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When rocks are subjected to deep burial, tectonic forces such as folding, and high pressures and temperatures, the textures and mineral compositions begin to change. This process called metamorphism.
Metamorphism is the solid-state transformation (no melting) of a rock mass into a rock of generally the same chemistry but with different textures and minerals.
Usually the metamorphic rock is different from the original rock, called the parent rock or protolith. Factors Controlling Metamorphism Most metamorphic reactions occur because each mineral is stable only within a certain range of temperature, pressure, and chemical environment. If temperature or pressure rises above that range, or if the chemical environment changes, a new mineral forms.
Deformation creates a banded structure called foliation. Both the texture and the minerals can change as a rock is metamorphosed. The mineralogy of a metamorphic rock reflects its metamorphic grade. Metamorphic grade is the temperature and pressure at which it formed. The increase in the Earth‟s temperature with depth from its surface is called the geothermal gradient. Consequently, the metamorphic grade of many rocks is related to the depth. Metamorphic grade is often expressed by the relative terms low-, medium-, and high-grade metamorphism.
Types of metamorphism and metamorphic rocks
1. Contact metamorphism: Contact metamorphism occurs where hot magma intrudes cooler country rock. The country rock may be of any type sedimentary, metamorphic, or igneous. A metamorphic
halo around a pluton can range in width from less than a meter to hundreds of meters. Contact metamorphism commonly occurs without deformation. The hornfels is a hard, dark, fine grained rock usually formed by contact metamorphism of shale.
2. Burial metamorphism: Burial metamorphism results from deep burial of rocks in a sedimentary basin. Burial metamorphism occurs without tectonic deformation. Because of the lack of deformation, rocks formed by burial metamorphism often retain sedimentary structures. Shale and siltstone become harder and better lithified to form argillite. Quartz sandstone becomes quartzite. Burial metamorphism converts limestone and dolomite to marble.
3. Regional metamorphism: Regional metamorphism occurs in and near a subduction zone. It is the most common type of metamorphism. Regionally metamorphosed rocks are strongly foliated and are associated with mountains and igneous rocks. Shale consists of clay minerals, quartz, and feldspar and is the most abundant sedimentary rock. As regional metamorphism begins, the clay minerals break down and are replaced by mica and chlorite. These new, platy minerals grow perpendicular to the direction of tectonic forces. As a result, the rock develops slaty cleavage and is called slate. With rising temperature and continued deformation, the micas and chlorite grow larger, and form a rock called phyllite. As temperature continues to rise, the mica and chlorite grow large enough to be seen by the naked eye, and foliation becomes very well developed. Rock of this type is called schist. At high metamorphic grades, light- and dark-colored minerals often separate into bands that are thicker than the layers of schist to form a rock called gneiss (pronounced “nice”). At the highest metamorphic grade, the rock begins to melt, forming small veins of granitic magma. When metamorphism decrease and the rock cools, the magma veins solidify to form migmatite, a mixture of igneous and metamorphic rock.
All metamorphic rocks that formed under identical temperature and pressure are grouped into a metamorphic facies. Metamorphic facies differ from one another, because they form under different conditions of temperature and pressure. Each facies is given a name derived from a mineral and/or texture commonly found in rocks of that facies.
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