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volcanose learning goals.docx

14 Pages

Earth and Ocean Sciences
Course Code
EOSC 114

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Volcanoes learning goals Volcanoes For Magma to rise through the crust it must be less dense than the crust, Runny enough to flow and hot enough to stay liquid, all this depends on its composition and temperature. At depth magma has less than 10% gas dissolved in it (H20, CO2, SO2, Cl) this means decreased viscosity and decreased density. The gas becomes less soluble as pressure decreases What is magma?: Any subsurface body of melt / crystals / bubbles / rock fragments resulting from melting of rock (gases dissolved and exsolved). What is lava?: Masses, sheets, or other bodies of magma at the Earth's surface How is magma created? By the melting of pre-existing rock in the Earth's interior (mantle & crust). Explain magma density: - Units: g cm -3 or km m -3. - Becomes less dense with rising temperature. - Becomes more dense with cooling and pressure. Explain magma viscosity: - The resistance to flow. - Controls the flow, deformation, and eruption of magma. - The composition of the magma affects the viscosity (especially H2O). - There is a higher viscosity (gooey) when it is cooler, and more dense. - There is a lower viscosity (runny) when it is hotter and less dense. What happens when magma rises? -They crystallize and exsolve volatiles (gases). volcanic rocks and explain the differences between the magmas they came from Basalt – Andesite – Dactite – Rhyolite Mafic – Intermediate – Felsic Dark light Hot Cool (1200-1400) (600-1000) Low Viscosity High viscosity Non Explosive(lava) Explosive( pyroclastic) What are felsic rocks?: - Rocks that come from felsic magma. - Contain high silica content. - Rich in low density minerals. - Low density. - E.g: Granite, Rhyolite. What are mafic rocks?: - Rocks that come from mafic magma. - Contain low silica content. - Rich in denser materials. - Dark color. - Denser. - E.g: Gabrro, Basalt. What are intrusive rocks?: Magma that freezes in the crust and is never erupted. - Slow cooling, large crystals. What are extrusive rocks? - Magma erupted at the surface. - Quick cooling, tiny crystals or glass. - E.g: Basalt, Rhyolite. magma erupt explosively (as pyroclastic material) and some magmas erupt effusively (as lava) - Viscosity and Gas content are very important in determining the eruption style (explosive or effusive) As magma rises, Pressure decreases. Bubbles begin to form. But viscosity fights bubble growth. Pressure rises inside the bubbles until the strength of the liquid magma is overcome and fragmentation occurs and pyroclastic material (tephra) is produced. - Mafic has low gas content + fluid = quiet eruptions, gas escapes, pressure released= safe - Felsic has high gas content +gooey = explosive eruptions, Gas kept under increasing pressure = Dangerous. When is magma erupted explosively (pyroclastic)? - When there is a larger silica content (65-75%). - When it is cool (600-1000 C). - When there is higher viscosity. - When it is felsic. When is magma erupted non-explosively/effusively (lava)? - When there is a smaller silica content (25-55%). - When it is hot (1200-1400 C). - When there is lower viscosity. - When it is mafic. What is pahoehoe lava? - Basaltic lavas with low viscosity and high eruptive temperatures that move a few kilometres per hour which harden with a smooth commonly ropy surface texture. What is a’a lava? - Cooler, more viscous basaltic lava flows that move only a few meters per day and harden with a rough, blocky surface texture of volcanic eruptions and how they are related to magma properties Hawaiian: Low viscosity basaltic magma, lava flows and or fire fountaining, non explosive Strombolian: Basaltic/andesitic magma, bombs, lavas – mildy explosive. Vulcanian: Viscous andesitic/rhyolitic magma, sustained explosions of ash, very explosive. Pelean: Dome collapse, generation of block and ash flows, violently explosive Plinian: Andesitic/rhyolitic ash, Sustained column of ash, pyroclastic flows – violently explosive. Phreatomagmatic: Contact between magma and water – violently explosive the world's active volcanoes Volcano location is controlled by: Plate tectonic boundaries or Hot spots these plate boundaries. Divergent: On continental crust: Stratovolcanoes, cindercones ,between oceanic plates: mid ocean ridges Suduction zones: beneath continental crust: Stratovolcanoes, Cinder Cones, calderas beneath oceanic crust, Stratavolcanoes, Cinder Cones, Calderas Hot Spots: Under continental crust: Calderas, stratovolcanoes, cinder cones, Under oceanic crust: shield volcanoes, cinder cones. What are the three types of plate boundaries? - Divergent boundaries. - Convergent boundaries. - Transform boundaries. Explain divergent boundaries. - Occur where two plates move away from each other and new lithosphere is produced. - Occurs in mid-ocean ridges. Explain convergent boundaries. - Occur where two plates collide head on. - A higher density oceanic plate is drawn down beneath a lower density continental plate through a process called subduction. - Convergent boundaries are subduction zones. Explain transform boundaries. - Occur where two tectonic plates slide horizontally past each other. - Most transform faults are located beneath oceans but some occur on continents. What types of volcanoes occur in divergent boundaries? - Shield volcanoes. What types of volcanoes occur in convergent boundaries? - Stratovolcanoes (composite volcanoes). lcanoes(that(occur(at(hot(spots. What is a hot spot? - Mafic magma originating from a plume of hot mantle rock. Shield volcanoes, they are stationary occurs here. - Located on the Cascadia subduction zone. - Stratovolcanoes occur at subduction zones. - The dominant volcanic rock scribe(the(morphology, dominant rock type and typical Eruption style of the different types of volcanoes Cinder cones: pyroclastic ejecta, Mafic, small volcanoes that never grow and usually erupt for years then never again. FREQUENT Cinder cone: – small mafic eruptions, affects small area – usually erupts once for a few years then stops – you can watch from a safe distance – Example: Paricutin, Mexico Shield Volcano: Lava erupts from fissure, runs down gentle slopes, cooling, erupt often, mafic lava flows, flows pahoehoe followed by aa, generally not very explosive. FREQUENT/PERMANET • Shield volcano: – broad edifice made of mafic lava flows – predictable – you can get very close – Example: Hawai Stratovalcano: Interbedded lava flows, pyroclastic flows, lahars, usually intermediate or felsic, frequently explosive often viscous magma, may erupt many times and stay active for 100,000 years. Mt st, Helens. • Strato-volcano: FREQUENT – large steep edifice, many unpredictable explosive eruptions – erupt intermediate and felsic lava and ash – erupt every 10 – 1000 years – Example: Mt. Bake Calderas: created from large, explosive eruptions of felsic pyroclastic material. The caldera is created when the roof of the magma chamber collapses. RARE, MOST DEVASTING • Supervolcano: – felsic, extremely powerful explosive eruption – probably won’t erupt in your lifetime, – eruption would be a global disaster, – Example: Yellowstone Describe cinder cones. - They are mafic. - The dominant rock type is basalt. - Usually erupt for a few years and then never again. - Frequent eruption. Describe shield volcanoes. - They are mafic. - The dominant rock type is basalt. - Lava erupts from fissure, runs down gentle slopes, and then cools. - Has a pahoehoe lava flow followed by an aa lava flow. - Generally not very explosive. - Frequent eruption. Describe stratovolcanoes. - They are felsic or intermediate. - They are frequently explosive, and it is often viscous magma. - May erupt many times and stay active for 100,000 years. - Interbedded lava flows, pyroclastic flows, and lahars. - Frequent eruption. Describe calderas. - They are felsic. - Created from large explosive eruptions of felsic pyroclastic material. - The caldera is formed when the roof of the magma chamber collapses. - Rare eruption but devastating. size of a volcanic eruption is estimated - The Volcanic Explosivity Index (VEI) - Takes into account the volume of material erupted, the height of the eruption cloud, the duration of the main eruptive phase, and other parameters to assign a number on a linear scale from 0 to 9. va flows, fire fountains, lava bombs and volcanic ash are and how they form Lava flows: Basalt lava flow in a channel, due to its low silica content it has high temperature and a low viscosity, usually flows fairly slowly. Pahoehoe is faster than aa lava and limited volume and often channelized flows faster than aa lava. AA lava below cooled surface the lava is fairly coherent and much hotter and has larger volume flows and thicker. Fire fountaining: sometimes basaltic lava can contain significan amount of gas, then small explosive eruptions form fire fountains, As partially liquid drops fall back to the ground they may coalesce to form a lava flow. Pyroclstic Falls ( ash Fall): During explosive volcanic eruptions ash falls downwind of the volcano, in the case of very large eruptions the ash may be deposited over a vast area. L
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