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ENVS 2250 (9)
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Unit 3 Notes.docx

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Department
Environmental Sciences
Course Code
ENVS 2250
Professor
Steve Sadura

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Natural Disasters Unit 3 February 2, 2013 Volcanoes Introduction  Estimated 1500 active volcanoes around the globe (550 on land and the rest underwater)  Lava – when molten rock spills out onto the earths surface  Magma – when molten rock is underground  Properties such as; viscosity (how easily it flows) and gas contents are examples of what dictates a peaceful or explosive eruption o Differences are closely related to the position of volcanoes relative to the plate boundaries they sit on  It eruptions are large the effects can be felt on a regional/global scale – i.e. Tsunami  The most important global risk is when volcanic debris reaches the stratosphere o Causing climate change Composition of Magma  Shallow magma storage is typically at 3-4 km depth but in areas of thick crust o Sometimes as deep as 30 km  Magma is a mixture of liquid, solid crystals and dissolved gasses  Bowen determined that certain minerals form at specific temperatures during the cooling of basaltic lava Olivine crystalizes at the highest temp and Quartz crystalizes at the lowest temp During cooling and crystallization becomes depleted in elements like magnesium and iron and becomes enriched in elements like silicon Pressure is highest deep in the earth, so as magma moves upward through the crust, gas is released  Gas can also be freed through the process of crystallization  The longer the process of crystallization continues, the more pressure builds up  Once the volume of gas bubbles in the magma gets high enough, the magma chamber becomes unstable, pressure exceeds the strength of the rocks holding it in, and the volcano erupts Plate Settings Oceanic Crust  Most volcanoes sit on oceanic rust and erupt most of the lava at the present time  Huge amounts of basaltic lava are spewed out at ocean spreading centers o Ex. Mid-Atlantic Ridge o Producing about 20km of new oceanic crust each year  Almost all of this volcanism takes place underwater Natural Disasters Unit 3 February 2, 2013  Some volcanoes on oceanic crust are not associated with the plate boundary and form over hot spots  Receives magma almost directly from the mantle without many changes along the way (because OC is thin) o Means that it is still hot o Has lots of iron and magnesium o Low silica content and high temp LOW VISCOSITY (flows easily)  Oceanic volcanoes don’t have steep slopes o Ex. Pouring water on a plate vs. liquid honey o Honey has higher viscosity forms a mound on the plate o Water has lower viscosity forms an almost flat profile o SHIELD VOLCANOS – Sometimes high but are very wide Type of eruption  After eruption, fluid basaltic lava flows back into the magma chamber easily  Volcanic vent doesn’t become solidly blocked with hardened lava  Doesn’t need a lot of pressure to break obstruction  FREQUENT but NOT EXPLOSIVE  2 ways an oceanic crust volcano can have a violent eruption 1. Phreatic eruption – When magma groundwater to produce steam under high pressure 2. When the process of crystallization progresses for long enough that silica- rich magma is produced Continental Crust  Cause most of the damage and loss of life  Most activity is at or near plate boundaries  Volcanism is mostly caused when plates collide in subduction zones, rather than at spreading centers  Margins of the Pacific Ocean ”The Ring of Fire”  In this case, magma is produced as the oceanic plate and its sediments are carried down to melt and be recycled  CC is much thicker than OC, so magma has a long way to rise before it is erupted  Therefore, undergoes lots of changes on the way to the surface  Most of this magma crystalizes at great depths, removing magnesium and iron  Enriched in silica  HIGH VISCOSITY  Produces steeper volcanoes because lava cannot run freely down the flanks – STRATOVOLCANOES Natural Disasters Unit 3 February 2, 2013 Type of Eruption  After eruption, the gooey, viscous, silca-rich magma cannot flow easily back down into the magma chamber  It solidifies in the vent forming a strong blockage  Takes a long time for enough pressure to build up to overcome the strength of the blockage o Can go several hundred years between eruptions  NOT FREQUENT but VIOLENT  Most occur at boundaries but some a related to hot spots in the mantle o Resurgent Caldera Eruption – the most dangerous geological phenomena Volcanic Hazards Volcanic Explosivity Index  A system for ranking volcanic eruptions  Logarithmic – each successive number on the scale describes an eruption 10 times more powerful than the one before Glowing Cloud Eruptions “Nuee ardente”  Ash flows/pyroclastic flows are avalanches of extremely hot, fast moving mixture of volcanic ash and superheated gases  So hot that they can glow  Heavier than air so moves quickly down the slope  Because of turbulence it acts like a heavy gas rather than a liquid or solid  frictional forces do not slow it down  Welded Tuff
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