Section Assessment

Section Summary

I Sedimentary rocks can be clastic, chemical, or biochemical.

I Clastic rocks form from sediments and are classified by particle size and shape.

I Chemical rocks form primarily from minerals precipitated from water in areas with high evaporation rates.

I Biochemical rocks form from the remains of once-living things.

I Sedimentary rocks provide geologists with information about surface conditions that existed in Earth's past.

Understand Main Ideas

1. iman4BHB State the type of sedimentary rock that is formed from the erosion and transport of rocks and sediments.

2. Explain why coal is a biochemical sedimentary rock.

3. Calculate the factor by which grain size increases with each texture category.

4. Analyze the environmental conditions to explain why chemical sedimentary rocks form mainly in areas that have high rates of evaporation.

Think Critically

5. Propose a scenario to explain how it is possible to form additional layers of evaporites in a body of seawater when the original amount of dissolved minerals in the water was enough to form only a thin evaporite.

6. Examine the layers of shale in Figure 6.12 and explain why shale contains no cross-bedding or ripple marks.

■Miiiw^ Earth Science

7. Assume that the volume of a layer of mud will decrease by 35 percent during sedimentation and compaction. If the original sediment layer is 30 cm thick, what will be the thickness of the shale layer after compaction and lithification?

144 Chapter 6 • Sedimentary and Metamorphic Rocks

Self-Check Quiz glencoe.com

144 Chapter 6 • Sedimentary and Metamorphic Rocks

Metamorphic Rocks Rocks
Figure 6.14 Strong forces were required to bend these rock layers into the shape they are today. Hypothesize the changes that occurred to the sediments after they were deposited.

Metamorphic Rocks

N4TTHa Metamorphic rocks form when preexisting rocks are exposed to increases in temperature and pressure and to hydrothermal solutions.

Real-World Reading Link When you make a cake, all of the individual ingredients that you put into the pan change into something new. When rocks are exposed to high temperatures, their individual characteristics also change into something new and form a completely different rock.

Recognizing Metamorphic Rock

The rock layers shown in Figure 6.14 have been metamorphosed (meh tuh MOR fohzd)—this means that they have been changed. How do geologists know that this has happened? Pressure and temperature increase with depth. When temperature or pressure becomes high enough, rocks melt and form magma. But what happens if the rocks do not reach the melting point? When high temperature and pressure combine and change the texture, mineral composition, or chemical composition of a rock without melting it, a metamorphic rock forms. The word metamorphism is derived from the Greek words meta, meaning change, and morphe, meaning form. During metamorphism, a rock changes form while remaining solid.

The high temperatures required for metamorphism are ultimately derived from Earth's internal heat, either through deep burial or from nearby igneous intrusions. The high pressures required for metamorphism come from deep burial or from compression during mountain building.

Igneous Intrusion Rocks

Section 3 • Metamorphic Rocks 145

Tony Waltham/Robert Harding World Imagery/CORBIS

Section 3 • Metamorphic Rocks 145

Tony Waltham/Robert Harding World Imagery/CORBIS

Fractional Crystallization
Figure 6.15 Metamorphic minerals form into many colors, shapes, and crystal sizes. Colors can be dark or bright and crystal form can be unique.

Metamorphic minerals How do minerals change without |

melting? Think back to the concept of fractional crystallization, |

discussed in Chapter 5. Bowen's reaction series shows that all min- |

erals are stable at certain temperatures and they crystallize from |

magma along a range of different temperatures. Scientists have dis- g covered that these stability ranges also apply to minerals in solid f rock. During metamorphism, the minerals in a rock change into 1

new minerals that are stable under the new temperature and pres- |

sure conditions. Minerals that change in this way are said to |

undergo solid-state alterations. Scientists have conducted experi- f ments to identify the metamorphic conditions that create specific | minerals. When the same minerals are identified in rocks, tists are able to interpret the conditions inside the crust during the |

rocks' metamorphism. Figure 6.15 shows some common meta- |

morphic minerals. |

A Reading Check Explain what metamorphic minerals are. |

Metamorphic textures Metamorphic rocks are classified |

into two textural groups: foliated and nonfoliated. Geologists use f metamorphic textures and mineral composition to identify meta- J

morphic rocks. Figure 6.16 shows how these two characteristics f are used in the classification of metamorphic rocks. s, s a

Foliated rocks Layers and bands of minerals characterize s foliated metamorphic rocks. High pressure during metamorphism causes minerals with flat or needlelike crystals to form with their long axes perpendicular to the pressure, as shown in Figure 6.17. This parallel alignment of minerals creates the layers observed in foliated metamorphic rocks.

Figure 6.16 Increasing grain size parallels changes in composition and development of foliation. Grain size is not a factor in nonfoliated rocks.

Figure 6.16 Increasing grain size parallels changes in composition and development of foliation. Grain size is not a factor in nonfoliated rocks.

Foliated Nonfoliated Chart

Composition

Metamorphic Rock Identification Chart

Composition

Z

T

R

A

U

R

E

Q

A

LE

P

o

o

IB HI

L E

P

F

SLATE

PHYLLITE

SCHIST

GNEISS

Quartz

QUARTZITE

Calcite or dolomite

MARBLE

Fine- to coarse-grained

Increased pressure and temperature

Increased pressure and temperature

Figure 6.17 Foliation develops when pressure is applied from opposite directions. The foliation develops perpendicular to the pressure direction.

Nonfoliated rocks Unlike foliated rocks, nonfoliated metamor-phic rocks are composed mainly of minerals that form with blocky crystal shapes. Two common examples of nonfoliated rocks, shown in Figure 6.18, are quartzite and marble. Quartzite is a hard, often light-colored rock formed by the metamorphism of quartz-rich sandstone. Marble is formed by the metamorphism of limestone. Some marbles have smooth textures that are formed by interlocking grains of calcite. These marbles are often used in sculptures. Fossils are rarely preserved in metamorphic rocks.

Under certain conditions, new metamorphic minerals can grow large while the surrounding minerals remain small. The large crystals, which can range in size from a few millimeters to a few centimeters, are called porphyroblasts. Although these crystals resemble the very large crystals that form in pegmatite granite, they are not the same. Instead of forming from magma, they form in solid rock through the reorganization of atoms during metamorphism. Garnet, shown in Figure 6.18, is, a mineral that commonly forms porphyroblasts.

Figure 6.18 As a result of the extreme heat and pressure during metamorphism, marble rarely contains fossils. Metamorphism does not, however, always destroy cross-bedding and ripple marks, which can be seen in some quartzites. Garnet porphyroblasts can grow to be quite large in some rocks.

Figure 6.18 As a result of the extreme heat and pressure during metamorphism, marble rarely contains fossils. Metamorphism does not, however, always destroy cross-bedding and ripple marks, which can be seen in some quartzites. Garnet porphyroblasts can grow to be quite large in some rocks.

What Quartzite Mineral Composition

Marble

Quartzite

Marble

Quartzite r ^

Garnet porphyroblast

Minerals in Metamorphosed Shale

Unification Low grade Intermediate grade High grade - ^Chlorite

-__White mica (mainly muscovite)

Kyanite i Sillimanite

Albite (sodium plagioclase feldspar)

■ Figure 6.19 Metamorphism of shale results in the formation of minerals that provide the wide variety of color observed in slate.

Grades of Metamorphism

Different combinations of temperature and pressure result in different grades of metamorphism. Low-grade metamorphism is associated with low temperatures and pressures and a particular suite of minerals and textures. High-grade metamorphism is associated with high temperatures and pressures and a different suite of minerals and textures. Intermediate-grade metamorphism is in between low- and high-grade metamorphism.

Figure 6.19 shows the minerals present in metamorphosed shale. Note the change in composition as conditions change from low-grade to high-grade metamorphism. Geologists can create metamorphic maps by plotting the location of metamorphic minerals. Knowing the temperatures that certain areas experienced when rocks were forming helps geologists locate valuable metamorphic minerals such as garnet and talc. Studying the distribution of metamorphic minerals helps geologists to interpret the metamorphic history of an area.

Types of Metamorphism

The effects of metamorphism can be the result of contact metamorphism, regional metamorphism, or hydrothermal metamorphism. The minerals that form and the degree of change in the rocks provide information as to the type and grade of metamorphism that occurred.

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Responses

  • Macario Greco
    What can metamorphic rocks be used for?
    8 years ago
  • SAGRAMOR BURROWS
    Which rocks form metamorphic rocks?
    8 years ago
  • mentha boffin
    What is quartzite mineral composition?
    7 years ago
  • marco
    What is the factor for grain size increase with each texture category?
    4 years ago
  • myrtle
    Can ripple marks be preserved in metamorphism?
    4 years ago

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