Objectives

Objectives• Compare and contrast intrusive and extrusive

igneous rocks.

What are igneous rocks?

• Describe the composition of magma.

• Discuss the factors that affect how rocks melt and crystallize.

– igneous rock

– lava

– extrusive

– intrusive

Vocabulary– partial melting

– fractional crystallization

– Bowen’s reaction series

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

What are igneous rocks?• Igneous rocks are rocks that are formed from

the crystallization of magma.


Types of Igneous Rocks• Extrusive igneous rocks are fine-grained igneous

rocks that cool quickly on Earth’s surface.


• Intrusive igneous rocks are coarse-grained igneous rocks that cool slowly beneath Earth’s surface.

• Granite is the most common intrusive igneous rock.

Types of Igneous Rocks• Careful study of granite rock formations revealed

that they cut across other rock formations.


• These cross-cutting relationships are evidence that the granite was intruded, or forced into, existing rocks.

Magma cools slowly beneath Earth’s surface and forms course-grained igneous rocks such as granite.

Composition of Magma• Magma is often a slushy mix of molten rock,

gases, and mineral crystals.


• The elements found in magma are the same major elements found in Earth’s crust: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), potassium (K), and sodium (Na).

Composition of Magma• Magmas are classified as

basaltic, andesitic, and rhyolitic, based on the amount of SiO2 they contain.


• Of all the compounds found in magma, silica (SiO2) is the most abundant and has the greatest effect on magma characteristics.

• Silica content affects melting temperature and also impacts how quickly magma flows.

Origins of Magma• In the laboratory, most rocks must be heated

to temperatures of 800°C to 1200°C before they melt.


• These temperatures are found in the upper mantle and lower crust.

• Scientists theorize that the remaining energy from Earth’s molten formation and the heat generated from the decay of radioactive elements are the sources of Earth’s thermal energy.

Origins of Magma

Factors That Affect Magma Formation


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

– Temperature generally increases with depth in Earth’s crust, a phenomenon known as the geothermal gradient.

Origins of Magma



– Pressure increases with depth as a result of the weight of overlying rock.

– As pressure on a rock increases, its melting point increases.

– Rocks and minerals often contain small percentages of water.

– As water content increases, the melting point decreases.

Origins of Magma



– Mineral content also impacts how magma is formed as different minerals have different melting points.

– In general, oceanic crust is rich in iron and magnesium and therefore melts at higher temperatures than continental crust, which contains higher levels of silicon and aluminum.

– For rocks to melt, the right combination of temperature, pressure, and composition must be present.

Origins of Magma



– Granite’s higher water content and mineral composition cause it to melt at a lower temperature than basalt.

How Rocks Melt

Partial Melting


– Because different minerals have different melting points, not all parts of a rock melt at the same time.

– Partial melting is the process whereby some minerals melt at low temperatures while other minerals remain solid.

How Rocks Melt

Partial Melting


– If temperatures are not great enough to melt the entire rock, the resulting magma will have a different chemistry from that of the original rock.

– This is one way in which different types of igneous rocks form.

How Rocks Melt

Fractional Crystallization


– When magma cools, it crystallizes in the reverse order of partial melting—the first minerals to crystallize from magma are the last minerals to melt during partial melting.

– Fractional crystallization is the process wherein different minerals form at different temperatures.

Bowen’s Reaction Series• Bowen’s reaction series illustrates the

relationship between cooling magma and mineral formation.


• Bowen discovered two main patterns, or branches, of crystallization.– The first pattern is characterized by a continuous, gradual change of

mineral compositions in the feldspar group.

– The second pattern is characterized by an abrupt change of mineral type in the iron-magnesium groups.

Bowen’s Reaction Series



Feldspars


– In Bowen’s reaction series, the right branch represents the feldspar minerals, which undergo a continuous change of composition.

– As magma cools, the first feldspars to form are rich in calcium.

– As cooling continues, their calcium-rich compositions change to sodium-rich compositions.


Feldspars


– In some instances, as when magma cools rapidly, the calcium-rich cores are unable to react completely with the magma.

– The result is a zoned crystal that has sodium-rich outer layers and calcium-rich cores.


Iron-Rich Minerals


– The left branch of Bowen’s reaction series represents the iron-rich minerals.

– These minerals undergo abrupt changes during fractional crystallization.


Iron-Rich Minerals


– As minerals form in the order shown in Bowen’s reaction series, elements are removed from the magma.

– Silica and oxygen are left over at the end of the reaction series.

– When the remaining melt, enriched with silica and oxygen, finally crystallizes, quartz is formed.

– Geologists hypothesize that under certain conditions, newly formed crystals can be separated from magma.

– This stops the chemical reactions between the magma and the minerals.

– Crystal separation can occur when crystals settle to the bottom of the magma body, and when liquid magma is squeezed from the crystal mush to form two distinct igneous bodies with different compositions.


Crystal Separation


– In some magma bodies, the minerals form into distinct bands in the order shown in Bowen’s reaction series resulting in a layered intrusion.


Layered Intrusions


– Geologists are uncertain how these layers form.

– Layered igneous intrusions can be valuable sources of rare metals.


Layered Intrusions


– Some have very high concentrations of elements such as platinum, chromium, nickel, or gold.

Section Assessment

1. Match the following terms with their definitions.

___ extrusive

___ intrusive

___ lava

___ fractional crystallization


A. coarse-grained igneous rocks that cool slowly beneath Earth’s surface

B. magma that flows out onto Earth’s surface

C. fine-grained igneous rocks that cool quickly on Earth’s surface

D. the process wherein different minerals form at different temperatures

C

A

B

D

Section Assessment

2. Describe partial melting.


Partial melting is the process whereby some minerals melt at low temperatures while other minerals remain solid.

________ In Bowen’s reaction series feldspars undergo abrupt changes.

________ The melting point of basalt is higher than granite.

________ Increased pressure decreases the melting point of rock.

________ Quartz is the last mineral to crystallize.

Section Assessment

3. Identify whether the following statements are true or false.


false

true

false

true

Objectives• Classify different types and textures of igneous rocks.

Classifying Igneous Rocks

• Recognize the effects of cooling rates on the grain sizes of igneous rocks.

• Describe some uses of igneous rocks.

– felsic

– mafic

– ultramafic

– porphyritic

Vocabulary– pegmatite

– kimberlite

Classifying Igneous Rocks• Igneous rocks are broadly classified as intrusive

or extrusive.


• Igneous rocks are further classified by their mineral compositions.

• Physical properties such as grain size and texture serve as clues for the identification of various igneous rocks.

Mineral Composition• The three main groups of igneous rocks—felsic,

mafic, and intermediate—are classified according to their mineral compositions.


– Felsic rocks, such as granite, are light-colored and have high silica contents.

– Mafic rocks, such as gabbro, are dark-colored, have lower silica contents, and are rich in iron and magnesium.

– Intermediate rocks, such as diorite, have some characteristics of both felsic and mafic rocks.

Mineral Composition


Mineral Composition

Ultramafic Rocks


– Ultramafic rocks are unusual in that they have low silica contents and very high levels of iron and magnesium.

– Some scientists theorize ultramafic rocks are formed by the fractional crystallization of olivine and pyroxene.

– Another hypothesis is that ultramafic rocks represent pieces of the upper mantle that have been brought close to Earth’s surface.

Grain Size• In addition to differences in their mineral

compositions, igneous rocks differ in the sizes of their grains.


Cooling Rates

– When lava flows on Earth’s surface, it cools quickly and there is not enough time for large crystals to form.

– Extrusive igneous rocks have no visible mineral grains.

– When magma cools slowly beneath Earth’s surface, there is sufficient time for large crystals to form.

– Intrusive igneous rocks may have crystals larger than 1 cm.

Texture• Often, it’s easier to observe the sizes of mineral

grains than it is to observe their shapes.


• Many mineral grains have interlocking edges.

• As the grains crystallize from magma, they grow together and form irregular edges.

• During fractional crystallization, the minerals that form early in the process float in a liquid and have space in which to grow distinct crystal shapes.

Texture

Porphyritic Texture


– A rock that has a porphyritic texture is characterized by large, well-formed crystals surrounded by finer-grained crystals of the same mineral or different minerals.

– Porphyritic textures indicate a complex cooling history wherein a slowly cooling magma suddenly began cooling rapidly.

Igneous Rocks as Resources• Igneous rocks have several characteristics that

make them especially useful as building materials.


– The interlocking grain textures of igneous rocks help to give them strength.

– Many of the minerals found in igneous rocks are resistant to weathering.

Ore Deposits• Ores are minerals that contain a useful substance

that can be mined at a profit.


• Valuable ore deposits are often associated with igneous intrusions.

• These deposits sometimes occur as veins.

Ore Deposits

Veins


– Some important metallic elements that are not included in common minerals are gold, silver, lead, and copper.

– These elements, along with the dissolved silica, are released at the end of magma crystallization in a hot, mineral-rich fluid that fills cracks and voids in the surrounding rock.

– This fluid solidifies to form metal-rich quartz veins, such as the gold-bearing veins.

Ore Deposits

Pegmatites


– Vein deposits may contain other valuable resources in addition to metals.

– Pegmatites are veins of extremely large-grained minerals.

– Ores of rare elements, such as lithium and beryllium, are found in pegmatites.

– Because these veins fill cavities and fractures in rock, minerals grow into voids and retain their shapes.

Ore Deposits

Kimberlites


– Diamond is a valuable mineral found in rare, ultramafic rocks known as kimberlites.

– Kimberlites are a variety of peridotite and likely form deep in the crust at depths of 150 to 300 km or in the mantle.

– Minerals found in kimberlites can form only under very high pressures.

– Geologists hypothesize that kimberlite magma is intruded rapidly upwards towards Earth’s surface, where it forms long, narrow, pipelike structures.

Section Assessment

1. Match the following terms with their definitions.

___ felsic

___ mafic

___ ultramafic

___ porphyritic


A. igneous rocks that have low silica content and very high levels of iron and magnesium

B. igneous rocks that are light-colored, have high silica contents, and contain quartz and feldspars

C. texture characterized by well-formed crystals surrounded by finer-grained crystals

D. igneous rocks that are dark-colored, have lower silica contents, and are rich in iron and magnesium

B

D

A

C

Section Assessment


2. What characteristic would indicate that kimberlites are formed deep within the crust or in the mantle?

They likely form deep within the crust or in the mantle because diamonds and other minerals found in kimberlites can only form under very high pressure.

Section Assessment

3. Why are some of the world’s most beautiful crystals found in pegmatites?


These veins fill cavities and fractures in rock which allows minerals to grow into voids and retain their shapes.

Chapter Resources Menu

Chapter Resources Menu

Study Guide

Section 5.1

Section 5.2

Chapter Assessment

Image Bank

Section 5.1 Main Ideas• Igneous rocks are formed by the cooling and

crystallization of magma. Intrusive rocks form inside Earth’s crust, and extrusive rocks form on Earth’s surface. Extrusive rocks, which cool more rapidly than intrusive rocks, are generally more fine grained.

• Magma is a slushy mix of molten rock, gases, and mineral crystals. The elements found in magma are the same major elements found in Earth’s crust: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), potassium (K), and sodium (Na).

Section 5.1 Study Guide

Section 5.1 Main Ideas• Silica (SiO2) is the most abundant compound in magma.

Magmas are classified as basaltic, andesitic, and rhyolitic, based on the amount of SiO2 they contain.

• Different minerals melt and crystallize at different temperatures in the processes of partial melting and fractional crystallization. Minerals crystallize from magma in a sequential pattern known as Bowen’s reaction series.


Section 5.2 Main Ideas• Igneous rocks are classified as felsic, mafic, intermediate,

and ultramafic, depending upon their mineral compositions. Felsic rocks such as granite are light-colored, have high silica contents, and contain quartz and feldspars. Mafic rocks such as gabbro are dark-colored, have lower silica contents, and are rich in iron and magnesium. Intermediate rocks have moderate silica levels. Ultramafic rocks have low silica contents and very high levels of iron and magnesium. Igneous groups can be further identified by crystal size and texture.

• Early forming minerals may have well-shaped crystals, while later-forming minerals have irregular shapes. Porphyritic textures contain both large and small crystals.


Section 5.2 Main Ideas• Igneous rocks such as granite are often used as building

materials because of their strength, durability, and beauty.

• Valuable ore deposits and gems are often associated with igneous intrusions. Ores of rare elements such as lithium and beryllium are found in veins of extremely large-grained minerals called pegmatites. Diamonds are found in rare types of igneous intrusions known as kimberlites.


1. The process wherein different minerals form at different temperatures is called ____.

a. partial melting

b. Bowen’s reaction series

c. fractional crystallization

d. crystal separation

Multiple Choice

Chapter Assessment

Partial melting is when some minerals melt at low temperatures while others remain solid. Bowen’s reaction series describes in what order minerals form. Crystal separation is a hypothesis that answers questions about Bowen’s reaction series.

2. Andesitic magma’s SiO2 concentration is ____.

a. 30 percent c. 60 percent

b. 45 percent d. 70 percent

Basaltic magma has an SiO2 concentration of 50 percent. Rhyolitic magma has an SiO2 concentration of 70 percent.

Multiple Choice

Chapter Assessment

Multiple Choice

3. What minerals are associated with the left branch of Bowen’s reaction series?

a. iron-magnesium c. intrusive

b. feldspars d. felsic

Chapter Assessment

Feldspars are represented on the right branch of Bowen’s reaction series. Intrusive describes igneous rocks that cool slowly beneath Earth’s surface. Felsic rocks are one classification of igneous rocks.

Multiple Choice

4. What is the texture of ultramafic rocks?

a. glassy c. coarse-grained

b. fine-grained d. very coarse-grained

Chapter Assessment

Ultramafic rocks include peridotite and dunite which both have a coarse grain.

Multiple Choice

5. Extrusive igneous rocks generally have small mineral grains because ____.

a. they cool slowly.

b. the minerals in extrusive rocks do not form crystals under any condition.

c. they cool quickly.

d. there is too much water in the magma.

Chapter Assessment

By quickly cooling, the minerals in extrusive igneous rocks do not have enough time to form large crystals.

Short Answer

6. What characteristics make igneous rocks useful as building materials?

Chapter Assessment

Igneous rocks are useful as building materials because the interlocking grain textures of igneous rocks help to give them strength and many of the minerals found in igneous rocks are resistant to weathering.

Short Answer

7. What causes a porphyritic texture to form in certain rocks?

Chapter Assessment

A porphyritic texture indicates a complex cooling history wherein a slowly cooling magma suddenly began cooling rapidly.

True or False

8. Identify whether the following statements are true or false.

______ Diamonds are found in kimberlites.

______ Fe-Mg minerals undergo a continuous change

according to Bowen’s reaction series.

______ Silica is the most common compound in magma.

______ A rock that melts at 1000ºC on the surface of Earth may not melt until 1200ºC at a depth of 100 km.

______ Fractional crystallization occurs in the reverse

order of partial melting.

Chapter Assessment

true

false

true

true

true

Image Bank

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Chapter 5 Images

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