Problemsolving Lab

Interpret Scientific Illustrations

Which metamorphic minerals will form? The minerals that form in metamorphic rocks depend on the metamorphic grade and composition of the original rock. The figure below and Figure 6.19 show the mineral groups that form under different metamorphic conditions.

Minerals in Metamorphosed Basalt

Lithification Low grade Intermediate grade High grade

Zeolite

Epidote

Amphibolel_

Pyroxene

(Sodium-rich) Plagioclase feldspar (Calcium-rich)

Analysis

1. What mineral is formed when shale and basalt are exposed to low-grade metamorphism?

2. Under high-grade metamorphism, what mineral is formed in shale but not in basalt?

Think Critically

3. Compare the mineral groups that you would expect to form from intermediate-grade metamorphism of shale, basalt, and limestone.

4. Describe the major compositional differences between shale and basalt. How are these differences reflected in the minerals formed during metamorphism?

5. Explain When limestone is metamorphosed, there is little change in mineral composition. Calcite is still the dominant mineral. Explain why this happens.

Granite intrusion

Granite intrusion

Contact Metamorphism For Limestone

Figure 6.20 Contact metamorphism from the intrusion of this granite batholith has caused zones of metamorphic minerals to form.

Apply what you know about contact metamorphism to determine the type of rock that is now present along the edge of the intrusion.

Regional metamorphism When high temperature and pressure affect large regions of Earth's crust, they produce large belts of regional metamorphism. The metamorphism can range in grade from low to high grade. Results of regional metamorphism include changes in minerals and rock types, plus folding and deforming of the rock layers that make up the area. The mountain shown in Figure 6.14 experienced regional metamorphism.

Contact metamorphism When molten material, such as that in an igneous intrusion, comes in contact with solid rock, a local effect called contact metamorphism occurs. High temperature and moderate-to-low pressure form mineral assemblages that are characteristic of contact metamorphism. Figure 6.20 shows zones of different minerals surrounding an intrusion. Because temperature decreases with distance from an intrusion, metamorphic effects also decrease with distance. Recall from Chapter 5 that minerals crystallize at specific temperatures. Metamorphic minerals that form at high temperatures occur closest to the intrusion, where it is hottest. Because lava cools too quickly for the heat to penetrate far into surface rocks, contact metamorphism from extrusive igneous rocks is limited to thin zones.

Hydrothermal metamorphism When very hot water reacts with rock and alters its chemical and mineral composition, hydrothermal metamorphism occurs. The word hydrothermal is derived from the Greek words hydro, meaning water, and thermal, meaning heat. As hot fluids migrate in and out of the rock during metamorphism, the original mineral composition and texture of the rock can change. Chemical changes are common during contact metamorphism near igneous intrusions and active volcanoes. Valuable ore deposits of gold, copper, zinc, tungsten, and lead are formed in this manner. The gold deposited in the quartz shown in Figure 6.21 is the result of hydrothermal metamorphism.

Vocabulary

Science usage v. Common usage Intrusion

Science usage: the placement of a body of magma into preexisting rock

Common usage: joining or coming into without being invited

Figure 6.21 When the hydrothermal solution in the quartz cooled, gold veins formed.

Figure 6.21 When the hydrothermal solution in the quartz cooled, gold veins formed.

Hydrothermal Vein

Section 3 • Metamorphic Rocks 149

Ken Lucas/Visuals Unlimited

Section 3 • Metamorphic Rocks 149

Ken Lucas/Visuals Unlimited

Economic Importance of Metamorphic Rocks and Minerals

The modern way of life is made possible by a great number of naturally occurring Earth materials. We need salt for cooking, gold for trade, other metals for construction and industrial purposes, fossil fuels for energy, and rocks and various minerals for construction, cosmetics, and more. Figure 6.22 shows two examples of how metamorphic rocks are used in construction. Many of these economic mineral resources are produced by metamorphic processes. Among these are the metals gold, silver, copper, and lead, as well as many significant nonmetallic resources.

Metallic mineral resources Metallic resources occur mostly in the form of metal ores, although deposits of pure metals are occasionally discovered, many metallic deposits are precipitated from hydrothermal solutions and are either concentrated in veins or spread throughout the rock mass. Native gold, silver, and copper deposits tend to occur in hydrothermal quartz veins near igneous intrusions or in contact metamorphic zones. However, most hydrothermal metal deposits are in the form of metal sulfides such as galena (PbS) or pyrite (FeS2). The iron ores magnetite and hematite are oxide minerals often formed by precipitation from iron-bearing hydrothermal solutions.

^P Reading Check State what resources hydrothermal metamorphism produces.

Figure 6.22 Marble and slate are metamorphic rocks that have been used in construction for centuries.

Nonmetallic mineral resources Metamorphism of ultrabasic igneous rocks produces the minerals talc and asbestos. Talc, with a hardness of 1, is used as a dusting powder, as a lubricant, and to provide texture in paints. Because it is not combustible and has low thermal and electric conductivity, asbestos has been used in fireproof and insulating materials. Prior to the recognition of its cancer-causing properties, it was also widely utilized in the construction industry. Many older buildings still have asbestos-containing materials. Graphite, the main ingredient of the lead in pencils, may be formed by the metamorphism of coal.

Figure 6.22 Marble and slate are metamorphic rocks that have been used in construction for centuries.

150 Chapter 6 • Sedimentary and Metamorphic Rocks

(l)Altrendo Travel/Getty Images, (r)Pixtal/SuperStock

150 Chapter 6 • Sedimentary and Metamorphic Rocks

(l)Altrendo Travel/Getty Images, (r)Pixtal/SuperStock

The Rock Cycle

Metamorphic rocks form when other rocks change. The three types of rock—igneous, sedimentary, and metamorphic—are grouped according to how they form. Igneous rocks crystallize from magma; sedimentary rocks form from cemented or precipitated sediments; and metamorphic rocks form from changes in temperature and pressure.

Once a rock forms, does it remain the same type of rock always? Possibly, but it most likely will not. Heat and pressure can change an igneous rock into a metamorphic rock. A met-amorphic rock can be changed into another metamorphic rock or melted to form an igneous rock. Alternately, the metamorphic rock can be weathered and eroded into sediments that might become cemented into a sedimentary rock. In fact, any rock can be changed into any other type of rock. The continuous changing and remaking of rocks is called the rock cycle. The rock cycle is summarized in Figure 6.23. The arrows represent the different processes that change rocks into different types.

External processes

Deposition, burial, lithification

Weathering and erosion

Uplift

External processes

Weathering and erosion

Types Weathering And Erosion

Deposition, burial, lithification

Melting

Uplift

______

Igneous

rocks

and

fc

Cooling and crystallization

Cooling and crystallization

Melting

Magma

Internal processes

Figure 6.23 Rocks are continually being changed above and beneath Earth's surface. The rock cycle shows some of the series of changes rocks undergo.

Section 6.3 Assessment

Section Summary

I The three main types of metamorphism are regional, contact, and hydrothermal.

I The texture of metamorphic rocks can be foliated or nonfoliated.

I During metamorphism, new minerals form that are stable under the increased temperature and pressure conditions.

I The rock cycle is the set of processes through which rocks continuously change into other types of rocks.

Understand Main Ideas

1. iman<TTfla Summarize how temperature increases can cause metamorphism.

2. Summarize what causes foliated metamorphic textures to form.

3. Apply the concept of the rock cycle to explain how the three main types of rocks are classified.

4. Compare and contrast the factors that cause the three main types of metamorphism.

Think Critically

5. Infer which steps in the rock cycle are skipped when granite metamorphoses to gneiss.

6. Predict the location of an igneous intrusion based on the following mineral data. Muscovite and chlorite were collected in the northern portion of the area of study; garnet and staurolite were collected in the southern portion of the area.

mmSHSfr Earth Science

7. Gemstones often form as porphyroblasts. Gemstones are described in terms of carat weight. A carat is equal to 0.2 g or 200 mg. A large garnet discovered in New York in 1885 weighs 4.4 kg and is 15 cm in diameter. What is the carat weight of this gemstone?

Self-Check Quiz glencoe.com

Section B • Metamorphic Rocks

Self-Check Quiz glencoe.com

Section B • Metamorphic Rocks

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Responses

  • SOL
    Which steps in the rock cycle are skipped when granite metamorphoses to gneiss?
    8 years ago
  • sara
    What mineral is formed when shale and basalt are exposed to lowgrade metamorphism?
    8 years ago
  • Kian
    How are basalt and shale differences reflected in the minerals formed during metamorphism?
    8 years ago
  • Palmira Romano
    What mineral is formed when shale and basalt are exposed to low grade metamorphism?
    8 years ago
  • belisarius
    What stays the same in a metamorphic rock although mineral composition might change glencoe?
    6 years ago
  • leon
    How differences reflect in basalt and shale during metamorphism?
    6 years ago
  • MELBA
    What minerals is formed when shale and basalt are xposed to lowgrade metamophism?
    5 years ago
  • eija-riitt
    What material is formed when shale and basalt are exposed to low grade me?
    5 years ago
  • luise
    What resource hydrothermal metamorphism produces?
    4 years ago
  • karin
    What minerals are formed when shale and basalt are exposed to low grade metamorphism?
    4 years ago
  • giuditta
    What minerals are formed when shale and basalt are exposed to lowgrade metamorphism?
    12 months ago

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