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.

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.

III.

Textures and Structures

The heat and pressure that form metamorphic rocks often deform the rock, giving rise to a variety of textures and structures collectively referred to as fabric. Some common metamorphic rocks can be identified according to their fabric. Regional metamorphism often produces a fabric quality called foliation, while rocks formed by contact metamorphism are generally nonfoliated.

A.

Foliated Rocks

Foliation is similar in appearance to the grain of wood. It occurs because certain minerals in a parent rock naturally form in parallel planes. Foliation may also occur when different minerals are sandwiched together and compressed, or when rock is fractured along parallel lines. Slate, phyllite, schist, and gneiss are examples of foliated rocks.

Slate is a fine-grained metamorphic rock formed from shale or clay sedimentary rock that has been exposed to low temperature and pressure. Slate is rich in silicates, which naturally form into planes. The low heat does not “overcook” the rock, so the foliation is very smooth in appearance. Greater pressure forms phyllite, which has a slightly coarser grain size than slate. The surface of a phyllite is visibly scaly and often has a silvery luster. More pressure, and subsequent heat, produces schist, a more coarsely foliated rock. Schist is usually foliated because of a planar mineral, but it may also be layered because of completely different mineral compositions. Foliation differs from layering, as the mineral grains in a foliated rock crystallize into parallel planes, whereas the mineral grains in a layered rock do not line up parallel with one another. More heat and pressure produce gneiss, a very coarse rock. The extreme foliation in gneiss is mainly due to the separation of different minerals that occurs at high pressure and temperature.

B.

Nonfoliated Rocks

Nonfoliated rocks are produced mainly by contact metamorphism, or heat from cooling magma. Contact heat generally results in a finer recrystallization of the parent rock, so little foliation is visible. Quartzite is typically a tough, hard, light-colored rock in which all the sand grains of a sandstone or siltstone have recrystallized into a fabric of interlocking quartz grains. Marble is a softer, more brittle rock in which the dolomite or calcite of the limestone parent rock has recrystallized. Hornfels is a common metamorphic rock formed when basalt or shale is exposed to heat from magma.

IV.

Mineral Reactions

The mineral structure of metamorphic rocks depends both on the type of parent rock and on the amount of heat and pressure present when the rocks formed. To define the types of mineral changes that may occur, geologists organize metamorphic rocks into several metamorphic facies, or groups. This idea has two basic principles: for rocks formed under the same metamorphic conditions, different mineral assemblages represent different parent rock compositions. For a given parent rock composition, different mineral structures imply different physical conditions.

The possible range of metamorphic conditions is divided into several different facies. Each facies group is defined by a specific mineral assemblage in a known example that is constant over a given range of temperature and pressure. As temperature or pressure increases, the parent rock will generate different mineral assemblages. Finding rocks that belong to certain facies in an area helps geologists to determine the geologic history of that area.

A.

Metamorphic Facies

Metamorphic facies are formed according to one of three processes: contact metamorphism, subduction-zone metamorphism, and regional metamorphism. Each of these processes occurs over a range of pressure and temperature to produce the different facies.

Low pressure (1 kilobar, or kb) and moderate to high temperatures of 300° to 850° C (600° to 1,560° F) produce hornfels facies during contact metamorphism. High pressures (5 to over 8 kb) and low to moderate temperatures of 250° to 600° C (480° to 1,100° F) form blueschist facies. Blueschist facies is typical of subduction-zone metamorphism as tectonic plates fold over one another. Five other groups, the greenstone, greenschist, amphibolite, granulite, and eclogite facies, are formed by regional metamorphism, such as the bending or buckling of continental plates into mountain ranges. Low pressure (1 kb) and low temperatures of 100° to 550° C (200° to 1,020° F) form greenstone and greenschist. Moderate pressures of 1 to 2 kb and medium temperatures ranging from 550° to 750° C (1020° to 1,380° F) form amphibolite. High pressures (over 10 kb) and extremely high temperatures of 700° to over 900° C (1,300° to 1,700° F) produce the granulite and eclogite facies.

B.

How Minerals Change Through Facies

Metamorphism is not a single isolated event. Rather, it is the cumulative effect of all the continuous changes that have occurred to a rock. To get to its present state, a metamorphic rock follows a pressure-temperature-time (PTT) path. Both the PTT path a rock has followed and the chemical composition of the parent rock primarily determine the mineral assemblages in metamorphic rocks.

For instance, at moderate temperature and pressure, the mineral contents of a basaltic parent rock react to form new minerals, including actinolite, a greenschist. At increased heat and pressure, the feldspar becomes richer in anorthite and the ferromagnesian minerals (minerals that contain iron and magnesium) react to form amphibole, giving its name to the amphibolite facies. As the heat and pressure increase again, orthopyroxene forms and the granulite facies begins. At very high pressures, the basaltic parent rock passes through the blueschist facies, forming high-pressure, low-temperature minerals such as glaucophane and jadeite. At still higher pressures, garnet and pyroxene occur, characteristic of the eclogite facies.

Pelitic parent rocks—claylike sediments with a large amount of aluminum—may give rise to an extensive set of mineral structures, but most of them consist of quartz, muscovite (see Mica), and three or four other minerals. Because of the complexity of these mineral structures, pelitic rocks are very important in determining the PTT paths of metamorphic rocks. Sandstone parent rocks consist predominantly of quartz, which recrystallizes on metamorphism. The principal changes in quartzites are therefore textural rather than mineralogical. In limestone parent rocks, the carbonate minerals recrystallize extensively to form marble.

V.

Geologic Importance

The different facies series are strongly related to plate tectonic movement. Metamorphic areas give important information on convergent-plate margins, where downward motion of the plates induces metamorphism.

Convergent plates cause crustal thickening and an increase in pressure at low temperature. This often results in blueschist metamorphism, but additional heating may change the blueschist to greenschist- and amphibolite-facies metamorphism, depending on the mineral composition of the colliding plates.

The collision of oceanic and continental plates results in subduction, or burial, of cold oceanic crust. Such subduction produces high-pressure, low-temperature conditions, which result in the formation of blueschists. In contrast, the collision of continental plates produces thickening and extensive regional metamorphism of the greenschist-amphibole-granulite type. Continental collisions are typically involved in large-scale tectonic activity, and detailed variations in pressure and temperature are often the result of large-scale thrusting and nappe development (the thrusting of rocks over other rocks). Evidence of such conditions is clearly seen in certain mountainous regions, including the Adirondack Mountains of northern New York State, in the United States, and the Alps in Europe.

Contact-metamorphic rocks are much more limited in extent and relate to more local aspects of geology. Nevertheless, they provide important information concerning the heat content, heat flow, and cooling history of both large-scale and small-scale igneous intrusions.

VI.

Economic Importance

Metamorphic rocks are an important source of building materials. Slate and marble are commonly used as finishing stone in buildings. Low-grade metamorphism of ultramafic igneous rocks (dark igneous rocks composed mostly of magnesium and iron) produces serpentine, a group of sheet-silicate minerals (crysotile, lizardite, antigorite) that are the principal sources of asbestos.

Metamorphism of impure limestones produces talc, a very soft silicate mineral that is an important mineral filler in paints, rubber, paper, asphalt, and cosmetics. High-grade marbles and granulites are important sources of sapphires and rubies, particularly in Sri Lanka and Myanmar (known as Burma until 1989).

Metamorphism is also an important process in concentrating specific elements to form deposits of ore bodies. In this way, low-grade gold occurrences may be concentrated during metamorphism, forming economically important gold deposits. This is the case in some deposits located in the Abitibi region of Quebec, Canada.