Geography 1111 Lecture 24 Notes
Geography 1111 Lecture 24 Notes GEOG 1111
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This 5 page Class Notes was uploaded by Bridget Notetaker on Monday October 10, 2016. The Class Notes belongs to GEOG 1111 at University of Georgia taught by Hopkins in Fall 2015. Since its upload, it has received 11 views. For similar materials see Intro to Physical Geography in Geography at University of Georgia.
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Date Created: 10/10/16
Geography 1111 Lecture 24 Notes Volcanism: the process by which magma and gases are transferred from the Earth’s interior to near or on the surface o 2 basic types: Extrusive and Intrusive Extrusive volcanic activity: o Volcano is a landform with a vent, or fissure (crack) on the surface which is the end of a conduit originating from below the crust in the Asthenosphere, or mantle o If the exit point is a vent, then usually a mountain landform is the result, but if the exit point is a fissure, then mountains usually do not form, only features of low relief o Magma often collects in a magma chamber below the vent or fissure before being expelled o Crater: the surface depression at the summit of the volcano, often the vent or opening is in the crater A volcano's level of activity may be categorized as either: o Active wherein the volcano is currently erupting, has erupted during recorded history, or still has a high potential to erupt o Dormant refers to a volcano that is in repose (rest), but still has the potential to erupt in the future o Extinct refers to a volcano that has little or no potential to erupt again o Most convergent and divergent plate boundaries have high volcanic activity Hot spots are also areas of high volcanic and earthquake activity o Lava is the term used for the molten rock issued from a volcano that is now on the surface, as opposed to the term magma used for the molten rock that is still underground Two general types of material (lava): o Mafic or basaltic lava: high in Mg and Fe and composed of <50% silica which makes it less viscous, meaning it flows readily (more easily) This leads to less gas being trapped within the lava mass, leading to a less explosive reaction o Felsic: lava is richer in silica (>50%) which makes it more viscous, meaning it flows slower This leads to more gas being trapped within the lava mass and more explosive eruptions Pyroclastics or tephra are solid fragments of magma expelled explosively from a volcano o Quite common with felsic magma and explosive eruptions o Bombs (> 1 m), pumice, scoria, cinders (1-5 in), lapilli, ash, and dust (Based on size; bombs largest, dust/ash smallest) Two General Types of Eruptions: o Effusive: eruptions are relatively gentle, non-violent events because they are associated with mafic magma which flows more readily and gases can more readily escape, leading to less explosive eruptions If the eruption is through a vent, it may form a shield volcano Ex: islands of Hawaii, Galapagos islands and Iceland If the eruption is through a fissure, then it may form a plateau or flood basalt Ex: Columbia Plateau in the state of Washington (on land) and from mid-oceanic ridges under water to form sea floor o Explosive: relatively violent events because they are associated with felsic magma which flows more slowing often solidifying in the conduit and thus trapping gasses and increasing internal pressure Explosive eruptions are most commonly found along subduction zones and often have a high content of pyroclastics These eruptions are more commonly through a vent, and form a Ex: Mt. St. Helens, Mt. Rainier, Mt. Pinatubo, Mt. Vesuvius Other events that may accompany an explosive eruption: o Lahar is a special type of mudflow and the result of a volcanic eruption During the eruption of a volcano that is snowcapped, the heat of the magma melts the snow which mixes with any ash, soil, etc. on the mountain This mix of ash, mud, and water then comes down the mountain side and may travel for dozens of miles Ex: One formed by the November 13, 1985 eruption of Nevado del Ruiz in Colombia killed over 20,000 of the ~29,000 towns people Armero, by covering the town in up to 100' of mud o Nuée ardentes or Pyroclastic flow is a cloud of hot volcanic gas and ash that moves down the flanks of a volcano Ex: A flow created during the 1902 eruption of Mont Pelee on the island Martinique, West Indies annihilated the town of St. Pierre, killing approximately 30,000 people o Phreatic eruptions: violent, explosive eruptions made more severe by water entering the magma, increasing pressure from steam and increasing the explosive power of the erruption Ex: Krakatau (1883), Tambora (1815), Santorini (1645 BC) Other volcanic features: o Cinder cone: a volcano that consists of cinder-sized tephra, with little or no lava and are usually less than 200m high Ex: Sunset Crater (AZ) o Volcanic dome is a mound of lava which may form inside a crater and be a cap over the vent It may also be the only surface feature from an eruption o Caldera is the term given a large, more or less circular depression or basin associated with a volcanic vent, with a diameter often many times greater than the original vent(s) This may be the result of collapse or subsidence, or may also result from an explosive event Ex: Crater Lake (Oregon, USA), Yellowstone National Park (Wyoming, USA) Mantle Plumes and Hot Spots: o Mantle plume is a buoyant mass of hot mantle material (magma) that rises to the base of the lithosphere It commonly produces volcanic activity and structural deformation on the surface Current theory suggests these plumes may originate at the core-mantle boundary as mantle material becomes hotter than surrounding material and thus rises o Hot spot is the surface expression of a mantle plume that has either formed or found an opening in the crust, creating a conduit to the surface As the tectonic plate (surface) moves over this 'plume' it may form a series of features on the surface, sometimes including shield volcanoes Ex: Hawaii is an oceanic hot spot, while the Yellowstone National Park area is a continental hot spot; Iceland is also the result of a mantle plume associated with a mid-oceanic ridge (divergent plate boundary) Hydrothermal (Geothermal) Features: o These are found in areas where water below the surface has been heated to temperatures higher than commonly found on the surface o This may be the result of water flowing deep below the surface and heated as a result of the geothermal gradient (temperature increases with depth at approximately 3°/100m) o Or it may be the result of contact with a shallow magma chamber, sometimes associated with mantle plumes and hot spots o Types: hot springs, geysers, fumaroles, etc. Intrusive volcanic activity is when magma cools to form igneous rock below the Earth’s surface and thus is only exposed by erosion or uplift o These are known as Plutonic Landscapes Pluton: the exposed portion of a body or mass of intrusive igneous rock o Two Main Types: Discordant plutons: are those which disrupt or change, often melting, the existing geologic structure into which it intrudes; it often creates the space it then occupies Batholith: is a large discordant mass of intrusive igneous rock, measuring over an area of at least 100km2 o Ex: Stone Mt. (GA) is the exposed portion (pluton) of a larger mass; as are the Black Hills of South Dakota and the Sierra Nevada Mts. along the California/Nevada border One batholith in Idaho is nearly 41,000 km Dike is a relatively narrow mass of cooled magma that cuts across preexisting strata or other structural features of the surrounding rock in a vertical orientation o May be a few feet in diameter to several kms wide, long, and deep o The largest known dike is in Zimbabwe and is 600 km long and averages 10 km wide Volcanic neck: is the solidified magma that originally filled the vent and neck of an ancient volcano and that has been exposed by erosion o EX: Shiprock (New Mexico), Devil’s Tower (Wyoming) Concordant plutons: are intrusive magma feature that didn’t destroy (melt) the existing geologic structure; it may move the existing rock or simply conformed to it by filling existing spaces Laccolith: is a concordant pluton which has arched (pushed) up the strata into which it was injected, and so forms a lens-shaped body with horizontal floor but that is smaller than a batholith Sill: is a relatively narrow mass of cooled magma that was injected between preexisting strata of the surrounding rock in a horizontal orientation o It can range from a few cm to hundreds of meters thick and may extend for several kms
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