Three Classes of Rock

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Lesson

ESSENTIAL QUESTION

How do rocks form?By the end of this lesson, you should be able to describe the formation and classification of sedimentary, igneous, and metamorphic rocks.

Three Classes of Rock

Wind and water have eroded the softer rock surrounding Ship Rock, an igneous landform in New Mexico.

7

J S6E5.c Rock formation and the rock cycle

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Answers

474 Unit 5 Earth’s Surface

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4 Apply As you learn the definition of each vocabulary term in this lesson, create your own definition or sketch to help you remember the meaning of the term.

Vocabulary Terms• rock • texture

• composition

3 Apply Use context clues to write your own definition for the words composition and texture.

Example sentence:The composition of the trail mix was 50% nuts, 30% dried fruit, and 20% granola.

composition:

Example sentence:Because glass is smooth, flat, and shiny, it has a much different texture than wood does.

texture:

Active Reading

1 Predict Check T or F to show whether you think each statement is true or false.

T FAll rocks form deep beneath Earth’s surface.

Some rocks are made up of materials from living things.

Some rocks take millions of years to form.

All rocks are made up of the same kinds of minerals.

Some rocks form from particles of other rocks.

2 Identify How do you think rocks might form as a result of the volcanic activity shown here?

Engage Your Brain

Quick Labs• Stretching Out• Observing Rocks

S.T.E.M. Lab• Modeling Rock Formation

475Lesson 7 Three Classes of Rock

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6-8-E_CNLAESE589398_44Afinal12-7-10LKell

Feldspar 65%

Quartz 25%

Mica 10%

Composition of a Sample of Granite

6-8-E_CNLAESE589398_45Afinal pass12-7-10LKell

Composition of a Sample of Limestone

Calcite 95%

Aragonite 5%



A Rocky WorldHow are rocks classified?A combination of one or more minerals or organic matter is called rock. Scientists divide rock into three classes based on how each class of rock forms. The three classes of rock are igneous, sedimentary, and metamorphic. Each class of rock can be further divided into more specific types of rock. For example, igneous rocks can be divided based on where they form. All igneous rock forms when molten rock cools and solidifies. However, some igneous rocks form on Earth’s surface and others form within Earth’s crust. Sedimentary and metamorphic rocks are also divided into more specific types of rock. How do scientists understand how to classify rocks? They observe their composition and texture.

By Mineral CompositionThe minerals and organic matter a rock contains determine the composition, or makeup, of that rock, as shown below. Many rocks are made up mostly of the minerals quartz and feldspar, which contain a large amount of the compound silica. Other rocks have different compositions. The limestone rock shown below is made up mostly of the mineral calcite.

5 Identify As you read, underline two properties that are used to classify rock.

Active Reading

6 Graph Fill in the percentage grid on the right to show the amounts of calcite and aragonite in limestone.

Do the Math

Granite is made of silica minerals.

Limestone is made of carbonate minerals.

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Answers 5. See students’ pages for annotations.

6. Students should color 95 boxes to repre-sent calcite and 5 boxes to represent aragonite.


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By TextureThe size, shape, and positions of the grains that make up a rock determine a rock’s texture. Coarse-grained rock has large grains that are easy to see with your eyes. Fine-grained rock has small grains that can only be seen by using a hand lens or microscope. The texture of a rock may give clues as to how and where it formed. Igneous rock can be fine-grained or coarse-grained depending on the time magma takes to cool. The texture of metamorphic rock depends on the rock’s original composition and the temperature and pressure at which the rock formed. The rocks shown below look different because they formed in different ways.

This mudstone is made up of microscopic particles of clay.B

This sandstone formed from sand grains that once made up a sand dune.A

This breccia is composed of broken fragments of rock cemented together.C

7 Describe Observe the sedimentary rocks on this page and describe their texture as coarse-grained, medium-grained, or fine-grained.

Visualize It!

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Answers 7. A: medium-grained; B: fine-grained;

C: coarse-grained





The Furnace BelowWhat are two kinds of igneous rock?Igneous rock forms when hot, liquid magma cools into solid rock. Magma forms when solid rock melts below Earth’s surface. Magma flows through passageways up toward Earth’s surface. Magma can cool and harden below Earth’s surface, or it can make its way above Earth’s surface and become lava.

Intrusive Igneous RockWhen magma does not reach Earth’s surface, it cools in large chambers, in cracks, or between layers in the surrounding rock. When magma pushes into, or intrudes, surrounding rock below Earth’s surface and cools, the rock that forms is called intrusive igneous rock. Magma that is well insulated by surrounding rock cools very slowly. The minerals form large, visible crystals. Therefore, intrusive igneous rock generally has a coarse-grained texture. Examples of intrusive igneous rock are granite and diorite. A sample of diorite is shown at the left.

8 Infer What evidence supports the claim that diorite is an intrusive igneous rock?

Deep Inside Earth The amount of time magma takes to cool determines the texture of an igneous rock.

Diorite is an example of intrusive igneous rock.

Magma chamber Magma chambers deep inside Earth contain pools of molten rock. Magma cools slowly in large chambers such as this.

Crystals Slow-cooling magma has time to form large mineral crystals. The resulting rock is coarse-grained.

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Answers 8. Diorite has crystals that I can see without

having to use a hand lens. So, diorite has a coarse-grained texture, which shows that it cooled beneath Earth’s sur-face surrounded by other rock.




Extrusive Igneous RockIgneous rock that forms when lava erupts, or extrudes, onto Earth’s surface is called extrusive igneous rock. Extrusive igneous rock is common around the sides and bases of volcanoes. Lava cools very quickly at Earth’s surface. So, there is very little time for crystal formation. Because there is little time for crystals to form, extrusive rocks are made up of very small crystals and have a fine-grained texture. Obsidian (ahb•SID• ee•uhn) is an extrusive rock that cools so rapidly that no crystals form. Obsidian looks glassy, so it is often called volcanic glass. Other common extrusive igneous rocks are basalt and andesite.

Lava flows form when lava erupts from a volcano. The photo above shows an active lava flow. Sometimes lava erupts and flows from long cracks in Earth’s crust called fissures. It also flows on the ocean floor at places where tension is causing Earth’s crust to pull apart.

Active Reading 9 Explain How does the rate at which magma cools affect the texture of igneous rock?

Basalt is an example of extrusive igneous rock.

Near or at Earth’s Surface Fine-grained igneous rock forms as lava cools quickly at Earth’s surface.

Intrusive Igneous Rock Both Extrusive Igneous Rock

10 Compare Use the Venn diagram to compare and contrast intrusive igneous rock and extrusive igneous rock.

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Answers 9. The less time that magma has to cool,

the smaller the crystals will grow. The resulting rock will be fine-grained.

10. Intrusive Igneous Rock: forms within Earth’s crust; crystal growth is slow; coarse-grained texture; Both: forms when molten rock (either magma or lava) cools and solidifies; Extrusive Igneous Rock: forms at or near Earth’s surface; crystal growth is rapid; fine-grained tex-ture.





Lay It On!What are three types of sedimentary rock?All the processes that form sedimentary rock occur mainly at or near the surface of Earth. Some of these processes include weathering, erosion, deposition, burial, and cementation. Based on the way that they form, scientists classify sedimentary rocks as clastic, chemical, and organic sedimentary rock.

Clastic Sedimentary RockClastic sedimentary rock forms when sediments are buried, compacted, and cemented together by calcite or quartz. The size of the sediment, or clasts, that makes up the rock is used to classify clastic sedimentary rocks. Fine-grained sedimentary rocks, in which grains are too small to be seen, include mudstone, siltstone, and shale. Sandstone, which is shown at the left, is a medium-grained clastic sedimentary rock with visible grains. Breccia and conglomerate are coarse-grained clastic sedimentary rocks made of large particles, such as pebbles, cobbles, and boulders.

Chemical Sedimentary RockChemical sedimentary rocks form when water, usually seawater, evaporates. Most water contains dissolved minerals. As water evaporates, the minerals in water become concentrated to the point that they precipitate out of solution and crystallize. Halite, or rock salt, is an example of chemical sedimentary rock. It is made of sodium chloride, NaCl. Halite forms when sodium ions and chlorine ions in shallow bodies of water become so concentrated that halite crystallizes from solution.

Sandstone

Halite

Horizontal layers of clastic sedimentary rocks and volcanic ash are exposed at Badlands National Park in South Dakota.

The Bonneville Salt Flats near the Great Salt Lake in Utah are made largely of halite. The salt flats are the remains of an ancient lake bed.

11 Identify How would you describe the texture of the halite shown below?

Visualize It!

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Answers 11. It has a smooth texture without the

appearance of any grains.

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The White Cliffs of Dover on the English sea coast are made up of the skeletons of the marine alga that is shown below.

Skeleton of a marine alga

Organic Sedimentary RockOrganic sedimentary rock forms from the remains or fossils, of once-living plants and animals. Most limestone forms from the fosssils of organisms that once lived in the ocean. Over time, the skeletons of these marine organisms, which are made of calcium carbonate, collect on the ocean floor. These animal remains, together with sediment, are eventually buried, compacted, and cemented together to form fossiliferous [fahs•uh•lif•er•uhs] limestone.

Coquina is a fossiliferous limestone that consists of the shells of marine mollusks that have been cemented together by calcite. Chalk is a soft, white limestone that is made up of the skeletons of microorganisms that collect in huge numbers on the floor of the deep ocean.

Coal is another type of organic sedimentary rock. It forms when plant material is buried and changes into coal as a result of increasing heat and pressure. This process occurs over millions of years.

Active Reading 12 Identify What are two types of organic sedimentary rock?

13 Compare Use the table to compare and contrast clastic, chemical, and organic sedimentary rock.

Three Types of Sedimentary RockClastic Chemical Organic

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Answers12. Sample answer: chalk and coal

13. Clastic: forms from sediment, or clasts, cemented together; classified as fine-grained, medium-grained, and coarse-grained; Chemical: forms from solutions of dissolved minerals; Organic: forms from the remains of animals or plants.





The Heat Is On!What are two types of metamorphic rock?As a rock is exposed to high temperature and pressure, the crystal structures of the minerals in the rock change to form new minerals. This process results in the formation of metamorphic rock, which has either a foliated texture or a nonfoliated texture.

Foliated Metamorphic RockThe metamorphic process in which mineral grains are arranged in planes or bands is called foliation (foh•lee•ay•shuhn). Foliation occurs when pressure causes the mineral grains in a rock to realign to form parallel bands.

Metamorphic rocks with a foliated texture include slate, phyllite, schist (SHIST), and gneiss (NYS). Slate and phyllite are commonly produced when shale, a fine-grained sedimentary rock, is exposed to an increase in temperature and pressure. The minerals in slate and phyllite are squeezed into flat, sheet-like layers. With increasing temperature and pressure, phyllite may become schist, a coarse-grained foliated rock. With further increases in temperature and pressure, the minerals in schist separate into alternating bands of light and dark minerals. Gneiss is a coarse-grained, foliated rock that forms from schist. Slate, phyllite, schist, and gneiss can all begin as shale, but they are very different rocks. Each rock forms under a certain range of temperatures and pressures, and contains different minerals.

14 Describe What happens to the minerals as gneiss forms from schist?

When shale is exposed to increasing temperature and pressure, different foliated metamorphic rocks form.

Sedimentary shale

Slate

Phyllite

Schist Gneiss

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Answers 14. The minerals recrystallize and separate

into distinct bands of dark and light min-erals.



Think Outside the Book Inquiry


The mineral grains in quartzite (top) and crystals in marble (bottom) do not form bands.

Marble is a nonfoliated metamorphic rock that forms when limestone is metamorphosed. Marble is used to build monuments and statues.

Nonfoliated Metamorphic RockMetamorphic rocks that do not have mineral grains that are aligned in planes or bands are called nonfoliated. Nonfoliated metamorphic rocks are commonly made of one or only a few minerals. During metamorphism, mineral grains or crystals may change size or shape, and some may change into another mineral.

Two common nonfoliated metamorphic rocks are quartzite and marble. Quartzite forms when quartz sandstone is exposed to high temperature and pressure. This causes the sand grains to grow larger and the spaces between the sand grains disappear. For that reason, quartzite is very hard and not easily broken down.

When limestone undergoes metamorphism, the limestone becomes marble. During the process of metamorphism, the calcite crystals in the marble grow larger than the calcite grains in the original limestone.

Active Reading 15 Apply What are two characteristics of nonfoliated metamorphic rocks?

16 Apply With a classmate, discuss how different types of rocks can be used as building or construction materials.

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Answers 15. Nonfoliated rocks do not have bands or

aligned minerals.

16. Students’ answers should demonstrate knowledge of the formation of both met-amorphic and sedimentary rock.



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20 Claims • Evidence • Reasoning While hiking in the mountains, you see a large outcrop of marble. State a process by which the metamorphic rock marble forms from the sedimentary rock limestone. Summarize evidence to support your claim and explain your reasoning.

Answers: 17 clastic, chemical, organic; 18 intrusive, extrusive; 19 foliated, nonfoliated

To complete this summary, fill in the blanks. Then, use the key below to check your answers. You can use this page to review the main concepts of the lesson.

Igneous rock forms from magma or lava that has cooled and hardened.

Metamorphic rock forms under high temperature or pressure deep within Earth’s crust.

Sedimentary rock may form from layers of sediment that are cemented together.

17 Sedimentary rocks can be classified into three groups:

, , and

18 Igneous rocks can be classified into two groups:

and 19 Metamorphic rocks can be classified into two groups:

and

Three Classes of Rock

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Visual Summary Answers 17. clastic, chemical, organic

18. intrusive, extrusive

19. foliated, nonfoliated

20. First, sediment and the remains of organisms had to build up in layers on the ocean floor. Then, over time these layers were buried and compressed, and then hardened into limestone. The limestone was buried deeper within Earth’s crust, where high temperatures or pressure caused the limestone to change into the marble, a nonfoliated metamorphic rock.


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.LessonLesson Review

VocabularyFill in the blank with the term that best completes the following sentence.

1 Sedimentary rocks that are made up of large pebbles and stones have a coarse-grained

2 Most granite has a of quartz, mica, and feldspar.

3 can be considered to be mixtures of minerals.

Key Concepts4 Summarize How does the cooling rate of

magma or lava affect the texture of the igneous rock that forms?

5 Describe How does clastic sedimentary rock form?

6 Explain What is the difference between foliated and nonfoliated metamorphic rock?

Critical ThinkingUse this photo to answer the following questions.

7 Identify What type of rock is shown here? What evidence supports your claim?

8 Describe How did this rock form? Explain your reasoning.

9 Infer Suppose this rock was exposed to high temperatures and pressure. What would most likely happen to it?

10 Infer What information can a foliated metamorphic rock provide you about the conditions under which it formed?

7


Lesson Review Answers 1. texture

2. composition

3. Rock

4. When magma cools slowly within Earth’s crust, crystals have a long time to form, so the crystals can grow to a large size. The resulting intrusive igneous rock has a coarse-grained texture. Lava on Earth’s surface cools rapidly, so crystals have only a short time to grow. Therefore, the crystals are small. The resulting extrusive igne-ous rock has a fine-grained texture.

5. Clastic sedimentary rock forms when sediments that are buried are compacted and cemented together.

6. Foliated rocks have minerals that are arranged in layers or bands. In nonfoliated rocks there is no regu-lar arrangement of the minerals.

7. Sample answer: The rock is an organic sedimentary rock. I can tell this because it is made up of marine fossils.

8. Marine organisms died, and their remains built up in layers on the ocean floor. Over time, these layers were buried and compressed, and then hardened to form fossiliferous limestone, an organic sedimentary rock.

9. The rock would most likely change into marble, a nonfoliated meta-morphic rock.

10. A foliated metamorphic rock can provide some information about the composition of the rock that was metamorphosed and the pres-sure under which it metamor-phosed.


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My Notes



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Three Classes of Rock

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