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Volcanoes- Volcano Typeso Stratovolcanoes Large, cone-shaped volcano Composed of alternating layers of lava and tephrao Shield Volcanoes Broad, slightly domed-shaped (like an inverted shield) Made by lateral flow of low-viscosity basaltic lava Have a low slope and cover large geographic areas Mauna Loa on Hawaiio Cinder Cone – conical piles of tephra- Eruptions to Remembero Mt. St. Helens – May 18, 1980 – 8:32 p.m. The blast devastated 600 km squared and killed 61 people Lahars plugged the Toutle River – closed the Columbia Ash fell in North Dakota – highways and rail lines stopped Destroyed timber valued at several 100 million dollars One side erupted, rather than erupting straight up- Volcanic Hazardso Earthquakeso Tsunamis – water explosions create giant waves- Mitigating Hazardso Danger assessment maps Delineate danger areas- Pyroclastic flows, lahars- Eruptive Styleo Effusive eruptions – produce lava flows Can produce huge lava fountains Commonly basaltic, lead to formation of shield volcanoes Pele’s hair and tears, bombso Basaltic Lava Flows Pahoehoe vs A’a- Pahoehoe – a Hawaiian word describing basalt with a glassy, ropy textureo Forms when extremely hot basalt forms a skin- A’a – is a Hawaiian word describing basalt that solidifies with a jagged, sharp,angular textureo Forms when hot flowing basalt cools and thickens Underwater, basalt cools instantly; it cannot flow It cools to form a rounded blob called a pillow Common on the mid-ocean ridge- Continental Hot-Spot Volcanoeso Continental hot-spot – cutes a continental plate Yellowstone – erupted and created a caldera- Flood Basalt Eruptionso Voluminous lava eruptions above a plumeSedimentary Rocks- Sedimentary Rockso Sediments are the building blocks of sedimentary rockso Sediments are diverse, as are the rocks made from themo 4 classes: Clastic – made from weathered rock fragments (clasts) Biochemical – cemented shells of organisms Organic – the carbon-rich remains of plants Chemical – minerals that crystallize directly from watero Clastic Sedimentary Rocks Clastic sedimentary rocks reflect several processes- Weather – generation of detritus via rock disintegration- Erosion – removal of sediment grains from rock- Transportation – dispersal by wind, water, and ice- Deposition – settling out of the transporting fluid- Lithification – transformation of loose sediment into solid rocko Burial, compaction, cementation Classified on the basis of texture and composition- Clast (grain) size – the average diameter of clastso Range from very coarse to very fineo Boulder, cobble, pebble, sand, silt, clayo With increasing transport, average grain size decreases- Clast composition – the mineral makeup of sedimentso May be individual minerals or rock fragmentso Mineral identities provide clues about… The source of the sediment The environment of deposition- Angularity and sphericity – indicate degree of transporto Grain roundness and sphericity increases with transport Well-rounded – long transport distances Angular – negligible transport- Sorting – the uniformity of grain sizeo Well-sorted – uniform grain sizeso Poorly sorted – wide variety of grain sizeso Sorting becomes better with distance from the source- Character of cemento Cement – minerals that fill sediment poreso Common cements; Silica Calcite Hematite Clay minerals- Coarse clastics – composed of gravel-sized clastso Breccia – composed of angular fragments Angularity indicates a lack of transport processing Deposited relatively close to sourceo Conglomerate – comprised of rounded gravel Indicates water transport- Clasts bang together forcefully in flowing water- Collisions round angular corners and edges of clasts Conglomerates are deposited at a distance from the sourceo Sandstone – clastic rock made of sand-sized particles Forms in many depositional settings Quartz is, by far, the dominant mineral in sandstones Sandstone varieties- Arkose – contains abundant feldspar- Quartz sandstone – almost pure quartz- Fine clastics – composed of silt and clayo Silt-sized sediments are lithified to form siltstoneo Clay-sized particles form shaleo Chemical Sedimentary Rocks Comprised of minerals precipitated from water solution Evaporates – created from evaporated seawater- Evaporation triggers deposition of chemical precipitates- Examples include halite (rock salt) and gypsum (dry wall) Travertine – calcium carbonate (CaCO3) precipitated from groundwater where it reaches the surface- Dissolved calcium reacts with bicarbonate- CO2 expelled into the air causes CaCO3 to precipitate Dolostone – limestone altered by Mg-rich fluids- Looks like limestone, except…o It has a sugary texture and a pervasive porosityo It weathers to a buff, tan color Replacement chert – nonbiogenic in origin- Many varietieso Flint – black or gray from organic mattero Jasper – red or yellow from Fe-oxideso Petrified wood – wood grain preserved by silicao Agate – concentrically layered ringso Biochemical and Organic Rocks These are sediments derived from living organisms- Biochemical – hard mineral skeletonso Biochemical limestone – CaCO3 skeletal (shell) remains Warm, tropical, shallow, clear, O2-rich, marine water Diverse organisms (plankton, corals, clams, snails, etc.) Many textural varietieso Chert – rock made of cryptocrystalline quartz Formed from opalline silica (SiO2) skeletons- Diatoms- Radiolarians Opalline silica added to bottom sediments dissolves Silica pore fluids solidify to form chert nodules or beds- Organic – cells of plants, algae, bacteria, and planktono Made from organic carbon Coal – altered remains of fossil vegetation Oil shale – shale with heat altered organic mattero Flint continued… Crytocrystalline: hardly classifies as a crystal structure Composition similar to the mineral quartzo Sedimentary Structures Features imparted to sediments at or near deposition- Layering- Surface features on layers- Arrangement of grains Help decipher conditions at or near time of deposition Sedimentary rocks are usually layered – “stratified”- Arranged in planar, close-to-horizontal beds- Bedding is often laterally continuous for long distances- Beds are often similar in composition, color, and texture- Bedding

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OSU EARTHSC 1100 - Volcanoes

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