GEOL 101 1nd Edition Lecture 9 Outline of Last Lecture I Evolution of Magma II Bowen s Reaction Series Outline of Current Lecture I Intro to Volcanoes II The Nature of Volcanoes III General Features IV Stratovolcanoes Current Lecture Intro to Volcanoes Volcanoes happen all over the world and are usually related with plate tectonics Located on the Ring of Fire mostly 3 main types of volcanoes Stratovolcanoes Shield volcanoes Cinder volcanoes These all have different styles to erupt and make different kinds of rocks Nature of Volcanoes There are 3 factors that result how violent or explosive an eruption can be Composition Temperature Dissolved Gasses These factors also control the viscosity resistance to flow of the lava or magma which then controls the eruption Magma with high viscosity is more explosive Magma with low viscosity is less explosive called quiescent These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Factors affecting viscosity Temperature o Hotter the magma the lower viscosity it has o Flows more Composition o Higher Silica content high viscosity Felsic Lava o Lower Silica content low viscosity Mafic Lava Dissolved Gasses water o Usually increase viscosity o Gases expand in the magma when it gets closer to the surface since the pressure is decreasing o The violent eruption is linked to how easy the gasses can escape In other words Low temperature high gas content high silica lava explosive volcanism High temperature low gas content low silica lava quiescent volcanism General Features Crater located at summit Steep walled depression at summit Usually less than 1 km in diameter However if bigger than 1 km it is called Caldera o Calderas form when the summit is collapsing after an eruption Vent Opening in crater that connects with magma through a pipe Pipes are circular tubes where magmas travel up from chamber Materials Extruded from Eruptions Lava Flows Basaltic lavas have much more fluids Types of Basaltic flows o Pahoehoe resembles twisted or ropey texture o A a blocky rough texture Dissolved Gases Mainly water vapor and carbon dioxide with some sulfur dioxide CO2 that is released if responsible for destroying trees or Kill Trees Pyroclastic materials fire fragments Ash and dust fine glassy fragments Pumice porous rock from foamy lava Lapilli pea sized to walnut sized material that can be called Cinders Larger than Lapilli o Blocks hardened and cooled lava o Bombs ejected hot lava Stratovolcanoes Composite Cones Classic large cone shaped thousands of feet high with sides that are steep Mt Rainer is over 14 000 feet above sea level Located on convergent boundaries where there are subduction zones Pacific Ring of Fire includes Andes Cascades Japan Indonesia New Zealand Composed of inter layered andesite and rhyolite lavas cinders ash and breccia Produce very explosive volcanism Mt Vesuvius Mt Pinatubo Stratovolcanoes are located at subduction zones this causes descending plates to trigger melting Magma then moves upward slowly and can form an island arc if it is on the ocean or a volcanic arc if not For subduction zones Continent plate convergence result in continental volcanic arc Ocean plate convergence result in oceanic island arc Stratovolcanoes produce Ash Clouds plumes of ash that get ejected into the atmosphere o Can travel for hundreds of miles o Material can be in the air for years and affect the climate Pyroclastic Flow high speed flows of hot ash rock and gas that travels down the sides of the mountain o Can move at fast speeds 100 200 km hr o Gases can reach 800 C o Google People of Pompeii Lahars Volcanic mud flows o Form by flash melting of glacier and snow caps or heavy rainfall o Forms a river of concrete that can flow at speeds of 50mph
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