ALL Geology notes for Exam 2

ALL Geology notes for Exam 2 - Geol 105 Exam 2 How melting...

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Geol 105 Exam 2 How melting occurs When hot mantle rises, its temperature may hit the melting point at shallow depths Dry pressure-release melting How rock melts If water is added to the mantle, its melting temperature will change Wet or flux melting (occurs in subduction zones) Volcanoes Worldwide Most volcanic activity erupts basalt and occurs on mid-ocean ridges Explosive volcanoes are largely in subduction zones Volcanoes in subduction zones form volcanic arcs Volcanic Explosivity Index (VEI) Ranges from 0-8 0: non-explosive, effusive -mainly lava flows 8: most explosive, highest eruption cloud -The most explosive volcanoes have magma with >65% silica -These are the most dangerous volcanoes Effusive-Hawaiian type eruption Not explosive, VEI=0 Has lava flows Lava is basaltic, very hot and fluid Fire fountain At start of eruption The little gas in the magma escapes easily Lava will spray into the air A’a flow : cooled crust of the flow forms a sharp, cinder-like surface Pahoehoe flow : smooth, ropy surface Hawaiian eruptions build shield volcanoes -Convex, like a warrior’s shield -Gentle slopes -Built from many fluid lava flows Explosive-Strombolian VEI of 1-3 Basaltic mama with slightly more silica and gas Short blasts lasting a few seconds Fragments erupted 100 to 1000 meters upwards Erupted lava fragments are scoria Centimeters in size, irregular, full of bubbles or vesicles
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Geol 105 Exam 2 Strombolian eruptions build cinder cones Cone a few hundred meter wide, 100s to 1000 m high Built from scoria and lava flows from a single vent Explosive- Vulcanian VEI = 3-5 Intermediate magma High gas contents Eruption cloud: 3-25 km high Can erupt large blocks (“bombs”) Explosive- Plinian VEI>5 High silica magma (>65%) High water and CO2 contents Eruption cloud >25 km high Large amounts of ash Explosive-Vulcanian and Plinian Vulcanian and plinian eruptions can erupt thick intermediate lavas… Or a violent eruption may produce a magma froth that cools to a glass foam (pumice) May also erupt ash (shards of glass and rock) Ash and pumice is mixed with hot gas in eruption clouds and pryoclastic flows Stratovolcanoes Multiple vulcanian and plinian eruptions will build stratovolcanoes Consist of alternating ash and lava layers Domes High silica magma may erupt as very viscous lava Builds up near vent s to form lava domes Mt. St. Helens lava domes in 1980 crater—location of current eruptions Calderas Very large eruptions may cause volcano to collapse Creates a caldera Most calderas form in subduction zone volcanoes. Example: Crater Lake, OR Hotspots under continents also produce high-silica magma Huge caldera forming eruptions may occur Example: Yellowstone Phreatomagmatic Eruptions Magma encounters ground or surface water Violent steam explosions Any type of magma
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Geol 105 Exam 2 Produces “tuff rings” Hydrothermal—fumaroles Steam vents All types of volcanoes Vent sulfur dioxide gas that will deposit sulfur at vent Basaltic Lava Flows Not dangerous to people unless directly in its path
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ALL Geology notes for Exam 2 - Geol 105 Exam 2 How melting...

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