Volcanoes CHAPTER 6 MATERIALS HAZARDS AND ERUPTIVE MECHANISMS CHAPTER 7 TYPES BEHAVIOR AND RISKS 1 Volcanoes Volcanism Refers to the rise of magma onto the earth s surface Magma a mixture of liquid rock mineral crystals and dissolved gases Volcanoes are conical or dome shaped landforms built by the emission of magma and its contained gasses onto the earth s surface 2 Locations of Volcanic Activity and Volcanic Hazards Volcanic activity is controlled by plate tectonics because plate movements relate to where sources of magma originate inside the earth Nearly all active volcanoes are located in one of three plate tectonic settings Subduction zones at convergent plate boundaries Example Volcanoes lining the trenches of the Pacific Ocean forming the Pacific Ring of Fire Rifting and sea floor spreading at divergent plate boundaries Example Volcanic eruptions at mid ocean ridges and in some rift zones on the continents like the East African Rift Valley Hot spots Example The Hawaiian Islands and the Galapagos Islands 3 Map of the World s Active Volcanoes Note that the majority of above sea active volcanoes about 66 occur in the Pacific Ring of Fire 4 Magma and the Driving Force Behind Eruptions Magma may be ejected onto the earth s surface as Lava Pyroclastics or tephra flour sized to boulder sized particles which are thrown in the air due to the built up pressure of gasses Above aa lava Right pahoehoe lava The violence of a volcanic eruption depends on the magma s viscosity and gas content The more viscous thick and more gaseous the magma the more explosive the eruption Refer to page 125 Above Gigantic eruption cloud of tephra 5 Viscosity Viscosity is a measure of a fluid s resistance to flow The main factor that determines the viscosity of magma is its silica SiO2 content The more silica in the magma the more viscous it is The more viscous silica rich the magma the more violent the eruption We are concerned then with three types determined by the chemical composition of the magma Pgs 146 148 1 Mafic or Basaltic magma low silicon and oxygen high iron and magnesium Simple silicate minerals Dark magmas Low Viscosity 2 Intermediate or andesitic magma intermediate silicon and oxygen intermediate iron and magnesium High Viscosity 3 Felsic or rhyolitic magma high silicon and oxygen low iron and magnesium Complex silicate minerals Pale magmas High Viscosity Refer to pages 126 127 Left The basic building block of all silicate minerals the silica tetrahedron Four oxygen atoms surrounding a single atom of silicon 6 Gas Content The gas content of a magma also relates to its behavior A magma with low gas content will tend to flow out of a volcano as relatively quiet lava A magma with high gas content will tend to blow apart violently upon erupting The higher the gas content the more violent the eruption The composition of the gases in magma are Water vapor Carbon Dioxide Minor amounts of sulfur dioxide hydrogen sulfide chlorine and flourine gases But it is the amount of dissolved water that typically inspires a volcano to violence Refer to pages 127 128 7 Steps to a Volcanic Eruption Volcanoes form wherever rock melts at depth and the magma can rise to erupt at the surface Rock deep in the earth may melt by Increasing its temperature Decreasing its pressure Adding water to lower the melting temperature Refer to page 125 Magmas that are generated deep within the Earth begin to rise because they are less dense than the surrounding solid rock As they rise they may encounter a depth or pressure where the dissolved gas no longer can be held in solution in the magma and the gas begins to form a separate phase makes bubbles and will continue to grow in size as pressure is reduced Refer to pages 127 128 8 Steps to a Volcanic Eruption Two Possibilities If the magma has a low viscosity the gas will easily expand to atmospheric pressure at the earth s surface and simply burst and a non violent eruption will occur usually as a lava flow If the magma has a high viscosity the gas will not be able to expand very easily creating a high pressure inside which will cause them to burst explosively on reaching atmospheric pressure This will cause and explosive volcanic eruption Refer to pages 127 128 9 Volcanic Explosivity Index The violence and size of a volcano s eruption is expressed by the Volcanic Explosivity Index VEI Values for the VEI range from 0 to 8 and are based on 1 2 3 the volume of material lava and particles erupted the height of the eruption column how long the eruption lasts The larger the VEI value the larger the eruption Refer to pages 129 133 This figure plots VEI values versus the eruption cloud height 10 Volcanic Explosivity Index Two main factors determine the size and explosivity of a volcano s eruption 1 The amount of time that has passed since the last eruption This figure shows that individual eruptions become more violent as the time gap between eruptions increases 11 Volcanic Explosivity Index 2 The viscosity thickness and gas content of the magma Both relate to plate tectonic setting Divergent boundaries MOR and hot spots both draw their magmas from the upper mantle This magma is called MAFIC basaltic MAGMA and is characterized by relatively low silica and low gas content the eruptions have low VEI The magma at convergent boundaries comes from melting of subducted oceanic plates Subduction forms FELSIC rhyolitic to INTERMEDIATE andesitic MAGMA and is characterized by relatively high silica and high gas content the eruptions have high VEI Refer to pg 147 12 Bottom line The world s most dangerous volcanoes are those at convergent plate boundaries 13 Types of Volcanic Hazards and Products The figure at right shows a composite of the potential hazards associated with a volcano The main hazards are lava flows ash falls pyroclastic flows lahars gas emissions 14 Lava Flows Lava is molten magma that flows out and onto the Earth s surface Lava flows are typically formed from low viscosity mafic magma that erupts at divergent boundaries and hot spots Refer to page 131 Fluid basalt lava flows are called Hawaiian type lava and can come in two varieties Smooth runny pahoehoe top and chunky aa bottom 15 Lava Flows The lower silica content and therefore low viscosity of mafic magma allows the lava to run down slopes easily Lava flow eruptions are fairly gentle and quiet They may cause property damage but rarely fatalities Refer to page 131 Refer to pg 134 135 16 Lava flows destroy whatever they
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