30271 Eve DeAngelis January 29, 2014.docx

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Earth & Environmental Sciences
ENVR 1104
Malcolm D Hill

Natural Disasters and Catastrophes ENVR 1104 CRN: 30271 1/29/14 Chapter 6: Volcanoes; Tectonic Environments and Eruptions • Magma = melted rock- less dense than other rocks; making it often rise to the surface • Lava = magma that has escaped to the earth’s surface • Rock melts by: – Increasing temperature of rock near its melting point – Decreasing pressure on plastic asthenosphere, near its melting point – Addition of water (steam) to rock, decreases its melting temperature • Magma Chamber: – Rising magma accumulates within the earth’s crust. Some of the magma may form volcanic eruptions to form volcanic cones, or lava flows – Viscosity= resistance to flow – Volatiles= dissolved gas content of a magma • Quantity of dissolved gases affects eruptive style (quiet vs. violent) • Chemistry of gases released during eruption affects health and climate impacts – Volume= the amount of magma available in a magma chamber for an eruption cycle; affects the geographic scale of the region impacted by the eruption • When minerals melt, some of the SiO cros4-linking survives in the liquid structure. The degree of cross-linking controls magma’s viscosity. • Basalt= formed by melting of mantle rock with the minerals olivine, pyroxene and Ca-feldspar; has a low dissolved gas content since mantle rocks do not usually have much H O2or CO va2or during melting. Basalt is the hottest magma type (1100-1200 C)- o Viscosity is the lowest of the three magma types – The gas bubbles can move freely, find neighboring bubbles, and grow larger. Larger bubbles can lift larger chunks of magma out of the top of the volcano – If the mantle melts without much gas (H O) be2ng involved, a ‘dry’ mafic (Mg-Fe-Ca-rich, magma (basalt) will form. – • Andesite – Formed when subducting oceanic plates release stored water; steam release triggers melting of the subducting plate plus the overlying mantle rock. Andesite has l
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