Metamorphic rocks are the third great class of rocks. These are what happens when sedimentary and igneous rocks become changed, or metamorphosed, by conditions underground. The four main agents that metamorphose rocks are heat, pressure, fluids and strain. These agents can act and interact in an infinite variety of ways. As a result, most of the thousands of rare minerals known to science occur in metamorphic ("shape-changed") rocks. Metamorphism acts at two scales, the regional scale and the local scale.
The Four Agents of Regional Metamorphism
Heat and pressure usually work together, because both rise as you go deeper in the Earth. At high temperatures and pressures, most rocks break down and change into a different assemblage of minerals that are stable in the new conditions. The clay minerals of sedimentary rocks are a good example. Clays are surface minerals, which form as feldspar and mica break down in the conditions at the Earth's surface. With heat and pressure they slowly return to mica and feldspar. Even with their new mineral assemblages, metamorphic rocks may have the same overall chemistry they had before metamorphism.
Fluids are an important agent of metamorphism. Every rock contains some water, but sedimentary rocks hold the most. First there is the water that was trapped in the sediment as it became rock. Second is the water that is liberated by clay minerals as they change back to feldspar and mica. This water can become so charged with dissolved materials that the resulting fluid is no less than a liquid mineral. It may be acidic or alkaline, full of silica (forming chalcedony) or full of sulfides or carbonates or metals, in endless variety. Fluids tend to wander away from their birthplaces, interacting with rocks elsewhere. That process, which changes a rock's chemistry rather than just its mineral assemblage, is called metasomatism.
Strain refers to any change in the shape of rocks due to the force of stress. Movement on a fault zone is one example. In shallow rocks, shear forces simply grind and crush the mineral grains (cataclasis) to yield cataclasite. Continued cataclasis yields the hard and streaky rock mylonite.
Under greater heat and pressure, when metamorphic minerals such as mica and feldspar begin to form, strain orients them in layers. The presence of mineral layers, called foliation, is important to observe when identifying a metamorphic rock. As strain increases, the foliation becomes more intense, and the minerals sort themselves into thicker layers. The foliated rock types that form under these conditions are called schist or gneiss, depending on their texture. Schist is finely foliated whereas gneiss is organized in wide bands of minerals.
The Basic Metamorphic Rock Types
The sedimentary rock shale metamorphoses first into slate, then into phyllite, then a mica-rich schist. The mineral quartz does not change under high temperature and pressure, although it becomes more strongly cemented. Thus the sedimentary rock sandstone turns to quartzite. Intermediate rocks that mix sand and clay — mudstones — metamorphose into schists or gneisses. The sedimentary rock limestone recrystallizes and becomes marble.
Metamorphism can be so intense, with all four factors acting at their extreme range, that the foliation can be warped and stirred like taffy, and the result is called migmatite. With further metamorphism, rocks can be turned into something hard to tell from plutonic granites. These kinds of rocks give joy to experts because of what they say about deep-seated conditions during things like plate collisions. The rest of us can only admire the laboratory skills needed to make sense of such rocks.
Contact or Local Metamorphism
A type of metamorphism that is important in specific localities is contact metamorphism. This usually occurs near igneous intrusions, where hot magma forces itself into sedimentary strata. The rocks next to the invading magma are baked into hornfels or its coarse-grained cousin granofels, another subject for specialists. Magma can rip chunks of country rock off the channel wall and turn them into exotic minerals, too.
Get more help identifying metamorphic rocks in the Rock Identification Tables.