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McMaster University
Earth Sciences
Sergei Basik

Pyroclastic Flow and Volcanic SurgeA pyroclastic flow is a mixture of hot volcanic ash and steam that pours downslope because it is too dense to rise It develops when rapidly erupting steam carries a large volume of ash in a column that rises high above the volcano When the rush of steam slows part of the column collapses and the cloud of ash pours down the flank of the volcano The main flow hugs the ground its less dense part becomes loose ash and becomes a part of turbulent air Fastmoving flows tend to hug valley bottoms but their high velocity can carry them over intervening hills and ridgesThey can race down the flank of a volcano at speeds from 50 to more than 200 kilometers per hour incinerating everything flammable in their path including forests and people In some cases a highspeed ashrich shock wave called a surge may race across the ground ahead of a pyroclastic flowSinkholesThere are three types1Dissolution Acid ground water seeps through the soil that is thin and permeable and it dissolves underlying limestone along fractures The upper parts of fractures widen to form lumpy karst surface The overlying soil percolates to create a surface depression Where the ground water is high or fractures become clogged with sediment the depression may fill to form a pond2Cover Subsidence When lots of permeable sediments that sit on top of limestone slowly fill expanding fractures in limestone can create a sink hole3Cover Collapse When sediments above the limestone have a lot of clay in it it becomes less cohesive and permeable so it doesnt percolate into underlying limestone This allows the soil cavity to grow larger leading to sudden collapse of the thinning roof Tsunami Earthquake Generated90Caused by sudden rise or fall of the sea floor lot of water displacedMost commonly on reverse or thrust faults on a subduction zone oceanic plate under lithospheric plateTwo plates can be stuck for many years at the subduction zoneContinental plate moves downward as oceanic plate is moving under it This causes the continental plate to bulgeThis bulge finally ruptures in an earthquake releasing energy that displaces a huge volume of water towards and away from the continent The height of tsunami increases as it reaches the shoreEl NinoNormally oceanic circulation flows westwards in pacific oceans due to trade winds High P cell over South America and low P cell over Indonesia Dry air descends over SA and moist air rises over Indonesia causes strong winds that push warm water west towards Indonesia Dry conditions along the west coast of SAEvery 6 years during winter the Pacific Ocean circulation reverses in a pattern called El NinoHigh P develops over Indonesia and Low P over SA This weakens the trade winds allowing warm water to flow east wards This causes extreme weather around the world Lots of rain on the west coast of SA and drought conditions in Indonesia and Australia
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