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04 volcanoes - Earth Sciences 1022a Lecture Brief 25 Sep 09...

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Earth Sciences 1022a Lecture Brief 25 Sep 09 VOLCANOES ETC. Volcanic eruptions occur where magma reaches Earth’s surface, often from gas buildup Materials Extruded :- hot, runny mafic lava tends to flow quietly downhill while cooler, viscous felsic magma tends to erupt violently, with pyroclastic material ; two lava types: hot, runny pahoehoe with a surface resembling coils of rope, and cooler, blocky aa ; pillow lavas form underwater as lava repeatedly breaks through quenched tube ends pyroclastic material ranges from dust and ash to streamlined bombs and large blocks nuée ardentes (pyroclastic flows) of devastating hot glowing clouds of ash and gas lahars of pyroclastics mixed with rain, ice, and snow Volcanic Structures: commonly cone-shaped mountains with a small crater at the top or a larger caldera (>1 km across) after magma chamber drains, rim and sides cave in types: shield (broad dome of successive lava flows); small cinder cone (mainly pyroclastic material); composite cone
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