dangers in performing the lab and remind you to work carefully. The symbols are explained in Appendix A. When you have completed the lab, be sure to clean up the work area. Follow your teacher’s instructions about proper disposal of wastes. Finally, be sure to wash your hands thoroughly after working in the laboratory.

Some investigations will be done in the “field.” The field can be any outdoor area, such as a schoolyard, a forest, a park, or a beach. Just as in the laboratory, good preparation helps you stay safe when doing science activities in the field. There can be many potential safety hazards outdoors, including severe weather, traffic, wild animals, or poisonous plants. Advance planning may help you avoid some potential hazards. Whenever you do field work, always tell an adult where you will be. Never carry out a field investigation alone.

At some point, an accident can occur in the lab. When any accident occurs, no matter how minor, notify your teacher immediately. Then, listen to your teacher’s directions and carry them out quickly. Make sure you know the location and proper use of all the emergency equipment in your lab room. Knowing safety and first aid procedures beforehand will prepare you to handle accidents properly.

The Work of Scientists ■ Section Summary

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