Metamorphic Rocks

Metamorphic Rocks Metamorphic rocks are produced from preexisting igneous, sedimentary, or from other metamorphic rocks. Every metamorphic rock has a parent rock – the rock from which it was formed. Metamorphism, which means to “change form,” is a process that leads to changes in the mineralogy, texture, and sometimes the chemical composition of rocks. Factors that might cause a rock to alter from one form to another includes changes in temperature, pressure (stress), and the introduction to chemically active fluids. Metamorphism often progresses incrementally, from slight changes (low-grade metamorphism) to substantial changes (high-grade metamorphism). An example of low-grade would be shale turning into slate when put under pressure (stress). In high-grade changes, slight melting may occur, as well as folds or obliteration of fossils or vesicles in the parent rock.

Examples of Metamorphic Rocks Quartzite Slate Gneiss Marble Phyllite Schist

What Drives Metamorphism? The agents of metamorphism include heat, pressure (stress), and chemically active fluids. During metamorphism, rocks may be subjected to all three metamorphic agents simultaneously. Heat is the most important of metamorphism because it provides the energy to drive chemical reactions that result in the recrystallization of existing materials and/or the formation of new materials. Earth’s internal heat comes mainly from radioactive decay within the Earth’s interior.

What Drives Metamorphism? Pressure, like temperature, increases as you get deeper into the Earth. Buried rocks are subjected to this pressure, called confining pressure, which causes the spaces between mineral grains to close, producing a more compact rock having a greater density. This pressure may ultimately cause minerals to recrystallize into new minerals that a display a more compact form. Unlike confining pressure, which “squeezes” rock equally in all directions (and does not fold or deform them), differential stress, where forces pushing on the rocks are unequal, can create folds and deformation. This is especially prominent at convergent plate boundaries.

Metamorphic Rock Textures Texture is used to describe the size, shape, and arrangement of grains within a rock. Most igneous and sedimentary rocks consist of mineral grains that have a random orientation. By contrast, deformed metamorphic rocks that contain platy minerals (micas) and/or elongated minerals (amphiboles) typically display some kind of preferred orientation in which the mineral grains exhibit a parallel or specific alignment. This preferred orientation of a rock’s minerals is called a foliated texture. Mica Schist Gedrite

Examples of Foliated Textures Various types of foliation exist, depending largely upon the grade of metamorphism and the mineralogy of the parent rock. We’ll look at three main types of foliation: rock or slaty cleavage; schistosity; and gneissic texture. Rock or Slaty Cleavage: This type of foliated texture describes a rock’s tendency to break or cleave along a specific crystal plane. Slate is a rock with excellent rock cleavage, as it breaks in flat slabs.

Examples of Foliated Textures Schistosity: This type of foliated texture describes a rock created with large, platy minerals (such as mica and chlorite) that have grown large enough to be seen by the unaided eye. In addition to platy minerals, schist often contains deformed quartz and feldspar grains that appear as flat, or lens-shaped, grains hidden among the mica grains.

Examples of Foliated Textures Gneissic Texture: During high-grade metamorphism, ion migrations can result in the segregation of minerals. Although foliated, gneisses will not usually split as easily as slates and schists. Gneisses that do cleave tend to break parallel to their foliation and expose mica-rich surfaces that resemble schist.

Foliated Metamorphic Rocks Slate Gneiss Schist Phyllite

Other Metamorphic Textures Not all metamorphic rocks exhibit a foliated texture. Those that do not are referred to as nonfoliated. Nonfoliated textures usually form in environments where parent rocks are composed of minerals that exhibit equidimensional crystals, such as quartz or calcite. Another texture common to metamorphic rocks consists of particularly large grains, called porphyroblasts, that are surrounded by a fine-grained matrix of other minerals. Porphyroblastic textures develop in a wide range of environments and result in very large specimens of certain minerals, such as garnets.

Nonfoliated Metamorphic Rocks Marble Quartzite

The exterior of the Taj Mahal is constructed mainly of the metamorphic rock marble.

Metamorphic Environments Hydrothermal metamorphism occurs when hot fluids circulate through fissures and cracks that develop in rock. This hot fluid chemically alters rocks and is closely related to igneous activity.

Metamorphic Environments There are a number of environments in which metamorphism occurs. Most are in the vicinity of plate margins, and many are associated with igneous activity. Contact or thermal metamorphism occurs when rocks immediately surrounding a molten igneous body are “baked” and therefore altered from their original state.

Metamorphic Environments Regional metamorphism occurs where rocks are squeezed between two converging lithospheric plates during mountain building.

The typical transition in mineralology that results from progressive metamorphism of shale.