Chapter 6 – Section 4: Metamorphic Rock Introduction This section describes how temperature and pressure changes and chemical processes change one type of rock to another. The section also explains the difference between regional and contact metamorphism and distinguishes between foliated and nonfoliated metamorphic rock.

15. Formation of Metamorphic Rock The process by which heat, pressure, or chemical processes change one rock type to another is called metamorphism. Most metamorphic rock forms deep within the Earth’s crust. All metamorphic rock forms from existing igneous, sedimentary, or metamorphic rock. During metamorphism, heat, pressure, and hot fluids cause some minerals to change into other minerals. Minerals may change in size, shape, or they may separate into bands that give the rock a layered appearance. Hot fluids from magma may circulate through the rock and either dissolve minerals or add minerals.

Formation of Metamorphic Rock (cont.) The type of rock that forms through metamorphism can indicate the conditions that were in place when the original rock changed. The different combinations of minerals that form are dependent on: The composition of the rock being metamorphosed The amount and direction of heat and pressure And the presence or absence of certain fluids Two types of metamorphism: One type occurs when small volumes of rock come into contact with magma The second type occurs when large areas of Earth’s crust are affected by the heat and pressure that is caused by the movement and collisions of Earth’s giant tectonic plates.

Formation of Metamorphic Rock (cont.) Contact Metamorphism Contact metamorphism occurs when magma comes into contact with existing rock and heat from the magma changes the structure and composition of the surrounding rock. In contact metamorphism only a small area of rock that surrounds the hot magma is changed by the magma’s heat. Hot chemical fluids moving through fractures may also cause changes in the surrounding rock.

Formation of Metamorphic Rock (cont.) Regional Metamorphism Regional metamorphism occurs over a large area. Can occur during periods of high tectonic activity, such as when mountain ranges form. The movement of one tectonic plate against another generates tremendous heat and pressure in the rocks at the edges of the tectonic plates. The heat and pressure cause chemical changes in the minerals of the rock. Most metamorphic rocks form as a result of regional metamorphism Volcanism and movement of magma often accompany tectonic activity, thus rocks that are formed by regional metamorphism may also form by contact metamorphism

16. Classification of Metamorphic Rocks Metamorphic rocks are classified by texture and chemical composition. Metamorphic rocks have either a foliated or nonfoliated texture. Foliated Rocks Foliation is a metamorphic rock texture in which minerals are arranged in planes or bands. Created by regional metamorphism Form in two ways: Extreme pressure causes minerals crystals to realign or regrow to form parallel bands Minerals that have different compositions separate to produce a series of alternating dark and light bands.

Classification of Metamorphic Rocks (cont.) Common foliated rocks include slate, schist, and gneiss. Slate forms when pressure is exerted on the sedimentary rock shale, which contains clay minerals that are flat and thin. Compressed into thin layers, which split easily into flat sheets. Phyllite is a fine-grained rock that is metamorphosed more than slate, but less than schist. Schist, a coarse-grained rock, forms when large amounts of heat and pressure are exerted on slate. Gneiss has bands of light and dark minerals. Intense heat and pressure may cause the minerals in schist to separate into bands as the minerals recrystallize.

Classification of Metamorphic Rocks (cont.) Nonfoliated Rocks Rocks that do not have bands or aligned minerals are nonfoliated. Formed through contact metamorphism Most nonfoliated metamorphic rocks share at least one of two main characteristics. The original rock may contain grains of only one mineral or contains small amounts of other minerals Thus the rock does not form compositional bands when metamorphosed Original rock may contain grains that are round or square, no long and short sides. They do not change position when exposed to pressure in one direction.

Classification of Metamorphic Rocks (cont.) Quartzite is a common nonfoliated rock. Forms when quartz sandstone is metamorphosed. Quartz is resistant to weathering, so quartzite remains after weaker rocks around it have eroded May form hills or mountains. Marble forms from the compression of limestone. Calcium carbonate in marble is susceptible to accelerated chemical weathering by acid rain. Hornfels is a fine-grained nonfoliated metamorphic rock with no specific composition. Anthracite coal is a highly metamorphosed form of coal, submetallic luster