22.3 Rocks and the Rock Cycle In 1993, Lynn Hill climbed the granite face of El Capitan without the aid of a rope except to provide safety in case of a

22.3 Rocks and the Rock Cycle

In 1993, Lynn Hill climbed the granite face of El Capitan without the aid of a rope except to provide safety in case of a fall. El Capitan is one of the largest and most impressive chunks of granite on Earth.


What are the three major groups of rocks?

Classifying Rocks

Rocks are classified into three major groups—igneous, sedimentary, and metamorphic—based on how they form.


The size, shape, and arrangement of the crystals and other particles that make up a rock give the rock its texture.

A rock’s texture reveals what the rock is made from and how and where it formed.

Classifying Rocks


How do igneous rocks form?

Igneous Rock

Igneous rock forms when molten material cools and solidifies either inside Earth or at the surface.


An igneous rock is a rock that forms from magma.

Magma is a mixture of molten rock and gases, including water vapor, which forms underground.

Magma that flows out of volcanoes is called lava.

Igneous Rock


An igneous rock that forms underground from hardened magma is called an intrusive rock.

An igneous rock that forms at Earth’s surface is called an extrusive rock.

Igneous Rock


Basalt is a fine-grained, extrusive rock. As this basalt cooled, the rock formed into hexagonal columns.

Igneous Rock


Extrusive rocks and intrusive rocks have differences in texture caused by differences in how the rocks formed.

• Intrusive rocks cool slowly underground, allowing their crystals to grow large. Large crystals give intrusive rocks a coarse-grained texture.

• Extrusive igneous rocks cool very quickly at the surface. Their crystals do not grow much before the rock cools. This gives extrusive rocks a fine-grained texture.

Igneous Rock


An igneous rock’s color gives a clue to its mineral composition.

• Gabbro, an intrusive rock, and basalt, an extrusive rock formed from magma rich in iron and magnesium. The rocks are dark and dense.

• Granite is a coarse-grained, intrusive rock with a high silica content. Granite is less dense and lighter in color than basalt and gabbro.

Igneous Rock


How are sedimentary rocks classified?

A sedimentary rock is a rock that forms over time as sediment is squeezed and cemented together.

Sedimentary Rock

Geologists classify sedimentary rocks into three main groups according to how they form: clastic rocks, chemical rocks, and organic rocks.


Sediment consists of small, solid pieces of material that comes from rocks or living organisms.

• The process of weathering breaks down rock at Earth’s surface, turning it into smaller pieces.

• Minerals dissolved in water are also sediment.

Sedimentary Rock


Sediment is often carried away by running water or wind to a new location, where it is deposited in layers.

As sediment piles up, the pressure causes the deeper sediment to be compressed.

Dissolved minerals in the water seep into the space between particles of sediment and form a kind of cement.

Sedimentary Rock


Colorful layers of sandstone, like those in this Utah canyon, formed over millions of years as water and wind laid down sediment.

Sedimentary Rock


Clastic Rock

Sedimentary rocks that form from the broken fragments of other rocks are called clastic rocks.

The fragments that make up clastic rocks are usually held together by cement.

Sedimentary Rock


Clastic rocks are classified mainly based upon the average size of the fragments that they contain.

• Conglomerate is made of gravel and pebbles.• Breccia is made up of sharp-edged fragments.• Sandstone is formed from grains of sand.• Mudstone is made primarily of mud or silt.• Shale is mudstone made of flat grains aligned

so that the rock can split into sheets.

Sedimentary Rock


Conglomerate is a sedimentary rock in which rounded pieces of other rocks are cemented together.

Sedimentary Rock


Chemical Rock

Chemical sedimentary rocks form when minerals precipitate out of solution.

• Rainwater dissolves many minerals on the land. These dissolved minerals are then carried into the ocean.

• As water evaporates from the ocean surface, the concentration increases until the minerals precipitate.

Sedimentary Rock


The strangely shaped rocks in Mono Lake, California, are made of tufa, a chemical rock composed of calcium carbonate.

Sedimentary Rock


Organic Rock

Some rocks form as the result of organic processes.

• Marine animals extract calcium carbonate from ocean water to form their shells and skeletons.

• The shells and skeletons sink to the ocean floor. • The fragments compact and cement together,

forming limestone.

Sedimentary Rock


The cliffs of Dover on the southern coast of England are composed of chalk, a type of fine-grained organic limestone.

Sedimentary Rock


How do metamorphic rocks form?

Metamorphic rock is rock that has been changed by temperature, pressure, or reactions with hot water.

Metamorphic Rock

Metamorphic rock forms when a rock is transformed by heat, pressure, or chemical reactions. Most metamorphic rocks form under high temperatures and pressures deep underground.


Metamorphism can result in a rock with a mineral content that is different from that of the original rock.

• Heat deep inside Earth allows the minerals to recrystallize, and small crystals to enlarge.

• Chemical changes occur, and new minerals may replace the original minerals.

Metamorphic Rock


Heat, pressure, and chemical reactions change the form of rocks. In this sequence, shale changes to slate. Additional heat and pressure may change slate into schist or gneiss.

Metamorphic Rock

Shale Slate Schist Gneiss


Metamorphism also changes the texture of rocks.

When pressure is applied from one direction, the particles may line up and lock together in layers or bands.

Metamorphic rocks with crystals arranged in parallel layers or bands are foliated rocks.

Metamorphic Rock


Some foliated rocks have a striped appearance. For example, when schist is subjected to heat and pressure, the result is gneiss.

Nonfoliated rocks don’t have bands. Their crystals are arranged randomly.

Metamorphic Rock


How can one type of rock change into another?

The rock cycle is a series of processes in which rocks continuously change from one type to another.

The Rock Cycle

In the rock cycle, forces within Earth and at the surface cause rocks to change form.


Constructive forces form new igneous rock.

Destructive forces break down rock, forming sediment.

Other forces push rock deep beneath the surface, where heat and pressure form metamorphic rock.

The Rock Cycle


Depending on their pathway through the cycle, rocks can wear away, undergo metamorphism, or melt and form new igneous rock.

The Rock Cycle



The Rock Cycle



The Rock Cycle



The Rock Cycle



The Rock Cycle


Assessment Questions

1. What type of rocks require weathering and erosion to form? a. sedimentary

b. metamorphic

c. igneous

d. sedimentary, metamorphic, and igneous



1. What type of rocks require weathering and erosion to form? a. sedimentary

b. metamorphic

c. igneous

d. sedimentary, metamorphic, and igneous

ANS: A



1. Igneous rocks are formed when rock is transformed by heat, pressure, or chemical reactions.

TrueFalse



1. Igneous rocks are formed when rock is transformed by heat, pressure, or chemical reactions.

TrueFalse

ANS: F, Metamorphic

Documents

22.3 Rocks and the Rock Cycle In 1993, Lynn Hill climbed the granite face of El Capitan without the aid of a rope except to provide safety in case of a