Metamorphic Rocks
Metamorphic rocks are any rocks that have been changed by heat, pressure, or a combination of these forces.
•Most metamorphic changes occur at temperatures of between 392°F-1,292°F (200°C-700°C). The extreme pressure at which rocks metamorphose is up to 6,000 times greater than atmospheric pressure and can occur at a depth of 12.4 mi (20 km).
•Contact metamorphism involves only heat. Regional metamorphism is brought about by heat and pressure.
•No melting of rock occurs during metamorphism. When melting takes place, magma, from which igneous rocks are formed, is created. • The chemical composition and structure of a rock can be changed by metamorphism. When large-scale faulting occurs, dislocation metamorphism takes place.
•Original structures, such as strata in sedimentary rocks, are removed during metamorphism.
•Fossils are sometimes found in slightly metamorphosed rocks such as slate. As the degree of metamorphism increases, fossils are destroyed.
•Garnet, a mineral much used as a semi-precious gemstone, is common in the metamorphic rock called schist.
•Some of the oldest rocks in the Earth's crust are highly metamorphosed gneisses.
Contact metamorphism
Contact metamorphism occurs when rocks are heated by magma or lava.
•A metamorphic aureole is the region around a mass of magma in which rocks have been altered.
•A lava flow can only metamorphose the rocks lying below it.
•The amount of contact metamorphism depends on the size of the igneous body producing heat and any fluids seeping from it. The metamorphic aureole around a large batholith may be a few miles wide.
•A small sill, dyke or lava flow may metamorphose rocks up to only a few inches away.
•A gradual metamorphic change takes place away from the igneous intrusion. The rocks furthest away are only slightly metamorphosed. Right next to the intrusion they may be highly altered.
Dark-colored "spots" and clusters of minerals are a common feature of clay and shale that have been altered by contact metamorphism. With great heat, a tough rock called hornfels is formed.
•When heated, limestone becomes crystalline marble. This new rock is a mosaic of calcite crystals, often with veins of green or blue minerals.
•Sandstone is changed to a hard, crystalline rock called metaquartzite.
Crushed rock
Large-scale faulting causes rocks along the fault surface to be crushed and metamorphosed.
•Fault breccia is a rock made of angular broken fragments. It is common along many fault lines. The crushed rock formed when thrust faults move deep in the Earth's crust is called mylonite.
•Mylonite is made from the dusty rock "flour" that is created as rocks are ground up along the fault plane. Larger fragments, often stretched out, are stuck into this.
•If thrust faults occur deep in the crust where temperatures are high, new minerals will grow in mylonite. Chlorite, mica, feldspar, and epidote are common examples. As the fault moves, rock flour and minerals are stretched out to give mylonite its typical texture. Mylonite tends to break in thin plates and slabs parallel to its texture.
Faulting
Faults are breaks in the rocks of the Earth's crust where the rocks move relative to each other. The same stratum will be at a different level on each side of the fault.
•A joint is a break in the rocks where no movement takes place.
•Both faults and joints are often the places where hot, mineral-rich fluids rise through the crust. Important mineral reserves occur in this way.
•The actual surface where the rocks break and move is called the fault plane. In many faults, such as normal and reversed faults, the fault plane is very steep. Thrust faults have a fault plane sloping at only a few degrees.
•When faults move, earthquakes occur. Movement on the San Andreas Fault that runs through California, threatens large cities such as Los Angeles.
•A normal fault is where the Earth's crust stretches and one mass of rock moves down a break-the fault plane.
•If two normal faults occur parallel to each other a block of the crust may sink between them. This is called a rift, or graben. The region of rift valleys in east Africa was formed in this way.
•Where a reverse fault occurs, the Earth's crust is made thicker by compression. One mass of rock is forced up the fault plane relative to the rocks on the other side.
•In a tear fault, there is virtually no vertical movement. Rock masses are moved sideways relative to each other.
Regional metamorphism
As the name implies, this type of metamorphism occurs over great areas of the Earth's crust.
•Regional metamorphism happens when mountain building, often associated with movement of the Earth's lithospheric plates, occurs.
•Pressure and heat are involved in altering rocks by regional metamorphism. The changes that take place during regional metamorphism may take tens of millions of years to occur. Some of the oldest rocks in the Earth's crust have been affected by regional metamorphism. These have been radiometrically dated at over 3,500 million years old. The deeper into the crust that rocks are taken by the processes of mountain building, the higher the degree, or grade, of metamorphism they suffer.
•Rocks formed by regional metamorphism are identified by their texture. Because of stresses in the rock, minerals are streaked out in layers.
•High-grade rocks-those that are altered most-are called gneiss. With increased temperature, caused by depth of burial, melting may occur and magma is created.
•At lower depths, where the temperature and pressure are lower, a rock called schist is formed.