Passing Plates I - The Theory By Trista L. Pollard

1 You have heard of dinner plates, dessert plates, and license plates. However, did you know that our earth has its own set of plates under its surface? In 1912, German meteorologist Alfred Wegener thought so. In fact, he was the first to propose the continental drift theory. 2 Wegener developed this theory after noticing that the bulge of Brazil on the eastern coast of South America appeared to match up with the dent of the southwestern coast of Africa. From his observation he argued that the two continents were once together, but they drifted apart. He also believed that our world was once a single continent called Pangea. Although Wegener stated that this continent existed throughout earlier geologic time periods, it eventually separated and drifted apart to the seven continents we have today. Wegener said that the continents were made of lighter rocks and that they rested on heavier crystal material. Wegener also pointed out that the fossils of a 270 million year old Mesosaur were found in eastern South America and western Africa. Even though other scientists used the idea of a land bridge to explain the fossils, Wegener held on to his continental drift theory. 3 Due to their makeup, Wegener believed that the continents were not rigidly fixed, but that they slowly moved about one yard per century. Ok, so our current continents shifted from their original positions. Was it magic? Well, this was the only point Wegener could not answer. In the area of earth science, Wegener was ahead of his time. Unfortunately, the scientific community did not take his theory seriously at first. 4 In 1929, an English scientist named Arthur Holmes, added more information to Wegener's theory. He believed that a convective flow of heated rock located in the Earth's mantle beneath the crust was behind the force that moved our continents. When this rock in the mantle heats up, it becomes less dense and rises to the surface. Once it reaches the surface, it cools and sinks back to the mantle where it will be reheated again. Holmes believed that this convective flow provided the force that caused the continents to drift. Even with Holmes' help, Wegener's continental drift theory was not accepted by other scientists until the late 1960's. 5 In the late 1960's (about 1967), J. Tuzo Wilson from the University of Toronto (Canada) was studying seafloor spreading in the Pacific Ocean. Wilson believed that the ocean floor had mid-ocean ridges with faults that were perpendicular to those ridges. He believed that the presence of these faults caused the tearing of the ocean crust from edge to edge. During his studies, he also invented the term plate which meant large masses of moving rock. Now the plate tectonic theory really starts shaking! Wilson proposed that our planet was divided into seven large plates and several smaller plates. The plates were part of the lithosphere which was the hard outer layer of the earth. The lithosphere is about sixty miles thick and rests on a softer layer called the asthenosphere. The asthenosphere is made up of a mixture of gases. These plates would move in relation to each other above hotter deeper zones. Along the boundaries of these shifting plates you have some of the world's most active volcanoes or plate-boundary volcanoes. 6 Now Wegener's theory is widely accepted by geologists and earth scientists around the world. Scientists now believe that the "super continent" Pangea was eventually divided into two continents called Laurasia (northern continental mass) and Gondwanaland (southern continental mass). From these two land masses there was the separation of North America, Eurasia, South America, Africa, and India. The last continents to separate were Australia and Antarctica. 7 So what are these seven major plates? They are the North American Plate, the Eurasian Plate, the Pacific Plate, the South American Plate, the African Plate, the Indian-Australian Plate, and the Antarctic Plate. In Passing Plates II, we will look at plate boundaries and faults. Copyright © 2014 edHelper

Passing Plates II - Who's Fault? By Trista L. Pollard

1 In Passing Plates I, we took a trip back through history to see where the plate tectonic theory was born. Let's journey deeper into the earth to learn about these plates. Most of the plate "action" occurs within the lithosphere, which includes the crust and upper mantle. The crust is the solid outermost layer of the earth, and the mantle is the layer under the crust where the plate movement occurs. All of this makes the lithosphere one restless place. The places where the plates meet in the lithosphere are called plate boundaries. Earthquakes usually occur in plate boundaries. There are three types of plate boundaries: divergent, convergent, and transform. 2 Divergent boundaries or "spreading zones" are found where two plates are moving away from each other. Most divergent boundaries are found in oceans where seafloor spreading occurs. The ocean floor has mid-ocean ridges and underwater mountain chains. In the centers of these mid-ocean ridges lie underwater volcanoes and other "hot spots." These hot spots are areas where magma rises from the asthenosphere. This rising mantle is referred to as an upswelling of magma. As the magma rises, plates in the mid-ocean ridges are forced apart. New earth material is added to the edges which means new oceanic lithosphere is born. 3 Convergent boundaries are found where lithospheric plates move toward each other. Eventually as these plates move toward each other, one plate subducts or overrides the other. This boundary is usually referred to as a subduction zone. As the subducting plate continues to move, the other plate is pushed downward toward the mantle where it will start to melt. The melting causes the plate to be reabsorbed into the earth. It is also the cause of some of the world's most destructive earthquakes. The northwest coasts of the United States, western Canada, and southern Alaska and Aleutian Islands have a subduction zone plate boundary. 4 The third boundary is known as a transform fault. Plates in this boundary are sliding horizontally past each other without much upward or downward movement. There is also no crust destroyed or produced by the movement of these plates. This boundary is also referred to as a strike-slip fault or fracture zone. The famous San Andreas Fault along the coast of California and northwestern Mexico is a transform fault. Earthquakes in this boundary tend to happen at shallow depths. 5 To understand plate boundaries, we also need to understand faults. Faults are a break or fracture in the earth's crust along where two blocks of crust have slipped or slid past each other. We know about strike-slip faults which move laterally. There are also normal faults and thrust (reverse) faults. Normal faults occur when there is pulling or tension as a result of a break in the rock. The rock which lies above the fault surface begins to move down in relation to the rock below the fault surface. Thrust or reverse faults happen when there is compression or squeezing. Rocks above the fault surface move upward in relation to the rock below the fault surface. Strike-slip faults can happen with either type of stress (pulling or compression). Divergent boundaries usually have normal faults. Thrust or reverse faults usually occur along subduction zones. 6 The one important finding by scientists about faults is that earthquakes tend to reoccur along faults. This makes sense because faults are areas where the earth's crust is the weakest. There used to be the belief that if stress were relieved on a fault, another earthquake would not occur. In fact, relieving stress in one area could add stress to another part of the fault. However, as Alfred Wegener and Arthur Holmes pointed out, the earth's surface is constantly shifting. Understanding plate boundaries and faults continues to help scientists as they monitor the earth's movement over time.�

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Principles of Relative-Age Dating By Patti Hutchison

1 Have you ever heard the phrase "older than dirt"? Can anything be older than dirt? The answer is yes! Dirt is formed as layers of rock are eroded away. Some layers of rock are older than others. A process called relative-age dating helps scientists determine how old these rock layers are. This has also helped them to know how old the earth is. 2 Relative-age dating helps scientists put earth's historic events in order. They compare one event or rock layer to another. There are many principles that help scientists with this process. 3 One principle is called original horizontality. This simply means that sediments are usually deposited in horizontal layers. As the layers build up, they cement together to form sedimentary rocks. 4 While looking at rock layers, scientists can use another principle to help determine relative age. This is called the law of superposition. This law assumes that each layer of sediments has been deposited at a different time. Layers on the bottom are deposited first. The younger layers are on the top. This principle only works, of course, for sedimentary rocks that have not been disturbed. 5 A principle that helps scientists determine relative ages of disturbed rock layers is called cross-cutting relationships. This rule is used with igneous intrusion, when there is a fault. A fault is a crack in a rock layer. Movement takes place at a fault. 6 This rule states that a fault is younger than the rock it cuts across. For example, scientists may discover a layer of schist. In a certain area, a layer of granite cuts across it along a fault. The principle of cross-cutting relationships helps scientists to know that the granite is younger than the schist. The granite has intruded into the schist. 7 Erosion can make relative dating difficult. There is a geological principle that helps scientists with these problems, however. When a layer of rock is eroded, some of the particles are carried away. The loose material on the surface may mix with the new layer deposited on top. This is called an inclusion. This shows that the rock layer on the bottom is older than the one on top. 8 Another way to determine relative age is by correlation. This is a way for scientists to match rock layers that are found far from each other. Geologists carefully examine the rocks. They look for unique fossils and other features. This helps them compare the layers. 9 The process of relative-age dating is based on the idea that sediments have been deposited in the same way since the beginning of time. Imagine standing on the bank of a river. The water flows by, depositing sediments along the bottom. This process has always happened the same way. This is called the principle of uniformitarianism. Scientists have been able to determine the age of earth using this principle. 10 Scientists have used relative-age dating to compare rock layers. They can tell if one rock layer is older or younger than another. The relative age of something is not exact. Scientists use other methods if they need to know the exact age of a rock. �

Absolute-Age Dating By Patti Hutchison

1 Absolute-age dating. No, it has nothing to do with dinner and a movie. Absolute-age dating is way for scientists to tell the exact age of a rock, fossil, or other object. There are several methods used to do this. 2 One way is called radiometric dating. This process involves measuring the rate of decay of radioactive isotopes. Isotopes are atoms of the same element that have the same chemical properties. They have different mass numbers. These isotopes are found in igneous and metamorphic rocks. They are also found in fossils and remains of organisms. 3 Radioactive substances give off protons (positive particles) and neutrons (neutral particles) at a steady rate. Eventually, these substances change into different elements. The new element is no longer radioactive. 4 For example, an isotope of uranium, U-238, will eventually decay into an isotope of lead. This change, over time, is called radioactive decay. In this case, the uranium is called the parent and the lead is called the daughter element. 5 Radioactive decay is a constant process. It is not affected by temperature, pressure, or other physical changes. As the number of parent atoms decreases, the number of daughter atoms increases. As scientists measure these changes, they can tell how old a substance is. 6 It takes a long time for most radioactive substances to decay completely. For this reason, scientists use a concept called half-life. This is the time it takes for half the original amount to decay. For example, the half life of uranium-238 is about 4.5 billion years. 7 Counting tree rings is another way to determine the age of an object or event. Trees grow faster during the spring months. They grow more slowly during the winter. A pair of winter and spring tree rings represents an annual ring. Scientists can date an event such as a drought or a forest fire by counting the annual rings on a cross-section of a tree. 8 Another type of absolute dating uses varves. Varves are bands of alternating dark and light sediments. Light colored sediments are usually deposited during summer months. Dark sediments are deposited during the winter. 9 Scientists study these varves, like tree rings, to find evidence of events that ran in a cycle, like the seasons. For example, they can compare varves in different lakes. This can help them find the ages of the lake sediments. 10 Key beds are another time-marker scientists use for absolute dating. A key bed is a layer of sediment. It was deposited during a short-lived event such as a volcanic eruption. Geologists can use radiometric dating to determine how old the rock layer is. They can then use this key bed to compare rock layers across a large area. 11 When Mount Saint Helens erupted in 1980, volcanic ash landed in several states. Over time, this ash will become a thin layer of clay. This is a key bed. It will mark the date of the eruption. 12 The earth has experienced awesome events over its long life. Asteroids, volcanic eruptions, and ice ages have changed the face of the earth. Species of organisms have come and gone. Scientists can tell when these events occurred using absolute-age dating.

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