Chapter 7: Metamorphism and Metamorphic Rocks

Fig. 7.21

OBJECTIVES

• Restate how metamorphic rocks relate to the two other rock groups (sedimentary and igneous).

• Describe how metamorphic rocks are produced by the action of heat, pressure, and fluids on preexisting parent rocks over the course of time.

• Recognize the different types of metamorphism, the processes that cause them, and the environments in which they occur.

• Identify how the textures of metamorphic rocks reflect the environment of their formation.

OBJECTIVES

• Compare and contrast the types of metamorphic rock and explain the basis of their classification.

• Explain how the mineral content of metamorphic rocks indicates the pressure and temperature conditions of metamorphism.

• Compare and contrast the plate tectonic settings where metamorphism takes place and the variable conditions of metamorphism that the different settings produce.

• Metamorphism means “a change in form.”

• Metamorphic rocks form when the texture or mineral composition of a rock changes.

• Heat, pressure, and fluids can cause metamorphism.

Metamorphism

Fig. 7.11

• Metamorphic rocks form at higher temperatures and pressures than sedimentary rocks.

• Metamorphic rocks form at lower temperatures than igneous rocks.

• The rock does not melt.

• Metamorphic processes generally cannot be observed directly.

Comparing Metamorphic, Igneous, and Sedimentary Rocks

• The following factors control metamorphism:• Heat• Pressure• Fluids• Rock composition• Time

Factors that Control Metamorphism

Fig. 7.1dGneiss

• Heating can make minerals less stable.

• Heating can accelerate the rate of chemical reactions.

Effects of Heating on Metamorphism

Heating causes increase in bond length, distortion of bonds, breaking of bonds, and formation of new bonds.

Figs. 7.4, 7.5

• Confining pressure acts on a rock equally in all directions.• Directed pressure acts more strongly in one direction than

in others.

Pressure and Metamorphism

Fig. 7.6

• Sources of metamorphic fluids are• Fluid trapped in sedimentary rock• Fluid introduced by tectonic and igneous processes• Fluid expelled when minerals react

• Parent rock composition determines what fluids are released and what minerals can react.

Fluids and Rock Composition

Amphibolite (parent rock = basalt)

Kyanite schist (parent rock = mudstone)

Fig. 7.8

• There are six main types of metamorphism:

• Regional metamorphism: occurs when large regions of crustal rocks are subjected to elevated temperatures and pressures; takes place during mountain-building

• Contact metamorphism: rocks around an igneous body produced as a result of heating by an adjacent body of magma

• Dynamic metamorphism: occurs along fractures in Earth’s crust where significant movement has taken place; involves crushing or smearing of rocks adjacent to fault zones

Types of Metamorphism

• Six main types of metamorphism (continued):

• Shock metamorphism: produced by the high-velocity impact of an extraterrestrial object such as a meteorite or an asteroid on Earth’s surface

• Hydrothermal metamorphism: occurs when rocks react with adjacent hot circulating fluids

• Burial metamorphism: caused by burial beneath a thick succession of overlying rock layers

Types of Metamorphism

• Regional metamorphism affects large areas of rock.

• Mountain chains are associated with regional metamorphism.

Regional Metamorphism

Metamorphic Zones: Regional metamorphism in Scotland

Fig. 7.24

• Contact metamorphism occurs in rocks that are heated by nearby magma.

• Contact metamorphism affects rocks over a small area.• The degree of metamorphism decreases with distance from

the magma chamber.

Contact Metamorphism

Fig. 7.10

• Dynamic metamorphism is associated with faulting.

• Dynamic metamorphism can produce crushing and smearing of rock layers.

Dynamic Metamorphism

Crushing produces fault breccia (a); smearing produces mylonite (b). The schematic diagram (c) shows the locations of various rocktypes typically produced by dynamic metamorphism in a fault zone. Fault breccia at shallow depths gives way to mylonites at depth.

Fig 7.11

• Shock metamorphism is characterized by distinctive, high-pressure minerals.

• Meteorite collisions produce shock metamorphism.

Shock Metamorphism

Shocked quartz from the Svasvesi impact structure, FinlandThe Barringer impact crater (Meteor Crater)

ArizonaFig. 7.12a

Photo: Martin Schmieder

• Hydrothermal metamorphism is caused by hot, circulating fluids.

• Hydrothermal metamorphism occurs primarily at mid-ocean ridges.

Hydrothermal Metamorphism

Hydrothermal ventPhoto: NOAA

• Burial metamorphism occurs at high pressure when rock layers are buried at depth.

Burial Metamorphism

Burial metamorphism is occurring beneath the thick sediments of the Mississippi Delta.

Photo: NASA

• The fabric of a metamorphic rock describes the geometric arrangement of mineral grains.

• Metamorphic rocks can have a foliated or nonfoliatedtexture.

Metamorphic Textures

Foliated (slate)

Nonfoliated (quartzite)

Fig. 7.22

Fig. 7.1a

• Foliated rocks are classified based on grain size and foliation.

• Nonfoliated rocks are classified based on origin or composition.

Classifying Metamorphic Rocks

Slate, with microscopic grains

Schist, with visible grains

Quartzite, composed of quartz

Marble, composed of calcite

Figs. 7.1 b, c

Fig. 7.20Fig. 7.22

• Slate, phyllite, schist, and gneiss are the four main types of foliated metamorphic rock.

• As metamorphic grade increases, grain size increases.

Foliated Metamorphic Rocks

Fig. 7.16

• Nonfoliated rocks can be classified based on either origin or composition.

• Hornfels, marble, quartzite, and amphibolite are common examples of nonfoliated rocks.

Nonfoliated Metamorphic Rocks

Hornfels QuartziteMarble Amphibolite

Figs. 7.19, 7.20, 7.22, 7.23

• Distinctive minerals characterize specific metamorphic temperatures and pressures.

• A metamorphic facies is a set of associated metamorphic rocks that formed under the same conditions.

Metamorphic Zones and Facies

Fig. 7.25

• Metamorphism can occur at divergent and convergent boundaries.

• Divergent boundaries are characterized by hydrothermal metamorphism.

• Contact and regional metamorphism are common at convergent boundaries.

Metamorphism and Plate Tectonics

Figs. 7.28b, 7.27b

Summary• Metamorphic rocks

• Form when a rock's mineral composition and texture change

• Are influenced by heat, pressure, fluids, parent rock composition, and duration of metamorphism

• Form through regional, contact, dynamic, shock, hydrothermal, or burial metamorphism

• Can be classified as foliated (layered) or nonfoliated• Can be classified based on mineral composition, origin,

or grain size• Contain minerals that can be used to infer the conditions

under which they formed• Form at divergent and convergent plate boundaries