GEOL 101 1nd Edition Lecture 15 Outline of Last Lecture I. Sedimentary RocksII. Detrital Sedimentary RocksIII. Chemical Sedimentary RocksIV. EnvironmentsV. Energy ResourcesOutline of Current Lecture I. OverviewII. Agents of MetamorphismIII. TexturesCurrent LectureOverview-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 returnsback to igneous rocks.-Metamorphic settings:- Contact metamorphismo Driven by rise in temperature within host rock- Hydrothermal metamorphismo Chemical alterations from hot iron-rich water- Regional metamorphismo Occurs during mountain buildingo Produces greatest volume of metamorphic rockThese 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 metamorphismAgents-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 gradiento 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 differentpressures 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 rockfrom which they formed- Sandstone that is made of quartz sands (SiO2) can be metamorphosed to make quartzite, which also contains quartzTextures-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 mineralo Examples: Parallel alignment of plates and/or elongated minerals Parallel alignment of flattened grains and pebbles Composition bandingo 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 aligno 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 cleavageo 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 regimeso Gneissic During high grades of metamorphism, ion migration results in the segregation of minerals Exhibit a distinctive bounded appearance o 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 textureo To indicate composition, mineral names are used- Schists may contain “accessory minerals”o Garnet, staurolite, sillimaniteo These accessory minerals may occur as porphyroblastso 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