Tsukikowerewolf
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Post: 54984197_1 created on Thu Oct 08, 2009 11:49 pmPosted: Thu Oct 08, 2009 11:49 pm
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Um, yeah, I need to study for my geology exam, and one of the ways I do that best is by typing up my notes. sweatdrop I figured I'd share them. smile
Okay, so, first of all, IGNEOUS ROCKS. Igneous Rocks - rocks that form as molten rock cools and solidifies. Magma - the parent material for igneous rocks (molten rock), that is formed by a process called "partial melting" - completely or partly molten material, which, on cooling, solidifies to form an igneous rock - most magmas consist of three distinct parts (a liquid component, a solid component, and a gaseous phase). Lava - magma that reaches the Earth's surface. Extrusive igneous rocks - (also known as "volcanic" wink igneous rocks that form when molten rock solidifies at the Earth's surface (for example, cooling after a volcanic eruption). Intrusive igneous rocks - (also known as "plutonic" wink igneous rocks that form at depth (that is, under the surface of the Earth). Melt - the liquid portion of magma that is composed of mobile ions of those elements commonly found in the Earth's crust (made up mostly of aluminum, potassium, calcium, sodium, iron, and magnesium). Volatiles - the gaseous components of magma that are materials that will vaporize (form a gas) at surface pressures - the most common volatiles found in magma are water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2). Crystallization - when magma is at its hottest, ions and groups of ions join together and break apart constantly - then, as magma cools, the ions begin to move more slowly and eventually join together into orderly crystalline structures - this process generates various silicate minerals that reside within the remaining melt. Texture - the overall appearance of the rock based on size, shape, and arrangement of its interlocking crystals. Three Factors contributing to the textures of igneous rocks - 1. the rate at which magma cools - 2. the amount of silica present - 3. the amount of dissolved gases in the magma - of these, the rate of cooling is the dominant factor, but, like all generalizations, this one has exceptions. Types of Igneous Textures Aphanitic (Fine-Grained) Texture - igneous rocks that form at the surface or as small masses within the upper crust where cooling is relatively rapid possess a very fine-grained texture - the crystals that make up aphanitic rocks are so small that individual minerals can only be distinguished with the aid of a microscope. Vesicular Texture - commonly seen in many aphanitic rocks are the voids left by gas bubbles that escape as lava solidifies - these spherical or elongated openings are called vesicles, and the rocks that contain them are said to have a vesicular texture - usually form in the upper zone of lava flow, where cooling occurs rapidly enough to "freeze" the lava, thereby preserving the openings produced by the expanding gas bubbles. Phaneritic (Coarse-Grained) Texture - when large masses of magma slowly solidify far below the surface, they form igneous rocks that exhibit a coarse-grained texture described as phaneritic - consist of a mass of intergrown crystals, which are roughly equal in size and large enough so that the individual minerals can be identified without the aid of a microscope. Porphyritic Texture - when magma containing some large crystals should change environments (for example, by erupting at the surface) the remaining liquid portion of the lava would cool relatively quickly - the resulting rock, which has larger crystals embedded in a matrix of smaller crystals, is said to have a porphyritic texture - the large crystals in such a rock are referred to as phenocrysts - the matric of smaller crystals is called groundmass - a rock with such a texture is termed a porphyry. Glassy Texture - during some volcanic eruptions, molten rock is ejected into the atmosphere, where it is quenched quickly - rapid cooling of this type may generate rocks having a glassy texture - glass results when unordered ions are "frozen" before they are able to unite into an orderly crystalline structure - one example would be obsidian. Pyroclastic (Fragmental) Texture - some igneous rocks are formed from the consolidation of individual rock fragments that are ejected during a very violent volcanic eruption - the ejected particles might be very fine ash, molten blobs, or large angular blocks torn from the walls of the vent during eruption - igneous rocks composed of these rock fragments are said to have a pyroclastic or fragmental texture. Pegmatitic Texture - under special conditions, exceptionally coarse-grained igneous rocks, called pegmatites, may form - these rocks, which are composed of interlocking crystals all larger than a centimeter in diameter, are said to have a pegmatitic texture - most pegmatites are found around the margins of large plutons as small masses or thin veins that commonly extend into the adjacent host rock. Sedimentary Rocks Sedimentary Rocks - Rocks formed at the earth’s surface by surficial processes - Include- biogenic, inorganic chemical, and clastic/detrital - comprise about 5% (by volume) of Earth’s outer 10 miles - amount is virtually insignificant, but is very important in figuring out the history of the Earth - most predominant on the Earth‘s surface - contain evidence of past environments - provide information about sediment transport - often contain fossils, etc. Sedimentary Rocks are important for economic considerations because they may contain coal, petroleum and natural gas, sources of iron, aluminum, and manganese, and uranium. Clastic sedimentary rocks - turning sediment into rock - many changes occur to sediment after it is deposited - diagenesis - chemical, physical, and biological changes that take place after sediments are deposited - occurs within the upper few kilometers of Earth’s crust, etc. Diagenesis - includes - recrystallization (development of more stable minerals from less stable ones) - lithification (sediments are transformed into solid rock by compaction and cementation and/or natural cements include calcite, silica, and iron oxide). Olivine is only stable at high degrees of heat. It becomes unstable at the Earth’s surface. Clay is the most predominant mineral on the Earth’s surface. They are chemically the most stable. Clay particles are very, very small. Every surface of every object in the universe has an electrical charge. Clay particles, though small, have a gigantic charge, causing them to stick together (lithification). Small particles - compaction. Sand grains are larger. Less charge. Must run water through it to glue the grains together (silicon and oxygen ions cement the grains together). Types of Sedimentary Rocks - Detrital Rocks - transported sediment as solid particles - Chemical rocks - sediment that was once in solution (precipitation of solutions) - Biogenic rocks - living organisms helped in the formation of the rock (like coal - made from detritus of dead trees; also limestone - made of calcite [calcium carbonate] - evolution and reproduction of shelled creatures - why calcite? - salt is too soluble and isn‘t very good protection/easy to crack open - quartz uses too much energy - calcite costs the least in energy and produces maximum protection) - corals make up the most important (limestones). Detrital sedimentary rocks - clay minerals, quartz, feldspars, micas - most important/chief constituents of detrital sedimentary rocks - smallest to largest - Clay, Silt, Sand, Granule, Pebble, Cobble, Boulder - Mud Rocks (Shale -made of clay, siltstone - silt, mudstone - silt and clay) - Sandstone is made of sand (duh) - conglomerate is gravel-sized. Shale - mud-sized particles in thin layers that are called laminea - most common sedimentary rock. Sandstone- sand-sized particles - forms in a variety of environments - predominant mineral is quartz. Conglomerate and breccia - both are composed of particles greater than 2mm in diameter- conglomerate consists largely of rounded pebbles and grains - formed in stream or river environments - lots of transportation of grains - cementation - breccia has angular grains - formed in a cave from collapsed cave roofs - water flows in - virtually no transportation of the grains, etc. Chemical Sedimentary Rocks - consist of precipitated material that was once in solution - precipitation of material occurs by inorganic processes and organic processes (biochemical origin) - precipitates from fluids - inorganic chemical sedimentary rocks - Common chemical sedimentary rocks - Limestone - most abundant chemical rock - composed chiefly of the mineral calcite - biogenic limestones and inorganic chemical limestones - seawater - Marine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms) - Inorganic limestones include travertine and oolitic limestone - lithographic limestone (for example) - biogenic limestones are the most common limestones - coquina limestones are from a particular seashell - fossiliferous limestone can have pretty much any fossils in it (dense, water cannot flow through it, etc.) - Common chemical sedimentary rocks - Dolostone - typically formed secondarily from limestone (made of dolomite - a carbonate, like calcite) - Chert - microcrystalline quartz - varieties include flint and jasper - one of the most important minerals in the universe - agate - Evaporates - evaporation triggers deposition of chemical precipitates - examples include rock salt and rock gypsum - Coal - different from other rocks because it is composed of organic material - stages in coal formation (in order) - plant material - peat - lignite (lithified peat/brown coal) - bituminous (black coal) - sedimentary rocks are classified according to the type of material - two major groups (detrital and chemical) - two major textures are used in the classification of sedimentary rocks - clastic (discrete fragments and particles - all detrital rocks have a clastic texture) - see Figure 7.18 - plant debris (coal - biogenic) - rock salt (inorganic chemical) - rock gypsum (inorganic chemical) - quartz (both biogenic and inorganic chemical - diatoms) - calcite (biogenic - chalk, fossiliferous limestone, coral limestone, coquina, travertine, crystalline limestone) - Travertine - water flows through limestone formations, calcite precipitates out of fresh water, etc. - Sedimentary Environments - geographic setting where sediment is accumulating - determines the nature of the sediments that accumulate (grain size, grain shape, etc.) - Types of sedimentary environments - continental - terrestrial processes predominant - dominated by stream erosion and deposition - glacial - wind (eolian) - Marine - marine processes predominant - shallow (to about 200 meters) - deep (seaward of continental shelves) - transitional - both marine and terrestrial processes predominant - subjective depending on detail - river environments are continental (sediments transported and deposited by flowing water) - two types of glacials (continental and alpine glaciers) - predominated by ice - wind (eolian) - sediment is moved by the wind - lakes - bases of mountains (beluvial deposits) - Marine processes - Shallow - continental shelf, continental slope, continental rise - Deep - Mid-ocean ridge, sea-mounts, hot-spot islands (shield volcanoes), abyssal plain - Great Barrier Reef is a limestone deposit - Transitional (shorelines) - tidal flats (marshes), lagoons, deltas (deposits at the mouth of a river), barrier islands (detached pieces of land on the continental shelf) - Sedimentary facies - different sediments often accumulate adjacent to one another at the same time - a package of sedimentary rocks specific to a sedimentary environment is called a sedimentary facies - each unit (facies) possesses a distinctive set of characteristics reflecting the conditions of a particular environment - merging of adjacent facies is a gradual transition - sea-level rises (sand on bottom, silt in middle, clay on top) - sea-level drops (clay on bottom, silt in middle, sand on top) - examples of sedimentary facies - sedimentary structures - provide information useful in the interpretation of Earth history - Types of sedimentary structures - strata or beds (most characteristic of sedimentary rocks) - bedding planes that separate strata - cross-bedding, etc. Things used in identifying rocks - Textures (grain size, grain shape, grain orientation), Composition (minerals found in rock), Structures (large-scale features found in a rock), etc. Sedimentary structures - provide information useful in the interpretation of Earth’s history - Types of sedimentary structures - Strata or Beds (most characteristic of sedimentary rocks) - Bedding Planes that separate strata - Cross-Bedding, etc. - Most fundamental is Bedding - sedimentary rocks are deposited in a flat, horizontal manner - natural breaks between strata in sedimentary rocks - all layers deposited at different times, but rocks deposited in layer all at once - Cross-Bedding - found only in sedimentary rocks - either water or wind flows and deposits sediment - can indicate tidal changes, etc. - ones deposited by wind are very thick - ones deposited by water are thin - Types of sedimentary structures - Graded Beds - Ripple Marks - Mud Cracks - Fossils - Graded Beds form through submarine landslides, like on the continental shelf’s edge down the continental slope and onto the continental rise - largest grains settle first and small grains settle last - gradation of grain size - Ripple Marks - created by water flow and tides - Mud Cracks - the sun dries wet mud that has been covered in very shallow water - Fossils - any evidence of former life - tell about environment the sediment was deposited in - trilobites (marine environments), etc. Metamorphism and Metamorphic Rocks Metamorphism and Metamorphic Rocks - sedimentary rocks form at the surface of the Earth - igneous rocks form in the mantle - the metamorphic rocks form in between them - Metamorphism - the solid-state (not molten, like igneous rocks) change of a rock due to a change in temperature, pressure, or chemically active fluids at conditions higher than those seen at or near the Earth’s surface (not like sedimentary rocks), producing a change in textures and mineralogy - Metamorphic rocks are produced from igneous rocks, sedimentary rocks, and other metamorphic rocks - change (usually increase) temperature and/or pressure - Metamorphism progresses incrementally from low-grade to high-grade - Types of Metamorphism - Contact (adjacent to igneous intrusions - high increase temperature, but not pressure - cooking the rock) - Dynamic (along subduction zones - high increase in pressure, but not a lot in temperature - isotherms - layers in subduction zone - isotherms get depressed) - Regional (convergent plate boundaries - mountains - high increase in both temperature and pressure) - Burial (in deep basins - like the Hudson Bay - rocks under sediment being added moderately increase in both temperature and pressure) - Hydrothermal (due to hot waters - hot water runs through rocks and boils them - increase in temperature - chemical properties) - Shock (meteorite strikes - high pressure increase very fast - no temperature change) - Agents of metamorphism - Heat - two sources of heat - Contact Metamorphism (heat from magma) - an increase in temperature with depth (geothermal gradient) - Pressure (static and differential stress) - increases with depth - confining pressure (or static pressure) applies forces equally in all directions - Rocks may also be subjected to differential stress which is unequal in different directions - Stress - Force per Surface Area (divided by surface area) - Pressure divided by Area (same thing) - Two kinds of Stress - Static and Dynamic (differential) - Static Stress - pressing around an object the same amount in all directions - does not change shape much - lithostatic stress (from rocks), hydrostatic stress (from water), etc, - Dynamic (differential) Stress - difference in amount of stress in each direction, changing the shape of the rock - increase in depth equals increase in stress (for Static Stress) - Dynamic (differential) Stress can occur at convergent plate boundaries - Static (hydrothermal, contact, burial metamorpism) - Dynamic (Regional, shock, dynamic metamorphism) - Chemically active fluids -mainly water, etc. Agents of Metamorphism - Chemically active fluids - Mainly water - enhances migration of ions - aids in recrystallization of existing minerals - causes metamorphism to occur - Sources of fluids - pore spaces of sedimentary rocks - fractures in igneous rocks - hydrated minerals such as clays and micas - increase of temperature and pressure causes water to emerge from rocks - The importance of parent rock - Most metamorphic rocks have the same overall chemical composition as the original parent rock - Mineral makeup determines, to a large extent, the degree to which each metamorphic agent will cause change - in other words, example, quartzite is made from quartz sandstone made of quartz (SiO2) - limestone is made at the beach - calcite and quartz mixes (quartz sand from sandstone and calcite from calcium-carbonate in sea-shells - makes Wollenstonite plus Carbon Dioxide) - SiO2 + CaCO3 à CaSiO3 + CO2 - what one starts with is what one has to end with - metamorphism of rocks with more than one mineral will cause more than one mineral to occur - metamorphism of rocks with only one mineral will only produce one mineral - Metamorphic textures - Texture - size, shape, and arrangement of mineral grains - foliation - massive (non-foliated) - cataclastic - Foliation is the most important (the parallel alignment of plating minerals) - Non-foliated (random orientation of minerals) - Cataclastic (crushed orientation) - Six types of metamorphism (Contact, Dynamic, Regional, Shock, Burial, and Hydrothermal) - Different kinds of Differential stress - pushed is COMPRESSION, pulled out is TENSIONAL, upper push, lower push, opposite directions is SHEER - Contact is static - Dynamic is differential (sheer) - Regional is differential (compression) - Shock is differential (compression) - Burial is static - Hydrothermal is static - Contact is massive, Dynamic is cataclastic, Regional is foliation, Shock is cataclastic (crushing rock really fast - think cataclysmic) - Burial is massive, and Hydrothermal is massive - See Figure 8.12 - mistakes in it, though - okay, so, anyways - five types of foliated - Slate, Phyllite, Schist, Gneiss, and Migmatite (partial melting of gneiss and schist) - Cataclastic (weakly foliated) - Mylonite and Metaconglomerate, also Fault Breccia - Non-foliated - Marble, Quartxite, Hornfels, Anthracite (metamorphosed coal) - Foliation is the result of directed stress - porphyroblastic texture (schist) - schist with large crystals in it - different minerals grow at different speeds - growth rate - zone of contact metamorphism is called a metamorphic aureole - Hydrothermal metamorphism - divergent plate boundaries - Regional metamorphism - convergent plate boundaries, etc. Regional Metamorphism - produces the greatest quantity of metamorphic rock - associated with mountain building - Metamorphic Zones - Barrow was the second son of a wealthy land-owner - joined the clergy - Scottish man - trained as a geologist - traveled from the south of Scotland to the north of Scotland - when in the South, found a sedimentary rock called a shale - upon walking North, he discovered a zone full of chlorite - then one full of biotite - then garnet then staurolite - then sillimanite - then granite at the Northernmost part of Scotland - increase of metamorphic grid (low grade to high grade) - specific first appearance of an index mineral at each place - a metamorphic mineral that forms at a specific set of temperature-pressure conditions - first appearance forms a line - this line is called an isograd - region between two adjacent isograds is called a zone - Appalachian Mountains and the Caladonian Mountains, as well as the Scottish Highlands, were once one mountain chain - same zones in same order - from Maine, chlorite, biotite, garnet, staurolite, silliminate, etc. - Most metamorphism occurs along convergent plate boundaries - Compressional stresses deform the edges of the plate - Formation of Earth’s major mountain belts including all those mountain changes, etc. Geologic Time - elative Age Dating, Absolute Age Dating, and The Geologic Time Scale - Relative Age Dating - not absolute, approximate - Absolute Age Dating - time-scale and zero-point (like a birthday in people) - age of events in a scale of years - datum points - going backwards in time - no date is exact - Relative Age Dating is not tied to a quantitative time scale (events, not rocks, are dated) - faulting, erosion, etc. are defined by Relative Age Dating - Law of Superposition - in an unreformed sequence of sedimentary rocks, the oldest rocks are deposited on the bottom and the youngest rocks are deposited on the top - Principal of Original Horizontality - Layers of sediment are generally deposited in a horizontal position - rock layers that are flat have not been disturbed - Principle of Corss-Cutting Relationships - Younger features cut across older features - Inclusions 0 An inclusion is a piece of rock that is enclosed within another rock 0 the rock containing the inclusion is younger - Unconformities - An unconformity is a buried erosional surface - three kinds - Angular Unconformity (deposition, folding and uplifting, erosion, and subsidence and renewed deposition - Disconformity (sedimentary rocks above and below, deposition of sediments, erosion, then more deposition - no deformation - sediment is parallel) - Nonconformity - igneous and metamorphic rocks below and sedimentary rocks above - Fossils - Evidence of Past Lief - Fossil - traces or remains of prehistoric life now preserved in rock - Fossil Range is the period between the first and last occurrences of a species - Absolute Age Dating - uses isotopes, etc. Okay, well, that's it for now. smile I really hope I do well on my geology exam! sweatdrop |
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