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From: Marilou Hawkins
Date: 10/8/02
Time: 3:51:37 PM
Remote Name: 207.65.229.58
Here is my final draft on the lithosphere, Elizabeth did the final draft for our sphere team. Elizabeth you did a good job. Way to go girl! Since this is my week to post for us I will do my best to have the first draft ready by Friday night. Please everyone feel free to critique this draft so that what I turn in is everybody's best work. The earth is divided into three different chemical layers: the core, composed mostly of iron and nickel, remains very hot after 4,5 billion years of cooling; the mantle (the middle layer), is rich in the elements iron, magnesium, silicon and oxygen; and the crust which is rich in the elements oxygen and silicon with lesser amounts of aluminum, iron, magnesium, calcium, potassium, and sodium. There are two types of crust: dense oceanic crust made of basalt, and continental crust made of lower density rock such as andesite and granite. The outermost layers of Earth can be divided by their physical properties into the lithosphere and asthenosphere. The lithosphere is the rigid outer layer made of crust and uppermost mantle. It is the ‘plate’ in plate tectonic theory. The mantle supports seven large plates and over 20 smaller plates of the lithospheric crust. These plates, because they sit atop the more fluid mantle (the asthenosphere), are moving at a rate of a few inches per year. Sometimes these plates collide or are forced apart. When this occurs, the crust is weakened and when magma escapes from these open areas, new material is added to existing plates. The asthenosphere is the part of the mantle that flows, that is, exhibits “plastic behavior” much like toothpaste in a tube. The flow of the asthenosphere is part of mantle convection, which plays a major part in moving the lithospheric plates. (http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/pla…) The lithosphere is the rigid inorganic portion of the Earth, which contains the crust, and 80 to 100 km of the mantle. Thickness of the lithosphere averages about 100km, however, the lithosphere can be just a few miles under the oceans and up to 300 km beneath the continents. (http://visearth.ucsd.edu/) The lithosphere includes the uppermost mantle, the rocks of seabed crust, the rocks of continental crust and their attendant soils. The rocks of the lithosphere are of three types: igneous (basalt, granite,), metamorphic (marble, slate), and sedimentary (limestone, sandstone). Soils are designated by particle size (gravel, sand, clay and silt), and by detailed descriptions of their color and the conditions that form them (ultisols, entisols, spodisols, etc). (Goode’s Atlas 19th Ed. P. 20) The lithosphere includes all the geologic forms on the Earth’s surface: sea floors, trenches, islands, plains, plateaus and mesas, valleys and basins, hills, mountains, and volcanoes, and all their parts. A volcano is a vent or opening in the earth’s crust through which magma, molten rock, crystals, and dissolved gases, from the upper mantle and lower crust erupts as lava; it is also the landform that is produce by the erupted material. As magma is formed, it begins to rise toward lower pressure regions. Before erupting, it tends to accumulate in magma reservoirs, or underground storage regions. Materials such as lava, tephra, or rock fragments, and gases are released from volcanic eruptions. Each eruption adds layers to the volcano. (http://encarta.msn.com/encnet/refpages/refarticle.aspc?refid=7615) There are three major types of volcanoes: Cinder cone, shield, and the composite or stratovolcano. The simplest type of volcano is the cinder cone. It is built from particles and blobs of congealed lava ejected from a single vent. As gas-charged lava is blown violently into the air, it breaks up into small fragments that solidify and fall as cinders or scoria around the vent to form a circular or oval cone. Most cinder cones have a bowl-shaped crater at the summit and rarely rise more than one thousand feet above their surroundings. Scoria forms when blobs of gas charged lava thrown into the air cool in flight and fall as dark volcanic rock containing cavities created by trapped gas bubbles. (http://vulcan.wr.usgs.gov/Glossary/CinderCone/description_cinder…) Shield volcanoes are built almost entirely of fluid lava flows. Flow after flow pours out in all directions from a central summit vent, or group of vents, building a broad gently sloping cone with a shape similar to a warrior’s shield. Lavas also commonly erupt from vents along fractures that develop on the flanks of the cone. Some of the largest volcanoes in the world are shield volcanoes (Kilauea, Mauna Loa). (http:Vulcan.wr.usgs.gov/Glossary?ShieldVolcano/description_shield …) Some of the earth’s grandest mountains are composite volcanoes, also called stratovolcanoes. Typically steep-sided cones, they are built of alternating layers of lava flows, volcanic ash, cinders, blocks, and bombs and may rise 8,000 feet above their bases. Most composite volcanoes have a crater at the summit, which contains a central vent or group of vents. Lavas either flow through breaks in the crater wall or issue from fissures on the flanks of the cone. Lava, solidified within the fissures, forms dikes that act as ribs which greatly strengthen the cone. The essential feature of a composite volcano is a conduit system through which magma from a reservoir deep in the earth’s crust rises to the surface. The volcano is built up the accumulation of material erupted through the conduit and increases in size as lava, cinders, ash, etc. are added to its slopes. Composite volcanoes tend to erupt explosively and pose a great danger to nearby life and property. Composite cones like Mount St. Helens and Mount Pinatubo are frequently found along subduction zones where one plate is being pushed under the edge of another plate. (http:Vulcan.wr.usgs.gov/Glossary?StratoVolcano/description_com …) Mount Pinatubo is a volcano located on the Philippine Island of Luzon. It erupted violently from June 12-15. Although it lay dormant for more than 500 hundred years, geologists recognized some signs a few months before the eruptions. The full blow of Mount Pinatubo was quite unexpected. Over 50,000 people were forced to evacuate and two major military bases were closed. Several hundred lives were lost as a result of the Mount Pinatubo eruption. (http://earthobservatory.nasa.gov/Study/AstronautPinatubo) E: The eruption began, April 2, 1991, with the ejection of small amounts of ash, which was followed three days later by high frequency volcanic quakes, indicating fracturing of rocks and movement of fault structures from pressure exerted by intruding magma and escaping steam. June 7, 1991, incidents of HFVQ’s increased from 26-178 daily to 1500-2000 daily, and low frequency quakes began, indicating that magma was nearing the surface. Harmonic tremors, indicating intensified seismic activity, began then and continued through the next day when scientists recorded an explosion type earthquake around 3:35 PM. June 9 (day 1) the eruption began with eight hours of ash-laden steam clouds being ejected, followed by pyroclastic flows that flowed into the Maraunot and Moraza rivers; intermittent occurrences of small pyroclastic flows persisted all morning. On day 4, three major explosions, the most powerful of which, ejected a cloud of ash that reached 20 kilometers above the vent, followed intense seismic activity. Ash pumice, and other large volcanic fragments were also ejected. Shortly before midnight a second series of explosions hurled clouds of ash and pyroclastic materials 25 kilometers into the air. Day 5, eruptions triggered heavy ash falls that blanketed most of Zambales, Tarlac, and Pampanga. Day 6, two major eruptions sent a column of debris to a maximum height of 30 kilometers, and pyroclastic flows 15 kilometers from the source. Day 7, eruptions produced a 40-kilometer high column and pyroclastic flows that advanced 16 kilometers from the center of activity. From 11:17 AM to 1:42PM eruptions came in such close succession that they appeared to be one continuous activity. E>L: The three main destructive agents were: ash fall, pyroclastic flow, and lahars (mudflows). Initially the ash tends to smother everything, but in the long term in acts a rich fertilizer for many crops. Pyroclastic flows (superheated mixtures of gases, volcanic fragments, ash pumice, crystals and glass shards), with their intense, penetrating heat, and enormous speeds, destroyed everything in their path. Lava floods created long-lasting damage down slope. Mount Pinatubo emitted around 8-10 km of material onto the surrounding slopes. The total elevation of the top of the mountain was lowered by around 150m. Forests and rice fields were destroyed by the eruption. Hundreds of cubic meters of loose sand and gravel fell on the upper slopes of the mountain. Lava floods created long-lasting damage down slope. The event also created post-eruption mudflows, or lahars. Lahars reshape the landscape, forcing the formation of new river channels, damming new lakes, and polluting waters far from the center of eruption and for months and years after the event. (http://earthobservatory.nasa.gov/Study/AstronautPinatubo)
L>E: It is the activities of the lithosphere that produced Pinatubo. Because the Philippine plate is being pushed under the Asian plate the Philippine Islands and their stratovolcanoes are created by the subduction zone created this tectonic plate movement. The sinking of the oceanic plate into the athenosphere generates magma under the continental crust. As the magma builds it rises through the weakest portions of the crust to form volcanoes. Because the magma in subduction zones is very thick its and gases are released explosively, most of the volcanoes in these subductive zones are stratovolcanoes, produced by repeated, explosive eruptions.
Conclusions: Volcanism is part of earth’s recycling process. Crustal plates are being constantly created and destroyed. Where one plate slips under another subduction zones are created, and volcanoes like Pinatubo develop over these subduction zones. While the eruption of volcanoes can be enormously destructive, they are the most dramatic part of an ongoing land building process. Much of the ejecta of volcanoes are highly valued as a producer of rich agricultural soils. This is exhibited globally by the numbers of farms on the sides of volcanoes, both dormant and active, in spite of the inherent danger. Pinatubo’s eruption shows how dangerous volcanoes are, and the persistence of farmers who return to their farms as soon as they can show how valuable volcanoes are to the soils of the lithosphere.