Metamorphic Rock

I

Introduction

Metamorphic Rock, type of rock formed when rocky material experiences intense heat and pressure in the crust of the earth. Metamorphic rocks are one of the three main groups of rocks. The other two groups are igneous rocks, which form when magma or molten lava solidifies, and sedimentary rocks, which form when wind or water deposit sediments and the sediments become compacted. Through the metamorphic process, both igneous rocks and sedimentary rocks can change into metamorphic rocks, and a metamorphic rock can change into another type of metamorphic rock. Heat and pressure do not change the chemical makeup of the parent rocks but they do change the mineral structure and physical properties of those rocks. By studying the composition and texture of metamorphic rocks, geologists can determine from what parent rocks the metamorphic rocks were formed.

II

Formation of Metamorphic Rocks

Forces within the earth create large amounts of heat and pressure, the factors that change igneous and sedimentary rocks into metamorphic rocks. Radioactive isotopes—forms of elements—generate heat within the earth as they decay. Magma (molten rock) moving from deep within the earth toward the surface also provides heat for metamorphism. Another source of heat within the earth that can lead to metamorphism is friction between rocks grinding past one another (along earthquake faults or at plate tectonic boundaries). In addition to heat, pressure within the earth contributes to the formation of metamorphic rocks by changing the texture and mineral density of rocks. See also Earthquake; Plate Tectonics.

A

Heat

Heat is the most important factor contributing to metamorphism. The temperature range over which metamorphic rocks form is approximately 150° C (300° F) to above 1,000° C (2,000° F), depending on composition of parent rock, pressure, and the presence of fluids such as water. At the upper range of temperature, metamorphic conditions stop as the rocks begin to melt, eventually forming igneous rocks. The melting temperature varies, from approximately 650° C (1,200° F) for rocks made of granite to well over 1,000° C (2,000° F) for rocks made of basalt.

Heat produced by radioactive decay may lead to the formation of metamorphic rocks. Radioactive isotopes within the earth emit heat as they decay, or disintegrate. Radioactivity is the process by which atoms of an element are transformed into new kinds of atoms, and heat is a by-product of this process. Some of the heat within the earth is produced by the radioactive decay of elements such as uranium, thorium, and potassium.

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Another way for heat to form metamorphic rocks is through the introduction of underground magma into an area of preexisting solid rock. When underground magma flows through a crack (called a dike) into areas of surrounding solid rock (known as country rock), there is a significant difference between the temperature of the magma and the temperature of the surrounding rock. On cooling, the magma introduces great amounts of heat into the country rock, usually leading to recrystallization and mineral reactions in the rocks nearby. This process is known as contact (or thermal) metamorphism. The magma itself cools to form igneous rock, but the nearby surrounding rock will likely be metamorphic in nature. The envelope of contact-metamorphosed rocks around a magma intrusion is called an aureole. The size of an aureole depends on the amount of heat provided by the intrusion. A narrow dike may have an aureole a few millimeters wide, whereas the aureole surrounding a batholith (a large intrusion of igneous rock) may stretch for many hundreds of meters.

Another source of heat is friction between bodies of rock as they grind against each other. Along earthquake faults, two bodies of rock—one on either side of the fault—may slide against each other as the strain (caused by rocks pushing against each other over many years) within the bodies of rock builds up. At plate boundaries, bodies of rock produce friction and heat as they slide against each other, as one plate moves under another plate, or as the two plates push directly against each other.

B

Pressure

One unit that scientists use to measure pressure is the bar. One bar is equal to the amount of pressure applied by the atmosphere to the surface of the earth at sea level (1 bar = 1.02 kg/sq cm, or 14.7 lb/sq in). Metamorphic rocks form under pressures of many kilobars, or thousands of bars. Rocks that are buried deep beneath many layers of rock experience lithostatic (Greek lithos, “rock”; statikos, “in place”) pressure, which causes the rocks to compress into a smaller, denser form.

Regional metamorphism results from increases in both pressure and heat below Earth’s surface. These increases occur below the surface of the earth, as tectonic plates come into contact with each other. Rock formed below the surface is generally igneous rock, which is formed from cooling magma. However, later deposits of rocks may bury sedimentary and extrusive igneous rocks, which form on the earth’s surface. Such burial often happens through subsidence, the settling associated with the development of sedimentary basins. It may also happen through tectonic overthrusting, as continental and oceanic plates fold up or down because of stress from movement or from contact with each other. The increased temperature and pressure in these areas cause mineralogical and textural changes in the original rock. This type of metamorphism develops on a much larger scale than contact metamorphism, usually over an area of hundreds or thousands of square kilometers.

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