Chapter Five: Volcanoes
Chapter Five: Volcanoes GLY-125-001
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This 6 page Class Notes was uploaded by Lauren Sadowski on Sunday October 11, 2015. The Class Notes belongs to GLY-125-001 at University of North Carolina - Wilmington taught by Nooner in Summer 2015. Since its upload, it has received 9 views.
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Date Created: 10/11/15
GLY125 Natural Hazards Chapter 5 Volcanoes 51 INTRODUCTION TO VOLCANISM Volcano activity is related to plate tectonics 23 of all the active volcanoes on earth are found around the ring of re because sinking plates at boundaries are mixing with magma Magma is the molted rock under the Earth but once it reaches the surface it is called lava Volcanoes form from different tectonic settings which makes every volcano different It is important to understand how the formation of magma and the plate tectonic settings affect volcanoes Knowing this information can understand the different types shapes and behavior of volcanoes HOW AND WHERE MAGMA FORMS I know this is going to be hard to forget since we were told this since elementary school but plate tectonics do not move around on a sea of magma Most magma comes from the asthenosphere Remember from previous chapters the asthenosphere is the weak layer in the mantle This ayer ows because it is close to the melting point This means that this layer is easier to melt than the layer above it the lithosphere There are three principle processes that create magma 1 Decompression melting Occurs when pressure exerted on hot rocks within the asthenosphere is decreased Decompression melting occurs and magma will begin to form even though the temperature stays the same You can nd this process predominately where Earth39s lithosphere is being stretched From chapter two we learned that the ocean ridges are constantly being stretched divergent bounda es The pressure of the planet is what keeps the magma in the center of the Earth however when the magma gets to close the surface the pressure is no longer as strong as it use to be and the magma pushes through the surface Also occurs with hotspots which are created by smaller decompression melting 2 Addition of volatiles lowers the melting temperature of rocks by breaking the chemical bonds within the minerals Volatiles are chemical compounds water carbon dioxide sulfur dioxide that evaporate easily and enter the atmosphere as gas Water and other volatiles enter as a uid to rocks that are close to the melting point and then the rock will begin to melt Magma formation from addition of volatiles are found at subduction zones which is what created the ring of re The Paci c Nazca Cocos Philippine and Juan de Fuca plates are sinking below tangent plates Volatiles that are trapped in the sediment on the sea oor are released as the plate is forced downward into the hotter mantle The volatiles play an important role in the behavior of a volcano 3 Addition of heat to rocks will caused them to melt if the temperatures rise above the melting temperature depending on their depth melts rocks as the magma moves through the mantle which creates more magma MAGMA PROPERTIES Magma is composed of the melted silicate rocks and other gases There are three types of magma that are differentiated by the amount of silica found in the magma gt Basaltic magma Most common type of magma Have relatively low silica contents 4555 perent Referred to as ma c in composition gt Rhyolitic magma Contains more than 65 percent of silica Referred to as felsic composition gt Andesitic magma ls intermediate in composition Contains 5565 percent silica Magma is less dense than rock which is why it oats to the surface and sometimes creates magma chambers that lay near the Earth39s surface Magma evolution is the processes of chemical changes magma goes through while rising to the surface VISCOSITY Greater amounts of silica make it more dif cult for lava to ow For those who are no familiar with viscosity it is the stickiness of a substance For an example honey has a higher viscosity that water Rhyolitic lava ows have high viscosity meaning it ows much slower It is normally restricted to the vent region and form steep domes Basaltic lava ows fast thin meaning it has low viscosity These differences affect the shape of the volcano The viscosity measures the severity of the eruption Obviously the fast moving lava is more dangerous than the slow moving lava Pyroclastic debris volcanic materials that are ejected from a volcano ash What to know for 51 1 What is magma and where does it come from 2 Describe the physical and chemical changes that cause rocks to melt 3 What is the relationship between plate tectonics and the location of volcanoes 4 What Is viscosity and what determines magma viscosity 5 Explain the relationship between magma composition viscosity and gas content 52 VOLCANO TYPES FORMATION AND ERUPTION BEHAVIOR Volcanoes are landforms that vary in size shape composition and eruptions Volcano Composition Volatile Shape Eruption Volcano Type Content Type Examples Stratovolca Andesite High Cone Combinatio Mount no or shaped with n of lava Shasta composite steep sides ows and Califronia cone built of lava pyroclastic Mount St ows and activity Helens pyroclastic Washington depos s Lava dome Andesite to Low to Dome Mostly Mount rhyolite moderate shaped effusive Lessen steep sided with lava39s California pilling up Mt Unzen near the dome Japan vent but can be explosive Shield Basalt Low Gentle arch Far Mauna Loa Volcano or shield traveling Hawaii shape with lava ows Kilauea shallow Hawaii slopes built up of many lava ows Cinder Cone Basalt Low to Cone Mostly SP Crater moderate shaped with tephra Arizona steep sides ejection Paricutin and summit lava ows Mexico crater Eldfell Iceland Continental Rhyolite High Broad uplift Tephra Yellowstone Calderas with large summit depression Craters Depression formed by explosion or collapse of volcano top 0 A few kilometes a mile in diameter Calderas 0 very lare crater formed from violent collapse of a cone 0 May be 20 or more kilometer in diameter and contain vents and hot sp ngs Vents Any opening for lava and debris 0 May be circular or thin ssures 0 Can produce ood basalts Hot springs Hot rocks heat groundwater The pressure pushes the water to the surface Geysers c When water is boiled underground and the steam explodes through the surface Caldera Eruptions Very rare extremely violent eruptions Produce a large amount of ash Referred to as the Supervolcano The last caldera eruption was at Yellowstone National park Volcanic Origins Mid ocean Ridges and Continental Rifts Basaltic magma directly form asthenosphere 34 of all lava extruded on earth If on land than shield volcanoes are formed Exampe Iceland at the Mid Atlantic Ridge Subduction zones composite comes are formed here most common along Paci c Rim Andesite magma with intermediate silica Over 80 percent of eruptions Exampe Cascade Mountains 0 Hot spots beneath the ocean basaltic magma forms shield volcanoes hot spots remain stationary Exampe Hawaii 53 GEOGRAPHIC REGIONS AT RISK FROM VOLCANOES Volcanoes are like earthquakes and are greatly affected by plate tectonics and most occur around the Ring of Fire But volcanoes can occur in other places due to other factors Hot spots Yellowstone park Or the volcanoes are on a midocean ridge So who is at risk Isolated areas of the US Southwest are at risk for volcano action because of Yellowstone National Park whereas the Eastern 23 of the US and Canada are safe from Yellowstone REMEMBER JUST BECAUSE THE EASTERN 23 OF US AND CANADA WILL NOT BE IMMEDIATELY AFFECTED BY THE VOLCANO BUT THE WORLD WILL BE AFFECTED BY THE ERUPTION 53 EFFECTS OF VOLCANOES Approximately 5060 volcanic eruptions each year around the world Most eruptions are harmless and cost little to no economic damage however when eruption take place near heavy populated areas the damage can be signi cant According to the book approximately 500 million people in the world live near volcanoes and with the population of humans only rising it is expected that more people will live near volcanoes Places such as Mexico Japan Philippines and Indonesia have the highest risk of Volcano eruptions Lava ow is one of the most familiar products of volcanic activity Pyroclastic activity the explosive volcanism in which magma and rocks that compose the volcano are blasted from a volcano vent and into the atmosphere Tephra pyroclastic debris Pyroclastic deposit when deposited hot tephra can be fused and form pyroclastic rock Ash fall occurs downwind of the volcano as the lighter ner particles are carried to the bottom of the volcano and rests on the Earth39s surface Ash falls cause lots of issues VVVV Crops are destroyed Water is contaminated by the particles and sediment from the volcano Buildings are damaged Health risks from drinkingeating contaminated food or breathing in gases ash sediment and particles from the volcanoes Pyroclastic ows debris from the volcano that ow 1 During the eruption ash is pushed into the air even though it has a higher density because of heat But will later settle after cooling Can still be hot when it reaches the ground and can start res destroy crops and buildings Lateral blasts form when the eruptions destroys a part of the volcano and magma starts leaking from the side of the volcano These are the fastest moving ows that destroy everything down the mountain Lava domes collapse most common volcanic process that causes pyroclastic ows Poisonous Gases The gases found in the debris of volcano are H20 water vapor C02 carbon dioxide H25 hydrogen sul de 502 sulfur dioxide H20 makes up 90 of the gases emitted from volcanoes
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