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Earth Rocks Midterm Notes

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Department
Earth Sciences
Course
Earth Sciences 1022A/B
Professor
Dr.S.R.Hicock
Semester
Winter

Description
Igneous Rocks: crystalize from magama that forms at high temperatures and pressure deep in the earth Magmum: rises through the crust and either reaches the surface by volcanoes (extrusive) or cools below the surface (intrusive) ▯ how it evolves... ▯ ▯ each mineral has its own cooling temperature. As certain minerals cool ▯ ▯ they affect the other minerals in the magma chamber. They can also go ▯ ▯ through magma composition where a rock can fall into the mix and melt ▯ ▯ into another rock, composing an intermediate of them both Texture: overall appearance of the rock (size, shape, arrangement of crystals...) ▯ Different kinds of textures: ▯ Aphanitic: fine, rapidly cools at the ground surface ▯ Phaneritic: coarse, slowly cools below the surface ▯ Porphyritic: large crystals surrounded by smaller ones (takes hundreds of of ▯ thousands of years to create) ▯ Glassy: quenched so fast that crystals had no time to form ▯ Pyroclastic: when magma was ejected hard into the air and fell as particles to ▯ the ground Naming Igneous rocks... composition of the rocks textures and minerals Felsic rocks (70% silica) - granite and rhyolite - are common in mountainous areas; granite is widely used for building stone, monuments, and headstones Intermediate rocks (60%) - andesite and diorite - found near subduction zones Mafic rocks (50%) - basalt is the most common rock - found mainly in upper oceanic crust and volcanic islands; found mainly in lower oceanic crust Ultramafic (45%) - peridotite - main rock in the upper mantle Intrusive Igneous Bodies: most magma crystallizes at depth to form plutons: dykes - cut rock layers where magma interjected fractures in the rock sills - layers are parallel where magma squeezed between layers laccoliths - concordant bodies (like sillis) that bulge in middle due to intrusion of viscous magma batholith - more than100 km exposed in cores of mountain belts; original magma chamber Volcanic eruptions: when magma reaches Earth’s surface, often from gas buildup ▯ Materials Extruded: hot, runny mafic lava tends to flow quietly downhill while ▯ ▯ ▯ ▯ cooler, viscous felsic magma tends to erupt violently, with Two lava types: hot, runny pahoehoe with a surface resembling coils of rope and cooler and blocky Pillow Lavas: form underwater as lava repeatedly breaks through quenched tube ends Pyroclastic Material: ranges from dust and ash to streamlined bombs and large blocks - nuée ardentes (pyroclastic flows) of devastating hot glowing clouds of ash and gas - lahars of pyroclastics mixed with rain, ice, and snow Volcanic Structures: commonly cone-shaped mountains with a small crater at the top or a larger caldera (>1 km across) after magma chamber drains, rim and sides cave in types of volcanoes: ▯ - large shield (broad dome of successive lava flows) ▯ - small cinder cone (mainly pyroclastic material) ▯ - composite cone (classic cone shape; mixture of flows and pyroclastic material) fissure eruptions and lava plateaus form where lava pours onto land from fissures in the crust - builds up thick piles of lava flows (plateaus) on continents Plate Tectonics and Volcanic Activity: The earths plates move in such a way that causes volcanic activity to occur. - generates basaltic magma that can differentiate into other types - seamounts where basaltic lava flows out from the sea floor - above mantle plumes located under the middle of lithospheric plates, resulting in intraplate volcanoes (e.g. Hawaiian Islands) Weathering: After rocks are formed, they are weathered when exposed at the surface; this prepares material for transport by erosional processes in the rock cycle - two main types of weathering include mechanical (disintegration) and chemical (decomposition) that act together and enhance the effectiveness of each other Mechanical: reduction of rocks to smaller pieces includes: ▯ frost wedging by ice forming in cracks in rocks, breaking them apart; unloading due to expansion on removal of overlying material; ▯ biologic activity - roots open fractures Chemical: Includes: ▯ dissolution- water in the weathering zone is weak carbonic acid that reacts with most minerals, turns them into substances stable at surface conditions ▯ oxidation - dark silicates react with oxygen to form rusty Fe, Mg oxides and clay minerals; ▯ hydrolysis - orthoclase in granite switches K+ for tiny H+ ions, turns to clay Rates of Weathering: depends on how fractured the rock is, the type of minerals, climate and rock characteristics. (e.g. granite is made of relatively stable minerals and does not weather easily whereas marble (soluble) and dark silicate minerals weather easily) dark minerals form under high pressure temperature conditions stable light minerals form under low pressure and temperature climate: weathering is faster, more intense in warm, wet climates; slow in polar areas Soil: results from weathering plus biologic activity ▯ soil type depends on five factors: ▯ ▯ parent material: type of bedrock, sediment determines rate of soil ▯ ▯ formation, fertility ▯ ▯ time: more time, greater intensity and depth of soil development on a ▯ ▯ parent material ▯ ▯ climate: most important; soil forms fast & deep in tropics, slow & shallow ▯ ▯ near poles ▯ ▯ plants, animals: decompose to humus, release nutrients, hold soil ▯ ▯ moisture, aerate ▯ ▯ topography: steep slopes erode easily - do not hold moisture or vegetation l l e▯ ▯ w ▯ ▯ ▯ ▯ ▯ ▯ ▯ ▯ ▯ Weathering continued... soil profile: develops downward from the ground surface as layered horizons: O decomposed humus (like compost) A mixture of mineral matter and humus E eluviated (downward-transported) fines and leaching of ions in solution B accumulation of oxides and clays (in wet climates) C weathered parent material (regolith) parent material Soil Erosion: serious problem worldwide; we need soils for food production - starts with raindrops displacing soil particles then sheet erosion washes them into rills, gullies, and eventually to streams that carry the soil away; accelerated by deforestation by farming, logging, construction etc – plants anchor, protect topsoil - soil erosion also clogs reservoirs, streams, reducing holding capacity, flood control ------------------------ Sedimentary Rocks: product of mechanical and chemical weathering after they’ve been settled. As debris from lakes, rivers seas etc accumulate the materials at the bottom are compacted and after long periods of time become cemented together by mineral matter. Diagenesis & Lithification: after deposition, sediment undergoes physical, chemical, and biologic changes as it gets turned into rock (hardened, consolidated); ▯ lithification includes... ▯ ▯ compaction - pore space is reduced and particles are pressed closer r e h t e g o ▯ ▯ t ▯ ▯ cementation – precipitate from solution onto grains, fill pores, cement ▯ ▯ grains together Detrital Sedimentary Rocks: - formed from rock fragments and minerals deposited by rivers, glaciers, wind, gravity - mainly made of clay minerals and resistant quartz Shale: is the most abundant sedimentary rock - made of silt and clay deposited in quiet water in lakes, river floodplains, lagoons - grains packed together so water and oil cannot flow through - good cap rock for oil and gas – also used for pottery, bricks, tile Sandstone: sand sized grains dominate. ▯ Sorting: if the grains in the rock are all about the same size then they are well ▯ sorted ▯ arkose: if lots of feldspar Conglomerate: solidified gravel from landslide, stream bed, or wave action - has rounded stones Breccia: has angular particles not moved far from source rocks Chemical Sedimentary Rocks: formed by weathered material in solution (liquid) precipitating from water or as biochemical rocks made of dead marine organisms ▯ Precipitation occurs in 2 ways: ▯ ▯ inorganic: (not life) evaporation and chemical activity ▯
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