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Chapter 5: Igneous Rocks

EARTH SCIENCE Geology, the Environment and the Universe

Section 5.1 What are igneous rocks?

Section 5.2 Classification of Igneous Rocks

CHAPTER

5 Igneous Rocks

Exit Click a hyperlink to view the corresponding slides.

Essential Questions

• How do igneous rocks form?

• How can the composition of magma be described?

• What are the factors that affect how rocks melt and crystallize?

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5.1 What are igneous rocks?

Review Vocabulary

• silicate: mineral that contains silicon and oxygen, and usually one or more other elements

• Igneous rocks are the rocks that form when molten material cools and crystallizes.

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New Vocabulary

lava

igneous rock

partial melting

Bowen’s reaction series

fractional crystallization

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Igneous Rock Formation

• Lava is magma that flows out onto Earth’s surface.

• Igneous rocks form when lava or magma cools and minerals crystallize.

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• The type of igneous rock that forms depends on the composition of the magma, which is often a slushy mix of molten rock, dissolved gases, and mineral crystals.

Composition of magma

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• Magma is classified as basaltic, andesitic, or rhyolitic, based on the amount of silica it contains. Silica affects melting temperature and impacts how quickly magma flows.

Composition of magma

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Please click the image above to view the interactive table.

egeu_ch05_t05_1.swf

egeu_ch05_t05_1.swf

• Magma can be formed either by melting of Earth’s crust or by melting within the mantle.

Magma formation


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• The four main factors involved in the formation of magma are temperature, pressure, water content, and mineral content.

Magma formation


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• Temperature generally increases with depth in Earth’s crust. This temperature increase is known as the geothermal gradient.

Magma formation


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• Pressure also increases with depth as a result of the weight of overlying rock. The increased pressure on a rock also increases the rock’s melting point.

Magma formation


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• Rocks and minerals often contain small percentages of water, which changes the melting point of the rocks. As water content increases, the melting point decreases.

Magma formation


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• Rocks melt at lower or higher temperatures because their component minerals have lower or higher melting points. In general, rocks that are rich in iron and magnesium melt at higher temperatures than rocks that contain higher levels of silicon.

Mineral content


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• The process whereby some minerals melt at relatively low temperatures while other minerals remain solid is called partial melting.

Partial melting


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• If temperatures are not high enough to melt the entire rock, the resulting magma will have a different composition than that of the original rock.

Partial melting


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• Canadian geologist N. L. Bowen demonstrated that as magma cools and crystallizes, minerals form in predictable patterns in a process now known as the Bowen’s reaction series.

Bowen’s Reaction Series

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• Bowen discovered two main patterns, or branches, of crystallization.


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• The left branch of Bowen’s reaction series represents the iron-rich minerals, which undergo abrupt changes as magma cools and crystallizes.

Iron-rich minerals


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• The right branch of Bowen’s reaction series represents the plagioclase feldspars, which undergo a continuous change of composition as magma cools.

Feldspars


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• When magma cools, the first minerals that crystallize are the last minerals that melted during partial melting. This process is called fractional crystallization.

Fractional Crystallization

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• In the basaltic intrusion of the Palisade Sills in the Hudson River valley of New York and New Jersey, small crystals formed in the chill zone as the outer areas of the intrusion cooled more quickly than the interior.

Visualizing Fractional Crystallization

and Crystal Settling

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Please click the image above to view the video.

5 Visualizing Fractional Crystallization and Crystal Settling.swf

5 Visualizing Fractional Crystallization and Crystal Settling.swf

• Under certain conditions, newly formed crystals can be removed from magma, ending the chemical reactions between the magma and the minerals.

Fractional Crystallization

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a. true

b. false

The presence of water decreases the

melting temperature of rock.

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5.1 Section Check

a. iron

b. potassium

c. silica

d. water

Which component has the highest

abundance in most of the magmas that

form on Earth?

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5.1 Section Check

a. The crystals grew quickly.

b. The crystals were partly dissolved.

c. The magma had high gas content.

d. The magma’s composition changed.

Crystals of plagioclase feldspar in an

igneous rock are visibly zoned. Which

explanation explains this observation?

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5.1 Section Check

Essential Questions

• What are the different types and textures of igneous rocks?

• How do cooling rates affect the grain sizes in igneous rocks?

• What are some of the uses of igneous rocks?

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5.2 Classification of Igneous Rocks

• fractional crystallization: a sequential process during which early formed crystals are removed from the melt and do not react with the remaining magma

Review Vocabulary

• Classification of igneous rocks is based on mineral composition and texture.

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New Vocabulary

intrusive rock

extrusive rock

basaltic rock

granitic rock

texture

porphyritic texture

vesicular texture

pegmatite

kimberlite

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Mineral Composition of Igneous Rocks

• When magma cools and crystallizes below Earth’s surface, intrusive rocks form.

• Magma that cools and crystallizes on Earth’s surface forms extrusive rocks.

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• Igneous rocks are classified by their mineral compositions.

• Basaltic rocks, also called mafic rocks, are dark-colored, have lower silica contents, and contain mostly plagioclase and pyroxene.

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• Granitic rocks, also called felsic rocks, are light-colored, have high silica contents, and contain mostly quartz and feldspar.

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• Rocks that have a composition of minerals that is somewhere between basaltic and granitic are called intermediate rocks.

• A fourth category, called ultrabasic, or ultramafic, contains rocks with only iron-rich minerals such as olivine and pyroxene and are always dark.


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• Rock type can be determined by estimating relative percentages of minerals in the rocks.

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Texture

• Texture refers to the size, shape, and distribution of the crystals or grains that make up a rock.

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• When lava flows on Earth’s surface, it cools quickly and there is not enough time for large crystals to form. The resulting extrusive igneous rocks have crystals so small that they cannot be seen without magnification.

Crystal size and cooling rates

Texture

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Crystal size and cooling rates

Texture

• When lava cools so quickly that crystals do not form, volcanic glass is the result.

• When magma cools slowly below Earth’s surface, there is sufficient time for large crystals to form. Thus, intrusive rocks can have crystals larger than 1 cm.

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• A porphyritic texture is characterized by large, well-formed crystals surrounded by finer-grained crystals of the same mineral or different minerals.

Porphyritic rocks

Texture

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• The spongy appearance that results from gas bubbles trapped in lava is called vesicular texture.

Vesicular rocks

Texture

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Thin Sections

• Scientists use a special microscope, called a petrographic microscope, to help them identify minerals by examining the mineral grains in sections of rock thin enough for light to pass through.

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Igneous Rocks as Resources

• Valuable ore deposits often occur within igneous intrusions. At other times, they may occur as veins in the rocks surrounding intrusions.

Veins

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• Veins of extremely large-grained minerals that can contain ores are called pegmatites.

Pegmatites

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• Diamonds and other minerals that can form only under very high pressure are found in rare, ultrabasic rocks known as kimberlites.


Kimberlites

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• The interlocking grain textures and the resistance to weathering of many of the minerals present in igneous rocks make them especially useful as building materials.


Igneous rocks in construction

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Which rock is the intrusive equivalent of

basalt?

a. granite

b. diorite

c. gabbro

d. obsidian

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5.2 Section Check

In which igneous rock should a person

prospect for diamonds?

a. granite

b. kimberlite

c. basalt

d. pegmatite

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5.2 Section Check

What information about an igneous rock

might a person learn from a description of

its texture?

Answer: A description of the texture of an igneous rock may provide information about the depths at which a rock was formed, and how quickly the magma cooled.

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5.2 Section Check

CHAPTER

5 Igneous Rocks

Resources

Chapter Assessment Questions

Standardized Test Practice

Click on a hyperlink to view the corresponding feature.

Study Guide

Earth Science Online

http://www.connected.mcgraw-hill.com/

• Igneous rocks are the rocks that form when molten material cools and crystallizes.

• Magma consists of molten rock, dissolved gases, and mineral crystals.

• Magma is classified as basaltic, andesitic, or rhyolitic, based on the amount of silica it contains.

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Study Guide

• Different minerals melt and crystallize at different temperatures.

• Bowen’s reaction series defines the order in which minerals crystallize from magma.

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Study Guide

• Classification of igneous rocks is based on mineral composition and texture.

• Igneous rocks are either ultrabasic, basaltic, intermediate, or granitic.

• The rate of cooling determines crystal size.

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5.2 SECTION


Study Guide

• Ores often occur in pegmatites. Diamonds occur in kimberlites.

• Some igneous rocks are used as building materials because of their strength, durability, and beauty.

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Study Guide

According to Bowen’s reaction series, which

mineral crystallizes at the highest temperature?

a. olivine

b. amphibole

c. quartz

d. potassium feldspar

CHAPTER

5 Igneous Rocks

Chapter Assessment

Which texture indicates that an igneous rock

cooled very quickly?

a. vesicular

b. coarse-grained

c. glassy

d. spongy

CHAPTER

5 Igneous Rocks

Chapter Assessment

A student is examining thin sections of a

gabbro and a granite. Which mineral should the

student identify in the gabbro but probably not

identify in the granite?

a. quartz

b. potassium feldspar

c. muscovite mica

d. olivine

CHAPTER

5 Igneous Rocks

Chapter Assessment

Magmas of several different compositions have

erupted from the same volcano. How can this

be explained by the concept of fractional

crystallization?

CHAPTER

5 Igneous Rocks

Chapter Assessment

Answer: Fractional crystallization is one way that different magmas can be produced at the same volcano. If magma is injected beneath the volcano and begins to crystallize, the composition of the remaining magma will change as the crystals form. The magma may then erupt from the volcano at any time.

CHAPTER

5 Igneous Rocks

Chapter Assessment

Why are igneous rocks sometimes used as

building stones?

Answer: Igneous rocks are hard and often durable. They can endure the outdoor environment on buildings or in statues. Many igneous rocks consist of crystals that have attractive colors, shapes, and arrangements. When cut and polished, these rocks make attractive floors, walls, and countertops.

CHAPTER

5 Igneous Rocks

Chapter Assessment

An igneous rock consists entirely of olivine.

Which composition does the rock have?

a. granitic

b. intermediate

c. basaltic

d. ultrabasic

CHAPTER

5 Igneous Rocks


The igneous rock pumice has many holes.

What do these holes represent?

a. dissolved crystals

b. gas pockets

c. pores between grains

d. fractures in the rock

CHAPTER

5 Igneous Rocks


The graph shows Earth’s geothermal gradient.

In which of the following depth ranges does

temperature increase most rapidly?

a. 0 to 1000 meters

b. 1000 to 2000 meters

c. 3000 to 4000 meters

d. 4000 to 5000 meters

CHAPTER

5 Igneous Rocks


a. rhyolite

b. granite

c. diorite

d. gabbro

Which rock formed from lava that cooled on

Earth’s surface?

CHAPTER

5 Igneous Rocks


Hot, water-rich fluids sometimes form during

the last stages of magma crystallization. Why

are these fluids important?

CHAPTER

5 Igneous Rocks


Answer: The fluids often include rare and valuable elements that have very low concentrations in Earth’s crust as a whole. Watery fluids sometimes form metal-rich veins or other metal-rich deposits that include these valuable elements.

CHAPTER

5 Igneous Rocks


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