GEOL 101 1nd Edition Lecture 15 Outline of Last Lecture I Sedimentary Rocks II Detrital Sedimentary Rocks III Chemical Sedimentary Rocks IV Environments V Energy Resources Outline of Current Lecture I Overview II Agents of Metamorphism III Textures Current Lecture Overview Metamorphism to change form Increased temperature and or increased pressure Produced from all three rocks in the Rock Cycle Metamorphism progresses incrementally from low grade to high grade Low grade rocks change slightly High grade rocks have been through dramatic changes During metamorphism the rock must remain in solid form but once it begins to melt it returns back to igneous rocks Metamorphic settings Contact metamorphism o Driven by rise in temperature within host rock Hydrothermal metamorphism o Chemical alterations from hot iron rich water Regional metamorphism o Occurs during mountain building o Produces greatest volume of metamorphic rock These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute o Rocks usually display zones of contact and or hydrothermal metamorphism Agents Heat Provides the energy necessary for chemical reactions that change existing minerals into new minerals Two sources of heat o Contact metamorphism heat from chamber An increase in temperature with depth due to the geothermal gradient o Pressure stress Increases with depth Confining pressure applies a force that is the same on all sides all around Rocks may also be subjected to differential stresses unequal or different pressures on different sides From tectonic movement Chemically active fluids Mainly water with other volatile components Enhances migration of ions Aids in recrystallization of existing minerals Sources of fluids Pore spaces sediment Fractures igneous rocks Hydrated minerals like clay and mica Parent Rock original rock from which the metamorphic rock formed Most metamorphic rocks have the same overall chemical composition as the parent rock from which they formed Sandstone that is made of quartz sands SiO2 can be metamorphosed to make quartzite which also contains quartz Textures Texture refers to the size shape and arrangement of grains within a rock Metamorphic rocks are classified into two categories Foliated texture planar arrangement of mineral o Examples o o o o o Parallel alignment of plates and or elongated minerals Parallel alignment of flattened grains and pebbles Composition banding Foliation can form in various ways including Rotate of platy and or elongated minerals Minerals in the direction of preferred orientation Changing the shape of equidimensional grains into elongated shapes that align Slaty cleavage where cracks can be split into thin tabular sheets Closely spaced planar surfaces in which rocks split Can develop in a number of ways depending on metamorphic conditions and parent rock More common in low grade metamorphic rocks but can occur in higher grades Slate often exhibits slaty cleavage Schistosity Platy minerals are discernible with the unaided eye and exhibit a planar or layered structure Rocks with this texture is referred to as schist Schists form under higher temperature and pressure regimes Gneissic During high grades of metamorphism ion migration results in the segregation of minerals Exhibit a distinctive bounded appearance From low grade to high grade Slate Phyllite Schist Gneissic Slate Excellent rock cleavage Most often generated from low grade metamorphism of shale mudstone or siltstone Phyllite Slightly higher grade metamorphism than slate Made of mica minerals which are not yet large enough to be seen without a microscope Described as having a glossy sheen or being plastic coated Exhibits rock cleavage Parent rock is slate or shale Schist Medium to coarse grained Platy minerals predominate commonly micas The term schist describes the texture o To indicate composition mineral names are used Schists may contain accessory minerals o Garnet staurolite sillimanite o These accessory minerals may occur as porphyroblasts o Parent rock is phyllite slate or shale Gneiss Medium to coarse grained Bounded appearance Often composed of white or light colored feldspar rich layers with bands of dark ferromagnesian minerals Parent rock can be schist phyllite slate or shale OR granite and other igneous rocks Nonfoliated texture non planar arrangement of minerals Next Lecture Porphyroblastic large crystals surrounded by smaller crystals Next Lecture