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GEOS 218 (2)
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Geological Disasters and Society Study Guide Exam 2 - 4.0ed the exam

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Department
Geosciences
Course
GEOS 218
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All Professors
Semester
Fall

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Geos 218 Study Guide Exam 2 Tsunamis -form when two plates subduct under another (earthquake), and the stress bounces vertically and then ripples out into huge waves -waves will slow when they approach shore, but traveling about 500 mph in open ocean -energy is distributed from surface to sea floor -reaches greatest heights when it reaches a narrow space or harbor; 1 meter wave in deep water becomes 8 meters in shallow -series of waves separated by 10 to 60 min. ***tsunami waves travel faster than wind waves ***LARGEST WAVE IS UNPREDICTABLE ***most destructive powers from momentum and long wavelength ***What event can produce the largest tsunami? -asteroids ***What is the most common way that tsunamis are made? -earthquakes ***In which of the following scenarios would tsunami tend to have the greatest destructive power? -A section of a coastline where there is a harbor and the bottom of the ocean bottom dips steeply. ***The Pacific Ocean has an average depth of about 5,500 meters, which yields a theoretical deep- ocean tsunami velocity of about _________ meters per second. -230 -velocity=(g times depth) ½ g: acceleration due to gravity Earthquake in January 1700 -the Juan de Fuca Plate is subducting under the North American Plate -we know there was an earthquake because: 1. trees in water, not trees that grow in water. Must have slid down from hills 2. massive tsunami recorded in Japan that same time 3. tree rings stopped growing that were in the ocean Tsunami Warning Systems -sensors at the bottom of the seafloor, away from wind waves -none in Indian Ocean before 2004, that's why there was a failure to report the Sumatra tsunami -few in Indian Ocean because there isn't enough money -more sensors in places with money *more people died in the 2004 Sumatra tsunami that all combined in the 20 century How to sense that a tsunami is coming: 1. feel an earthquake 2. see the sea level retreat from beach 3. hear a wave coming (usually very loud) 4. go to higher ground 5. go upstairs in a well built area ***Head toward deeper water if you were on a boat and a tsunami was coming ***Magnitude of EQ must be at least 7.5 for a “real” tsunami to happen Magmatism and Plate Tectonics Vesuvius, Pompeii: 79AD eruption -where the Eurasian Plate and theAfrican Plate collide -stratovolcano: clouds of hot gas (850 C), ash and pomice enveloped the city pyroclastic: raining fire Is Vesuvius safe today? -no, volcanoes can live on to be millions of years old in geological time -most volcanism at spreading centers (least dangerous), then subduction zones (most dangerous), then hot spots (Hawaii), none at transform faults -90% at plate boundary -80% at spreading centers -10% at subduction zones -10 % at hot spots Melting of rocks 3 ways to melt rocks: 1. decrease in pressure (spreading centers) MELTS THEASTHENOSPHERE -when plates spread apart, the magma rises up because there is less pressure ***most common way to melt rock 2. addition of water (subduction zones) MELTS CONTINENTAL CRUST -water is released when a plate subducts under the other, water is released and melts the rocks, therefore creating magma ***most dangerous 3. addition of heat (hot spots) MANTLE -hot pockets of magma underneath Earth's crust -least common way to melt rock ***elements compose minerals, minerals compose rocks -all rocks formed by cooling magma are igneous rocks ***oxygen and silicon most abundant minerals to make rocks ***Eruption of magma at mid-ocean ridge systems occurs mostly by... -decompressive melting of the asthenosphere ***Rhyolitic magmas rich in K, Al, Na, and SiO2 typically are produced by the melting of ____and are associated with ____eruptions. -continental crust, highly explosive ***Basaltic magmas rich in Fe, Mg, Ca and poor in SiO2 are generated mostly by the melting of the ______and are erupted mostly as ________. -asthenosphere, lava flow ***Gases that are not emitted commonly during eruptions are? -nitrogen plutonic rocks: magma cools slowly and solidifies UNDER the surface (big crystals) felsic rocks: silica rich/iron poor (granite, rhyolite) -very light color -mostly at subduction zones -continental rocks ***has the lowest temperature -***HIGHER WATER CONTENT ***MORE MINERALS HIGHER VISCOSITY volcanic rocks: magma erupts and cools quicklyAT the surface (small crystals) ***fine grained mafic rocks: silica poor/iron rich (gabbro, basalt) -dark color -more at spreading centers and hot spots -creates more of a “peaceful” eruption -oceanic rocks ***has the highest temperature -***LOWER WATER CONTENT Magma types from least vicious to most: -basalt, andesite, rhyolite ***The Earth's outer core is thought to be a highly viscous zone of liquid iron. However, liquid iron never rises from this zone to erupt as molten iron on the surface of the Earth. What is a plausible explanation for this observation? -the density of of liquid iron is higher than the overlying rocks ***Why are rhyolitic magmas generally more explosive than basaltic magmas? -The higher silica content and lower temperatures of rhyolitic magma make it difficult for the gas to escape. The 3 Vs of Volcanism 1. viscosity: resistance to flow -low viscosity=flows more easily ex: water, basalt -”safer” eruptions -high viscosity=explosive eruptions ex: honey, rhyolite *****VISCOSITY DEPENDS ON TEMPERATUREAND SILICA CONTENT -more mineral crystals, the higher the viscosity ***Magma viscosity increases with ________ -decrease in temperature, increase in silica, and increase in crystal content 2. volatiles: gases -all magma has volatiles -when pressure on magma goes down or when the temperature goes up, gases come out from the magma as bubbles solubility: amount of gas in magma ***ESCAPING GASES DRIVE VOLCANIC ERUPTIONS *hard to escape from high viscosity magma *easy for gas to escape from low viscosity magma magma: molten rock below surface lava: molten rock at the surface -water determines whether the eruption is peaceful or not nonexplosive: Iceland and Hawaii panoenoe: smooth ropy lava aa: rocky blocky rock from more viscous lava strombolian: somewhat explosive volcanian and plinian: very explosive ***undegassed magma is dangerous -andesitic=intermediate -can be dangerous ***as H2O content and viscosity goes up=pyroclastic debris ***as H2O and viscosity goes down=lava flow 3. volume: how much something comes out of the volcano Mt. St. Helens=1 km3 Pinatubo=6 km3 Tambora=80 km3 Yellowstone=1000 km3 ***The main process that causes volcanoes to explode violently is _______________. -a decrease in pressure triggering rapid expansion of the gases in a magma Shield Volcanoes: ***low viscosity, low volatiles, small volume ex: Hawaii volcanoes -very flat, with broad gentle slopes -relatively “safer”, VERY low viscosity -usually on
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