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Home , Metamorphism

Chapter 7: and Metamorphic Rocks

Fig. 7.21 OBJECTIVES

• Restate how metamorphic rocks relate to the two other 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 and explain the basis of their classification.

• Explain how the 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

• 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.

Fig. 7.11 Comparing Metamorphic, Igneous, and Sedimentary Rocks

• 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. Factors that Control Metamorphism

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

Gneiss Fig. 7.1d Effects of Heating on Metamorphism

• Heating can make less stable. Heating causes increase in bond length, distortion of bonds, breaking of bonds, and formation of new bonds.

• Heating can accelerate the rate of chemical reactions.

Figs. 7.4, 7.5 Pressure and Metamorphism • Confining pressure acts on a rock equally in all directions. • Directed pressure acts more strongly in one direction than in others.

Fig. 7.6 Fluids and Rock Composition • Sources of metamorphic fluids are • Fluid trapped in • Fluid introduced by tectonic and igneous processes • Fluid expelled when minerals react • composition determines what fluids are released and what minerals can react.

Amphibolite (parent rock (parent rock = = ) mudstone) Fig. 7.8 Types of Metamorphism

• 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

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

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

• Six main types of metamorphism (continued):

• : produced by the high-velocity impact of an extraterrestrial object such as a 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 Regional Metamorphism

• Regional metamorphism affects large areas of rock.

• Mountain chains are associated with regional metamorphism.

Metamorphic Zones: Regional metamorphism in Scotland

Fig. 7.24 Contact Metamorphism • 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.

Fig. 7.10 Dynamic Metamorphism • Dynamic metamorphism is associated with faulting.

• Dynamic metamorphism can produce crushing and smearing of rock layers. Fig 7.11 Crushing produces fault (a); smearing produces (b). The schematic diagram (c) shows the locations of various rock types typically produced by dynamic metamorphism in a fault zone. Fault breccia at shallow depths gives way to at depth. Shock Metamorphism • Shock metamorphism is characterized by distinctive, high- pressure minerals. • Meteorite collisions produce shock metamorphism.

Photo: Martin Schmieder

Shocked from the Svasvesi The Barringer impact crater (Meteor Crater) impact structure, Finland Arizona Fig. 7.12a Hydrothermal Metamorphism

• Hydrothermal metamorphism is caused by hot, circulating fluids.

• Hydrothermal metamorphism occurs primarily at mid-ocean ridges.

Hydrothermal vent Photo: NOAA Burial Metamorphism

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

Burial metamorphism is occurring beneath the thick sediments of the Mississippi Delta. Photo: NASA Metamorphic Textures

• The fabric of a metamorphic rock describes the geometric arrangement of mineral grains. • Metamorphic rocks can have a foliated or nonfoliated texture.

Foliated () Fig. 7.22 Nonfoliated (quartzite) Fig. 7.1a Classifying Metamorphic Rocks

• Foliated rocks are classified based on grain size and .

Slate, with microscopic Schist, with grains visible grains Figs. 7.1 b, c • Nonfoliated rocks are classified based on origin or composition.

Quartzite, , composed composed of of quartz Fig. 7.20 Fig. 7.22 Foliated Metamorphic Rocks

• Slate, , schist, and are the four main types of foliated metamorphic rock. • As metamorphic grade increases, grain size increases.

Fig. 7.16 Nonfoliated Metamorphic Rocks

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

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

Hornfels Marble Quartzite Amphibolite

Figs. 7.19, 7.20, 7.22, 7.23 Metamorphic Zones and

• Distinctive minerals characterize specific metamorphic temperatures and pressures.

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

Fig. 7.25 Metamorphism and

• Metamorphism can occur at divergent and convergent boundaries. • Divergent boundaries are characterized by hydrothermal metamorphism. • Contact and regional metamorphism are common at convergent boundaries.

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