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LIFE SCI 3C03 Chapter 3

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Life Sciences
Steven Brown

Chapter 3 – Volcanoes 3.1 Introduction to Volcanoes  Directly related to plate tectonics and most are located near plate boundaries.  Mid-ocean ridges and subduction zones are sites where molten rock, or magma, reaches the surface rd  2/3 of all active volcanoes on land are located along the Ring of Fire, which surrounds the Pacific Ocean (see figure 3.2).  Size, shape and behavior of volcanoes are closely related to their plate tectonic setting and to their magma chemistry and gas content.  Magmas that form volcanoes mainly contain 8 elements: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), magnesium (Mg), calcium (Ca), sodium (Na), and potassium (K). oxygen and silicon are the 2 most abundant Volcano Types  Different shapes and eruptive styles are formed depending on the chemistry and viscosity (fluidity) of magmas. (cold honey doesn’t flow much so it has a high viscosity, warm honey flows faster, therefore has a low viscosity)  Magma with high silica contents are cooler, more viscous, and have more dissolved gases than mamas with relatively low silica contents.  Some examples of high-viscosity magmas include dacite and rhyolite. Low-viscosity magmas: basalt and andesite  Rapid degassing of high-viscosity mamas triggers explosive eruption Shield Volcanoes  Largest and tallest volcanoes in the world; gently sloping slides and broad summits  Generally have non-explosive eruptions of very hot, low viscosity, basaltic magmas.  Basaltic magmas don’t go through fractionation, a process that involves crystallization of different minerals in a slowly cooling magma. As fractionation proceeds, the remaining magma becomes progressively enriched in silica, alumina and gases. This process basically changes the magma type (basalt/andesite etc)  Consist entirely of lava flow (erupts through the volcano vents) but tephra, which are fragmented debris, can also be erupted. Accumulation of tephra is known as pyroclastic deposits.  Lava can flow many kilometers away from a vent through underground lava tubes Composite Volcanoes aka stratovolcanoes  Erupt less frequently but are explosive (doesn’t just flow)  Lavas are relatively silica rich and viscous, and thus they rarely flow more than a few kilometers from vents  Responsible for most of the deaths and destructions caused by volcanoes Volcanic Domes are Steep-sided mounds of lava that form around vents from the eruption of highly viscous, silica-rich magmas. Cinder Cones aka scoria cones  Relatively small volcanoes made of nut- to boulder-sized pieces of red or black basalt.  Tephra from extinct cinder cones is the “lava rock” widely used in landscape  Found on the flanks of large volcanoes Maars are produced during the violent interaction of magma and groundwater Ice-contact Volcanoes (underwater volcanoes) aka tuyas  Usually melt large quantities of ice and thus producing high outburst floods termed jokilhlaups  Pillows formed due to rapid cooling of lavas  most of the time, the eruption melts through the ice above, producing lava flows that covered the pillow breccias  steep-sided flat-top volcanoes sometimes called tuyas Volcanic Features (craters, calderas, volcanic vents, geysers and hot springs)  Craters are depressions at the tops of volcanoes that are formed by explosion or collapse of the summit area. Couple of kilometers in diameter  Calderas are circular/oval depressions up to a few kilometers in diameter that form during explosive ejections of magma and subsequent collapse (large craters). caldera- forming eruptions rare but large and most deadly  Produced during the violent interaction of magma and groundwater  Volcanic vents are openings through which lava are pyroclastic debris erupt. Most circular, others are elongated cracks called fissures. Extensive fissue eruptions have produced huge accumulations of nearly horizontal lava flows called flood basalts  Hot springs: groundwater becomes heated when it comes into contact with hot rock. The water, now hot, discharges as surface as hot springs  Geysers are created as groundwater boils in an underground chamber to produce periodic, steam-driven releases of steam and hot water.  Resurgent Calderas are extremely violent are extremely large and violent calderas. Around 10 of these have erupted during the last million years. Volcanic Origins Subduction Zones  Oceanic crust, which is denser than continental crust and commonly covered by thick, wet sediments, is carried into Earth’s mantle. Rising heat and pressure rey out the subduction crust in a process called dehydration. The water expelled from the descending crust rises and changes the chemical composition of the overlying mantle. The presence of water lowers the melting temperature of the overlying mantle rocks and causes them to melt, forming magma that rises through the crust to erupt on the surface  Composite volcanoes (stratovolcanoes) occur at s
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