Metamorphic Rock

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.

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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.