Volcanoes Volcano Types o Stratovolcanoes Large cone shaped volcano Composed of alternating layers of lava and tephra o 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 Hawaii o Cinder Cone conical piles of tephra Eruptions to Remember o 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 Hazards o Earthquakes o Tsunamis water explosions create giant waves Mitigating Hazards o Danger assessment maps Delineate danger areas Pyroclastic flows lahars Eruptive Style o Effusive eruptions produce lava flows Can produce huge lava fountains Commonly basaltic lead to formation of shield volcanoes Pele s hair and tears bombs o Basaltic Lava Flows Pahoehoe vs A a Pahoehoe a Hawaiian word describing basalt with a glassy ropy texture o Forms when extremely hot basalt forms a skin A a is a Hawaiian word describing basalt that solidifies with a jagged sharp angular texture o 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 Volcanoes o Continental hot spot cutes a continental plate Yellowstone erupted and created a caldera Flood Basalt Eruptions o Voluminous lava eruptions above a plume Sedimentary Rocks Sedimentary Rocks o Sediments are the building blocks of sedimentary rocks o Sediments are diverse as are the rocks made from them o 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 water o 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 rock o Burial compaction cementation Classified on the basis of texture and composition Clast grain size the average diameter of clasts o Range from very coarse to very fine o Boulder cobble pebble sand silt clay o With increasing transport average grain size decreases Clast composition the mineral makeup of sediments o May be individual minerals or rock fragments o Mineral identities provide clues about The source of the sediment The environment of deposition Angularity and sphericity indicate degree of transport o Grain roundness and sphericity increases with transport Well rounded long transport distances Angular negligible transport Sorting the uniformity of grain size o Well sorted uniform grain sizes o Poorly sorted wide variety of grain sizes o Sorting becomes better with distance from the source Character of cement o Cement minerals that fill sediment pores o Common cements Silica Calcite Hematite Clay minerals Coarse clastics composed of gravel sized clasts o Breccia composed of angular fragments Angularity indicates a lack of transport processing Deposited relatively close to source o 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 source o 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 clay o Silt sized sediments are lithified to form siltstone o Clay sized particles form shale o 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 porosity o It weathers to a buff tan color Replacement chert nonbiogenic in origin Many varieties o Flint black or gray from organic matter o Jasper red or yellow from Fe oxides o Petrified wood wood grain preserved by silica o Agate concentrically layered rings o Biochemical and Organic Rocks These are sediments derived from living organisms Biochemical hard mineral skeletons o Biochemical limestone CaCO3 skeletal shell remains Warm tropical shallow clear O2 rich marine water Diverse organisms plankton corals clams snails etc Many textural varieties o 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 plankton o Made from organic carbon Coal altered remains of fossil vegetation Oil shale shale with heat altered organic matter o Flint continued Crytocrystalline hardly classifies as a crystal structure Composition similar to the mineral quartz o 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 reflects changing conditions during deposition These can be changes in o Energy conditions and hence grain size o Disturbance by organisms Bedding may also reflect non deposition or erosion uncomformities A series of beds are referred to as a strata Formation strata recognized on a regional scale Water flowing over loose sediment creates bedforms Bedforms are linked to flow velocity and sediment size o Asymmetric ripples unidirectional flow o Symmetric ripples wave