Metamorphic Rocks PDF

Deep within the crust, pressures can reach a crushing 10,000 atmospheres (10,000 times the pressure of the atmosphere on Earth at sea level) and temperatures can be over 600°C. What happens when rocks are subjected to such high pressures and temperatures? How do they change and are the changes predi...

Deep within the crust, pressures can reach a crushing 10,000 atmospheres (10,000 times the pressure of the atmosphere on Earth at sea level) and temperatures can be over 600°C. What happens when rocks are subjected to such high pressures and temperatures? How do they change and are the changes predictable? What factors influence the evolution of rock under these extreme conditions? This chapter answers these and additional questions re- garding rock metamorphism. Geologists use the term metamorphism to describe changes in mineralogy, texture, and/or composition that occur in rocks affect- ed by changes in temperature, pressure (also called stress), and/or chemically active fluids. In the world of rocks, metamorphism occurs predominantly by a phenomenon called solid-state reaction or a solid- state change. During metamorphism, minerals that recrystallize and remain solid are referred to as metamorphic minerals, if part, or all, of the rock melts, that part is classified as igneous. What Is Metamorphic Rock? Metamorphic rocks are sedimentary, igneous, or metamorphic rocks that take on a new texture and mineralogy in order to maintain equi- librium under changing conditions within the crust. That is, miner- als within a rock change in order to be stable under new conditions. In most cases, this occurs through recrystallization of minerals in the original (or "parent") rock (FIGURE 9.1). Recrystallization is the growth of new mineral grains in a rock by recycling old miner- als. Rarely does recrystallization involve only one mineral. Rather, a metamorphic rock is formed because a new assemblage of minerals results from recrystallization. Metamorphic rocks are a major source of building material, Slate and marble are commonly used as finishing stone in buildings and for statuary. (See the "Geology in Our Lives" box titled "Master of Marble.") Metamorphism of ultramafic rock (magnesium- and iron-rich igneous rocks) can produce chrysotile, the principal source of asbestos, which is an important component of fire-retardant materials, and talc (from metamorphism of dolostone), a very soft silicate mineral that is widely used as filler in paints, rubber, paper, asphalt, and cosmetics. Metamor- phism also has the effect of bringing together certain elements, such as metals, into dense accumulations that form valuable ores, such as the gold deposits in the Abitibi region of Quebec, Canada. Like the other types of rock we have studied, metamorphic rocks are named based on their texture and mineral composition. If a meta- morphic rock is heavily squeezed by tectonic forces, newly crystallizing minerals will align perpendicular to the maximum stress (as discussed in Section 9-2), thus creating textures known as foliation (planar structures) and lineation (linear structures). Different types of foliation and lineation give rise to different types of rock (as we will see later in the chapter). Because of tectonic forces that pull, squeeze, and shear the crust and often accompany metamorphism, these rocks typically acquire new texture as well as new mineralogy. The new minerals align as they form, layers develop, colour bands of minerals appear, and crystals may stretch and even rotate. Features of the parent rock, such as vesicles, fos- sils, and primary sedimentary structures (cross-bedding, stratification, etc.), usually disappear. The resulting metamorphic rock is a product of the composition of its parent rock, referred to as the protolith, and the conditions the protolith has experienced. Hence, it offers a window into the environment of the deep crust.