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Chapter 7 Prepared by Iggy Isiorho for Dr. Isiorho Metamorphism Index Metamorphism Metamorphism – The transformation of preexisting rock new rock as a result of high temperature, high pressure, or both, but without the rock melting in the process. Metamorphic rock – A rock produced by metamorphism. Parent rock – Original rock before being metamorphosed. Ductile – Capable of being molded and bent under stress. Factors of Metamorphic Rocks Composition of the Parent Rock Temperature Foliation Pressure Fluids Differential Stress Time Composition of the Parent Rock Usually no new elements or chemical compounds are added to the rock during metamorphism, except perhaps water. Therefore, the mineral content of the metamorphic rock is controlled by the chemical composition of the parent rock. Back Temperature Heat, necessary for metamorphic reactions, comes primarily from the outward flow of geothermal energy from the earth’s deep interior. The deeper a rock is beneath the surface, the hotter it will be. The particular temperature for rock at a given depth depends on the local geothermal gradient. A mineral is said to be stable if, given enough time, it does not react with another substance or convert to a new mineral or substance. Any mineral is stable only within a given temperature range. Minerals stable at higher temperatures tend to be less dense (or have a lower specific gravity) than chemically identical minerals stable at lower temperatures. The upper limit on temperature in metamorphism overlaps the temperature of partial melting of a rock. If partial melting takes place, the component that melts becomes magma; the solid residue remains a metamorphic rock. Pressure pressure – Pressure applied equally on all surfaces of a body; also called geostatic or lithostatic pressure. Any new mineral that has crystallized under high-pressure conditions tends to occupy less space than did the mineral or minerals from which it formed. Confining Back Differential Stress – A force acting on a body, or rock unit, that tends to change the size or shape of that body, or rock unit. Force per unit area within a body. Differential stress – When pressures on a body are not of equal strength in all directions. Compressive stress – A stress due to a force pushing together on a body. Shearing – Movement in which parts of a body slide relative to one another and parallel to the forces being exerted. Back Stress Foliation Foliation – Parallel alignment of textural and structural features. If the rock splits easily along nearly flat and parallel planes, indicating that preexisting, microscopic, platy minerals were pushed into alignment during metamorphism, we say the rock is slaty, or that it possesses slaty cleavage. If visible platy or needle-shaped minerals have grown essentially parallel to a plane due to differential stress, the rock is schistose If the rock became very ductile and the new minerals separated into distinct (light and dark) layers or lenses, the rock has a layered or gneissic texture. Back Fluids Hot water (as vapor) is the most important fluid involved in metamorphic processes, although other gases, such as carbon dioxide, sometimes play a role. The water may have been trapped in a parent sedimentary rock or given off by a cooling pluton. Water is thought to help trigger metamorphic chemical reactions. Water is a sort of intra-rock rapid transit for ions from one mineral, and then carries these ions elsewhere in the rock where they can react with the ions of a second mineral the new mineral that forms is stable under the existing conditions. Back Time The effect of time on metamorphism is hard to comprehend. Most metamorphic rocks are composed predominately of silicate minerals, and silicate compounds are notorious for their sluggish chemical reaction rates. Many laboratory attempts to duplicate metamorphic reactions believed to occur in nature have been frustrated by the time element. The several million years during which a particular combination of temperature and pressure may have prevailed in nature are impossible to duplicate. Back Classification of Metamorphic Rocks The kind of metamorphic rock that forms is determined by the metamorphic environment (primarily the particular combination of pressure, stress, and temperature) and by the chemical constituents of the parent rock. How do we classify the different types of metamorphic rocks? First consider the texture of a metamorphic rock. Is it foliated or nonfoliated If the rock is nonfoliated, it is named on the basis of its composition. If the rock is foliated, you must determine the type of foliation. For example, a schistose rock is called a schist. But this name tells us nothing about what minerals are in this rock; so we add adjectives to describe the composition. Types of Metamorphism Contact Metamorphism Regional Shock Metamorphism Metamorphism Contact Metamorphism metamorphism – Metamorphism under conditions in which high temperature is the dominant factor. Confining pressure may influence which minerals crystallize. However, the confining pressure is usually relatively low. The zone of contact metamorphism (also called an aureole) is usually quite narrow – generally from 1 to 100 meters wide. Contact Back Regional Metamorphism Metamorphism – Metamorphism that takes place at considerable depth underground. Depending on the pressure and temperature conditions during metamorphism, a particular parent rock may recrystallize into one of several metamorphic rock. Regional Back Metamorphism To demonstrate the relationship between regional metamorphism and plate tectonics, we will look at what is believed to take place at a convergent boundary in which oceanic lithosphere is subducted beneath continental lithosphere. Hydrothermal Processes Rocks that have precipitated from hot water or have been altered by hot water passing through are hard to classify. As described earlier, hot water is involved to some extent in most metamorphic processes. Beyond metamorphism, hot water also plays an important role creating new rocks and minerals. These form entirely by precipitation of ions derived from hydrothermal solutions. Hydrothermal minerals can form in void spaces or between the grains of a host rock. An aggregate of hydrothermal minerals, a hydrothermal rock, may crystallize within a preexisting fracture in a rock to form a hydrothermal vein. Hydrothermal Activity at Divergent Plate Boundaries Hydrothermal processes are particularly important at midoceanic ridges (which are also divergent plate boundaries). As show in Fig. 7.17, cold seawater moves downward through cracks in the basaltic crust and is cycled upward by heat from magma beneath the ridge crest. Very hot water returns to the ocean at submarine hot springs. Hot water traveling through the bassalt, gabbro, and ultramafic rocks of the oceanic lithopshere helps metamorphose these rocks while they are close to the diverging boundary. Metasomatism – Metamorphism coupled with th introduction of ions from an external source. Metasomatism If metasomatism is associated with contact metamorphism, the ions are introduced from a cooling magma. Some important commercially mined deposits of metals such as iron, tungsten, copper, lead, zinc, and silver are attributed to metasomatism. Sources of Water Where does the water come from? The following is a logical explanation. Ground water seeps downward from the earth’s surface through pores and fractures in rocks; however, the depth to which surface-derived water can penetrate is quite limited. Plate tectonics can account for water at deeper levels in the lithosphere as seawater trapped in the oceanic crust can be carried to considerable depths through subduction Back to the Beginning
