What Makes Metamorphic Rocks So Unique

Metamorphic rocks are the third great class of rocks. They occur 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 almost infinite variety of ways. As a result, most of the thousands of rare minerals known to science occur in metamorphic rocks. Metamorphism acts at two scales: regional and local. Regional-scale metamorphism generally occurs deep underground during  orogenies, or mountain-building episodes. The resulting metamorphic rocks from the cores of large mountain chains like the Appalachians. Local metamorphism happens at a much smaller level, usually from nearby igneous intrusions. It is sometimes referred to as contact metamorphism. A gneiss boulder showing characteristic mineral banding. Grant Dixon / Lonely Planet Images / Getty Images How to Distinguish Metamorphic Rocks The main feature that identifies metamorphic rocks is that they are shaped by great heat and pressure. The following traits are all related to that. Because their mineral grains grew together tightly during metamorphism, theyre generally strong rocks.Theyre made of different minerals than other kinds of rocks and have a wide range of color and luster.They often show signs of stretching or squeezing, giving them a striped appearance. The Four Agents of Regional Metamorphism Heat and pressure usually work together, because both increase as you go deeper into the Earth. At high temperatures and pressures, the minerals in most rocks break down and change into a different set 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 Earths 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 as before metamorphism. Fluids are an important agent of metamorphism. Most rocks contain some water, but sedimentary rocks hold the most. First, there is the water that was trapped in the sediment as it became rock. Second, there is 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, in essence, a liquid mineral. It may be acidic or alkaline, full of silica (forming chalcedony) or full of sulfides or carbonates or metal compounds, in endless varieties. Fluids tend to wander away from their birthplaces, interacting with rocks elsewhere. That process, which changes a rocks chemistry as well as 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 grinding yields the hard and streaky rock mylonite.   Different degrees of metamorphism create distinctive sets of metamorphic minerals. These are organized into metamorphic facies, a tool petrologists use to decipher the history of metamorphism. Foliated vs. Non-foliated Metamorphic Rocks Under greater heat and pressure, as  metamorphic minerals  such as mica and feldspar begin to form, strain orients them in layers. The presence of mineral layers, called  foliation, is an important feature for classifying  metamorphic rocks. As strain increases, the foliation becomes more intense, and the minerals may 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 noticeable, wide bands of minerals. Non-foliated rocks occur when heat is high, but pressure is low or equal on all sides. This prevents dominant minerals from showing any visible alignment. The minerals still recrystallize, however, increasing the overall strength and density of the rock. 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. Igneous rocks give rise to a different set of minerals and metamorphic rock types. These include serpentinite, blueschist, soapstone, and other rarer species such as eclogite. Metamorphism can be so intense, with all four factors acting at their extreme range, that the foliation can be warped and stirred like taffy; the result of this is  migmatite. With further metamorphism, rocks can begin to resemble  plutonic granites. These kinds of rocks give joy to experts because of what they say about deep-seated conditions during things like plate collisions. Contact or Local Metamorphism A type of metamorphism that is important in specific localities is contact metamorphism. This most often 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. Magma can rip chunks of country-rock off the channel wall and turn them into exotic minerals, too. Surface lava flows and underground coal fires can also cause mild contact metamorphism, similar to the  degree that occurs when baking bricks.