Ch 5 Volcanoes 5 1 Intro to Volcanoes most volcanoes exist on plate boundaries that s b c spreading sinking lithospheric plates produce magma within the earth volcano opening is called a vent 2 3ish of volcanoes are on ring of fire which is essentially the border of the pacific ocean volcanoes are also found on mid ocean ridges How magma forms rock melts most magma comes from the asthenosphere the asthenosphere is not melted but it is close to it which makes it prone to melting ways in which silicate rocks can melt decompression melting when pressure is released without a change in temperature the happens at divergent plate boundaries continental rifts hot spots at divergent plates continental rifts lithosphere is stretched so mantle upwells pressure is lowered which causes melting addition of volatiles violates break chemical bonds within rock which lowers the melting point causing rocks that are close to melting temp to melt volatiles are chemicals that evaporate easily volatiles are released which oceanic plates subduct addition of heat magma rises which heats overlying rocks often widespread degree depends on how fast magma rises basaltic magma low in silica andesitic and rhyoltic magma high in silica mafic rocks low in silica felsic rocks high in silica viscosity and volatile content are important in determining what kind of volcano forms Magma properties magma is made of melted silicate rock and dissolved gases Viscosity Volatile Content and Eruptive Behavior high concentration of dissolved volatiles will make a greater eruption viscosity is the resistance flow great amounts of silica make it more difficult for magma to flow temperature effects viscosity as lava cools viscosity grows and lava moves slower volatile increases with silica pyroclastic debris and tephra volcanic material ejected from a volcano a lot of it can form a pyroclastic deposit that can be cemented to form a pyroclastic rock Volcano Types Shield Volcanoes pipes common in hawaii iceland and Indian Ocean nonexplosive eruptions b c of low viscosity and low volatile content have a gentle slope they are very tall but their base is much longer than their height lava often travels under ground in lava tubes which turn into the equivalent of natural higher volatile content than shield cause a lot of destruction Composite Volcano Stratovolcano example mount st helen cone shape Volcanic Domes Cinder Cones scoria cones highly explosive eruptions small volcanoes built up by accumulation of tephra near volcanic vent tephra is called lava rock and is used in landscaping crater deppresion commonly found at the top of a volcano calderas giant depressions formed during explosive ejection of magma and subsequent volcanic vent any opening in which lava and debris errupts vents can be circular or be Volcanic Feature Craters Calderas and Vents collapse of the upper cone cracks called fissures Hot Springs and Geysers Caldera Eruptions Volcanic Orgins fissures can be big enough to be called flood basalts hot spring ground water comes into contact with hot water and springs up geyser ground water boils in chamber to produce periodic releases of steam water atleast ten in past million years often called super volcanoes explosion may be more than 100km3 the lesser magnitude volcanic activity can linger on for a million years Mid Oceanic Ridges volcanos at mid ocean ridge produce balsatic magma from asthenosphere magma mixes with little other than balsaltic ocean crust on land sheild volcanoes are formed Subduction Zones composite volcanoes andesitic in composition high silica and high volatile content Hot Spots Beneath Ocean shield volcanoes are formed from hot spots produced the chain of volcanic islands near including hawaii Hot Spots beneath Continents extremely explosive rhyolite magma 5 2 Geographic Regions at Risk from Volcanoes ring of fire has tons of volcanoes mid oceanic ridge highest risk place in north america is mountains of pacific coast and yellowstone large caldera explosion at yellowstone could be felt very widely across the US 5 3 Effects of Volcanoes lated areas 50 60 volcanos erupt per year not alot of people live very close to volcanoes but some do espeically in densely popu primary effects direct results of an eruption ex lava flow ash fall secondary effects caused by primary effects ex landslides floods fires tsunamis Lava Flow results when magma reaches surface and overflows the central crater or erupts from a volcanic vent along the flank of the volcano lava is named after which rocks they form basaltic andesitic and rhyolitic basaltic lava moves fast and is most common also called pahoehoe has a ropey surface when it moves faster its called aa and has a blocky surface when it hardens if lava flow hits water it produces black beaches whem tephra is blown out of a volcanic vent volcanic ash eruption fine grained rock and volcanic glass shatters and gas is blown into air the come back down and settle to produce and ash fall lateral blasts the volcanic ash and gas get blown to the side really fast avalanches of hot pyroclastic material coming out of the vent aka ash flows and nuee when it hardens Pyroclastic Activity Pyroclastic Flow ardentes Ash Fall covers ground with a layer of ash can destroy plants contaminate water and destroy buildings by piling up on the roof they burn everything that they cross can make it hard to breath destroy engines Poisonous Gases poisonous gases can be emitted during eruption especially CO2 book gives example of Lake Nyos in West Africa that was discussed in class sometime sulfur dioxide comes out of volcano which can produce acid rain volcanoes can produce a volcanic fog vog over areas of people Debris Flow Mudflow and Other Mass Movements lahar indonesian term for debris and mudflow when debris and tephra are mixed with water and go down hill it produces a lahar debris flows are coarse than mudflows Debris Flows hot volcanic material causes snow to melt and produces a flood that takes a lot of debris travels far and wide with it Mud Flow Landslide secondary effect landslides on sea floor can generate tsunamis massive mud flows sometimes occur people still live in areas covered by old flows no guarantee it wont happen again Mount St Helen From Lateral Blast to Lava Flows March 1980 explosion created by boiling ground water as it came in contact with hot rock bulge formed and the failed which created an avalanche massive lateral blast which moved more than 480 km vertical