How does metamorphism affect rocks

The two main types of metamorphism are both related to heat within Earth:. Quartzite is very hard and is often crushed and used in building railroad tracks.

Schist and slate are sometimes used as building and landscape materials. Skip to main content. Planet Earth. They have a significant amount of sheet silicate platy minerals and are classified by composition, grain size, and foliation type. Non-foliated — These have no evident planar fabric or foliation, crystallized under conditions where there was no differential stress, and are comprised of equant minerals only. These are classified mainly by the minerals present or the chemical composition of the protolith.

Foliated Metamorphic Rocks. Example - metamorphism of a shale, made up initially of clay minerals and quartz all of clay or silt size. Slate - Slates form at low metamorphic grade by the growth of fine grained chlorite and clay minerals. The preferred orientation of these sheet silicates causes the rock to easily break along the planes parallel to the sheet silicates, causing a slatey cleavage.

Note that in the case shown here, the maximum stress is applied at an angle to the original bedding planes, so that the slatey cleavage has developed at an angle to the original bedding. Because of the nearly perfect breakage along planes, slates are useful for blackboards and shingles.

Phyllite - Fine mica-rich rock, formed by low — medium grade metamorphism. In a phyllite, the clay minerals have recrystallized into tiny micas biotite and muscovite which reflect a satiny luster.

Phyllite is between slate and schist. Schist - The size of the mineral grains tends to enlarge with increasing grade of metamorphism.

Eventually the rock develops a near planar foliation caused by the preferred orientation of sheet silicates mainly biotite and muscovite. Quartz and Feldspar grains, however show no preferred orientation.

The irregular planar foliation at this stage is called schistosity. Schist often has other minerals besides micas. When these non-mica minerals occur with a grain size greater than the rest of the rock, they are called pophyroblasts. Gneiss As metamorphic grade increases, the sheet silicates become unstable and dark colored minerals like hornblende and pyroxene start to grow.

These dark colored minerals tend to become segregated in distinct bands through the rock, giving the rock a gneissic banding. Because the dark colored minerals tend to form elongated crystals, rather than sheet- like crystals, they still have a preferred orientation with their long directions perpendicular to the maximum differential stress. Granulite - At the highest grades of metamorphism all of the hydrous minerals and sheet silicates become unstable and thus there are few minerals present that would show a preferred orientation.

The resulting rock will have a granulitic texture that is similar to a phaneritic texture in igneous rocks. Migmatites — If the temperature reaches the solidus temperature first melting temperature , the rock may begin to melt and start to co-mingle with the solids. Usually these melts are felsic with the mafic material remaining metamorphic. Non-foliated Metamorphic Rocks. Non-foliated rocks lack a planar fabric. Absence of foliation possible for several reasons:.

Non-foliated rocks are given specific names based on their mineralogy and composition: Amphibolite - These rocks are dark colored rocks with amphibole usually hornblende as their major mineral.

They are usually poorly foliated and form at intermediate to high grades of metamorphism of basaltic or gabbroic protoliths. Hornfels - These are very fine grained rocks that usually form as a result of magma intruding into fined grained igneous rocks or shales. The magma causes a type of metamorphism called contact metamorphism to be discussed later. Quartzite - A rock made up almost entirely of quartz.

They are formed by metamorphism of quartz arenites sandstones. Since quartz is stable over a large range of temperatures and pressures, no new minerals are formed during metamorphism, and the only metamorphic effect that occurs is recrystallization of the quartz resulting in interlocking crystals that make up a very hard rock.

Marble - A limestone or dolostone made up only of calcite or dolomite will metamorphose to a marble which is made mostly recrystallized calcite or dolomite. The Recrystallization usually obliterates all fossils. Marbles have a variety of colors and are often complexly banded. They are commonly used as a decorative stone. Although textures and structures of the protolith are usually destroyed by metamorphism, we can still get an idea about the original rock from the minerals present in the metamorphic rock.

Minerals that form, do so because the chemical elements necessary to form them are present in the protolith. General terms used to describe the chemical composition of both the protolith and the resulting metamorphic rock are:.

Pelitic Alumina rich rocks, usually shales or mudstones. These start out originally with clay minerals and as a result of metamorphism, Alumina rich minerals like micas, chlorite, garnet, kyanite, sillimanite and andalusite form.

Because of the abundance of sheet silicates, pelitic rocks commonly form slates, phyllites, schists, and gneisses during metamorphism. Mafic - These are Mg and Fe rich rocks with low amounts of Si.

Minerals like biotite, hornblende and plagioclase form during metamorphism and commonly produce amphibolites. Calcareous - These are calcium-rich rocks usually derived from limestones or dolostones, and thus contain an abundance of Calcite. Marbles are the type of metamorphic rock that results. They are stable at different pressures and temperatures, and, as we will see later, they are important indicators of pressures and temperatures in metamorphic rocks Figure 7.

Pressure is important in metamorphic processes for two main reasons. First, it has implications for mineral stability Figure 7. Second, it has implications for the texture of metamorphic rocks. Rocks that are subjected to very high confining pressures are typically denser than others because the mineral grains are squeezed together Figure 7.

Because of plate tectonics, pressures within the crust are typically not applied equally in all directions. In areas of plate convergence, the pressure in one direction perpendicular to the direction of convergence is typically greater than in the other directions Figure 7. In situations where different blocks of the crust are being pushed in different directions, the rocks will be subjected to sheer stress Figure 7. Foliation is described in more detail later in this chapter. The presence of water is important for two main reasons.

The conditions required to form a metamorphic rock are very specific. The existing rock must be exposed to high heat, high pressure, or to a hot, mineral-rich fluid. Usually, all three of these circumstances are met. In order to create metamorphic rock, it is vital that the existing rock remain solid and not melt. If there is too much heat or pressure, the rock will melt and become magma.

This will result in the formation of an igneous rock , not a metamorphic rock. Consider how granite changes form. Granite is an igneous rock that forms when magma cools relatively slowly underground. It is usually composed primarily of the minerals quartz, feldspar, and mica. When granite is subjected to intense heat and pressure, it changes into a metamorphic rock called gneiss. Slate is another common metamorphic rock that forms from shale. Limestone, a sedimentary rock , will change into the metamorphic rock marble if the right conditions are met.

This happens due to geologic uplift and the erosion of the rock and soil above them. At the surface, metamorphic rocks will be exposed to weathering processes and may break down into sediment.