Metamorphic Rocks PDF

Metamorphic Rocks Metamorphic rocks are rocks that have undergone changes in: a) Texture b) Chemical composition Any rock (igneous, sedimentary, or metamorphic) can become a metamorphic rock. If rocks are buried de...

Metamorphic Rocks Metamorphic rocks are rocks that have undergone changes in: a) Texture b) Chemical composition Any rock (igneous, sedimentary, or metamorphic) can become a metamorphic rock. If rocks are buried deep in the Earth at high temperatures Due to increased: and pressures, they form new minerals and a) pressure textures all without melting. b) temperature c) chemical reactions in fluids Metamorphic rocks are classified on the basis of texture and composition into two types Foliated Nonfoliated. 2 Metamorphism is the transformation of existing rock to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of 150 to 200 °C (300 to 400 °F), and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. 3 Various forms of metamorphism exist, including regional, contact, hydrothermal, shock, and dynamic metamorphism. These differ in the characteristic temperatures, pressures, and rate at which they take place and in the extent to which reactive fluids are involved. Metamorphism occurring at increasing pressure and temperature conditions is known as prograde metamorphism, while decreasing temperature and pressure characterize retrograde metamorphism. 4 TEXTURAL CHANGES As a rock undergoes metamorphism, some mineral grains grow larger and others shrink. The shapes of the grains may also change. Metamorphism transforms limestone into a metamorphic rock called marble. Like the fossiliferous limestone, the marble is composed of calcite, but the texture is now one of large interlocking grains, and the fossils have vanished. 5 Minerals characterizing metamorphic rocks Andalusite Kyanite Sillimanite Staurolite Garnet 6  Metamorphic rocks: controlling factors 1- Parent rock composition No new material is added to rock during metamorphism Metamorphic rock will have similar composition to parent rock If parent material contains only one mineral, THE resultant metamorphic rock will only have one mineral --mineral will be recrystallized (texture changes)-- Limestone Marble 7  LIMESTONE Texture changes Note fragments of shells MARBLE LIMESTONE Note interconnecting grains 8  If parent material contains many minerals… …old minerals will recombine to form new minerals clay, quartz, mica, and volcanic fragments in a sandstone will combine to form new metamorphic minerals Example is garnet: which grows during metamorphism Garnet schist (metamorphic rock) Garnet growing 9 2- Temperature during metamorphism Heat originates from Earth’s deep interior All minerals are stable over finite temperature range Higher temperatures than that range cause melting 10  3- Pressure during metamorphism Pressure in the Earth acts the same in all directions pressure is proportional to depth in the Earth increases by ~1 kbar Pressure increases per 3.3 km with depth --grains pack together-- Volume decreases with depth High pressure minerals: more compact and dense 11 Tectonic forces - driven by plate motion! lead to forces that are not equal in all directions (differential stress) compressive stress (hands squeeze together) causes flattening at 90° to stress shearing (hands rubbing together) causes flattening parallel to stress 12 Flattened pebbles in metamorphic rock 13 4- Fluids hot water (water vapor) most important heat causes unstable minerals to release water water reacts with surrounding rocks and transports dissolved material and ions 5- Time metamorphism may take millions of years longer times allow new minerals to grow larger --coarser grained rocks 14 Classification of Metamorphic Rocks Based on ROCK TEXTURE Foliated (layered) e.g. slaty Non-foliated (non-layered) e.g. marble 15  FOLIATED metamorphic NON-FOLIATED metamorphic rocks are those in which the rocks do not display layers. minerals have been flattened A good example of non- and pushed down into parallel foliated rock is quartzite, the layers. The bands in foliated smooth-textured, metamorphic rock look like metamorphosed form of the pages in a book. mineral quartz. Examples of foliated rocks are A coarse-textured non- slate, shale, and gneiss. foliated rock is marble. 16 17 Foliated (layered) metamorphic rocks Results from differential stress (not equal in all directions) foliation 18  Under microscope Non-foliated Foliated 19 Foliated metamorphic rock: slate 20 Foliated metamorphic rock: phyllite (higher T, P than slate)  Foliated metamorphic rock: schist (higher T, P than phyllite) …new minerals grow -- garnet (large, roundish grains) Garnet Foliated metamorphic rock: gneiss (higher T, P than schist) banding of quartz/feldspar and ferromagnesian minerals Migmatites are very high grade metamorphic rocks that undergo a partial of melting. 24 25 26 27 Non-foliated (non-layered) metamorphic rocks results from pressure: equal in all directions named on the basis of their composition limestone (CaCO3) marble (CaCO3) 28 Non-foliated metamorphic rocks: quartzite Metamorphosed quartz sandstone Non-foliated metamorphic rocks: hornfels metamorphosed basalt  Types of metamorphism I. Contact metamorphism occurs adjacent to magma bodies intruding Cooler country rock -- “contact” produces non-foliated metamorphic rocks happens in a narrow zone of contact (~1 to 100 m wide) known as aureole forms fine-grained (e.g. hornfels) or coarse-grained (e.g. marble) rocks II. Regional metamorphism occurs over wide region and mostly in deformed mountain ranges produces foliated metamorphic rocks happens at high pressures and over a range of temperature increases in pressures and temperatures forms rocks of higher metamorphic grade III. Partial melting during metamorphism produces migmatites, which have both intrusive and metamorphic textures IV. Shock metamorphism occurs during impact events yields very high pressures forms “shocked” rocks around impact craters  Hydrothermal METAMORPHISM cold sea water encounters hot basalt, forms steam, alters minerals  Plate tectonics and metamorphism Regional metamorphism associated with convergent boundaries pressure increases with depth temperature varies laterally different P, T conditions yield different degrees of metamorphism temperatures cooler in down-going (subducting) plate (dashed purple line is isotherm -- line of equal T)