Metamorphism: Alteration of Rocks by Temperature and Pressure PDF

7/2/2024 Metamorphism: Alteration of Rocks by Temperature and Pressure 1) Causes of Metamorphism 2) Types of Metamorphism 3) Metamorphic Textures 4) Regional Metamorphism and Metamorphic Grade...

7/2/2024 Metamorphism: Alteration of Rocks by Temperature and Pressure 1) Causes of Metamorphism 2) Types of Metamorphism 3) Metamorphic Textures 4) Regional Metamorphism and Metamorphic Grade 5) Plate Tectonics and Metamorphism DURING THE ROCK CYCLE, rocks may be subjected to temperatures and pressures great enough to cause changes in their mineralogy, texture, or chemical composition. – Rocks are thus transformed as they encounter high temperatures and pressures deep in the Earth’s crust. Metamorphism is a dynamic process. Earth’s internal heat engine drives the plate tectonic processes that push rocks formed at Earth’s surface down to great depths, thereby subjecting them to high pressures and high temperatures. 7/2/2024 2 1  7/2/2024 1) Causes of Metamorphism When a rock is subjected to significant changes in temperature or pressure, it undergoes changes in its chemical composition, mineralogy, and texture, or all three, until it is in equilibrium with the new temperature and pressure. Most metamorphic rocks are formed at depths of 10 to 30 km, in the middle to lower half of the crust. Only later are those rocks exhumed, or transported back to Earth’s surface, where they may be exposed as outcrops. The heat and pressure in Earth’s interior and the fluid composition are the three principal factors that drive metamorphism. A rock’s metamorphic grade reflects the temperatures and pressures it was subjected to during metamorphism. 7/2/2024 3 Grade of Metamorphism Metamorphic rocks form under different pressure and temperature conditions. Chemically-active fluids may also play a role. 1. Low-grade metamorphic rocks are those formed under the lower temperatures and pressures of shallower crustal regions. 2. High-grade metamorphic rocks are those formed under the higher temperatures and pressures at greater depths. Metamorphic minerals: Some silicate minerals are found mostly in metamorphic rocks: kyanite, andalusite, sillimanite, staurolite, garnet, and epidote. Geologists use distinctive textures as well as mineral composition to help guide their studies of metamorphic rocks. 7/2/2024 4 2  7/2/2024 The Role of Temperature Heat can transform a rock’s chemical composition, mineralogy, and texture by breaking chemical bonds and altering the existing crystal structures of the rock. When a rock is subjected to higher T, atoms and ions recrystallize, linking up in new arrangements and creating new mineral assemblages. Many new crystals grow larger than those in the original rock. Temperature increases gradually with depth. The increase in temperature with increasing depth is called the geothermal gradient. Because different minerals crystallize and remain stable at different temperatures, a rock’s mineral composition serves as a geothermometer to gauge the temperature at which it formed. 7/2/2024 5 The geothermal gradient varies among plate tectonic settings, but pressure increases with depth at about the same rate everywhere. An isotherm is a line that connects zones of equal temperature. 7/2/2024 6 3  7/2/2024 The Role of Pressure Pressure, like temperature, changes a rock’s chemical composition, mineralogy, and texture. A solid rock is subjected to two basic kinds of pressure (stress): 1) Confining pressure is a general force applied equally in all directions. A rock descending to greater depths in Earth’s interior is subjected to progressively increasing confining pressure in proportion to the weight of the overlying mass. 2) Directed pressure, or differential stress, is force exerted in a particular direction. Directed pressure is usually concentrated within particular zones or along discrete planes. The compressive force exerted where lithospheric plates converge is a form of directed pressure, and it deforms the rocks near the plate boundary. 7/2/2024 7 Rocks subjected to differential stress may be severely distorted, becoming flattened in the direction the force is applied and elongated in the direction perpendicular to the force. The minerals in a rock under pressure may be compressed, elongated, or rotated to line up in a particular direction. These rocks in California, show both the banding and the folding characteristic of sedimentary rocks metamorphosed into marble, schist, and gneiss. 7/2/2024 8 4  7/2/2024 The Role of Fluids Metamorphic processes can alter a rock’s mineralogy by introducing or removing chemical components that are soluble in heated water. Hydrothermal fluids accelerate metamorphic chemical reactions because they carry dissolved CO2 as well as other chemical substances that are soluble in hot water under pressure. As hydrothermal solutions percolate up to the shallower parts of the crust, they react with the rocks they penetrate, changing their chemical and mineral compositions and sometimes completely replacing one mineral with another without changing the rock’s texture. This kind of change is called metasomatism. Many valuable deposits of copper, zinc, lead, and other metallic ores are formed by this kind of chemical substitution. Source of the fluids: These fluids come mainly from chemically bound water in clays. 7/2/2024 9 2) Types of Metamorphism Metamorphic processes are categorized on the basis of their geologic settings. 7/2/2024 10 5  7/2/2024 3) Metamorphic Textures The texture of a metamorphic rock is determined by the sizes, shapes, and arrangement of its constituent crystals. In general, the grain size increases as the metamorphic grade increases. 1. Foliation and Cleavage The most prominent textural feature of regionally metamorphosed rocks is foliation, a set of flat or wavy parallel cleavage planes produced by deformation of igneous and sedimentary rocks under directed pressure. These foliation planes may cut through the bedding of the original sedimentary rock at any angle or be parallel to the bedding. In general, as the grade of regional metamorphism increases, foliation becomes more pronounced. 7/2/2024 11 The planes of platy crystals are aligned parallel to the foliation; an alignment that is known as the preferred orientation of minerals. Directed pressure on rocks containing platy minerals causes foliation. 7/2/2024 12 6  7/2/2024 Foliated Rocks The foliated rocks are classified according to four main criteria: 1. Metamorphic grade 2. Grain (crystal) size 3. Type of foliation 4. Banding In general, foliation progresses from one texture to another with increasing metamorphic grade. As temperature and pressure increase, a shale may metamorphose first to a slate, then to a phyllite, a schist, a gneiss, and finally to a migmatite. 7/2/2024 13 Quartzite 2. Granoblastic Rocks Granoblastic rocks are nonfoliated metamorphic rocks composed mainly of crystals that grow in equant (equidimensional) shapes, such as cubes and spheres, rather than in platy or elongate shapes. These rocks result from metamorphic processes, such as contact metamorphism, in which directed pressure is absent, so foliation does not occur. Granoblastic rocks include hornfels, quartzite, marble, greenstone, amphibolite, and granulite. – Marbles are the metamorphic products of heat and pressure acting on limestones and dolomites. Greenstones are metamorphosed mafic volcanic rocks. Amphibolites are made up of amphibole and plagioclase feldspar. Granulite, a high-grade metamorphic rock that is also referred to as granofels, has a homogeneous granular texture (composed of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals). 7/2/2024 14 7  7/2/2024 3. Porphyroblasts Newly formed metamorphic minerals may grow into large crystals surrounded by a much finer grained matrix of other minerals. These large crystals, called porphyroblasts, are found in rocks formed both by contact and regional metamorphism. Porphyroblasts form from minerals that are stable over a broad range of pressures and temperatures. Garnet and staurolite are two common minerals that form porphyroblasts, although many others are also found. 7/2/2024 15 A summary of the textural classes of metamorphic rocks and their main characteristics: 7/2/2024 16 8  7/2/2024 4) Regional Metamorphism and Metamorphic Grade Geologists studying metamorphic rocks constantly seek to determine the intensity and character of metamorphism more precisely than is indicated by the terms of “low grade” or “high grade.” To make these finer distinctions, geologists “read” minerals as though they are pressure gauges and thermometers. Mineral Isograds: Mapping Zones of Change When studying a broad belt of regional metamorphism, we can see many outcrops, some showing one set of minerals, some showing others. Different zones within the belt may be distinguished by index minerals: abundant minerals that each form under a limited range of temperatures and pressures 7/2/2024 17 Index minerals define the different metamorphic zones within a belt of regional metamorphism. (a) Map of New England, showing metamorphic zones based on index minerals found in rocks metamorphosed from shale. (b) Rocks produced by the metamorphism of shale at different temperatures and pressures. 7/2/2024 18 9  7/2/2024 5) Plate Tectonics and Metamorphism Different types of metamorphism are likely to occur in different plate tectonic settings: 1. Continental interiors. Contact metamorphism, burial metamorphism, and perhaps regional metamorphism occur at different levels in the crust. 2. Divergent plate boundaries. Seafloor metamorphism and contact metamorphism around plutons intruding into the oceanic crust. 3. Convergent plate boundaries. Regional metamorphism, high-pressure and ultra-high-pressure metamorphism, as well as contact metamorphism. 4. Transform faults. In oceanic settings, seafloor metamorphism may occur. In both oceanic and continental settings, extensive metamorphism caused by shearing forces along transform faults may be found. 7/2/2024 21 EXERCISES-Examples of Questions 1. What are the causes of metamorphism? 2. What are the various types of metamorphism? 3. What do metamorphic rocks reveal about the conditions under which they were formed? 4. What is a porphyroblast? 5. What is the difference between a granite and a slate? 6. How are metamorphic rocks related to plate tectonic processes? 7. In which plate tectonic settings would you expect to find regional metamorphism? 7/2/2024 22 10