LEC 05-Volcanoes

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Unformatted text preview: derwater). Fluid mafic lava quietly extruded from crustal fractures called fissures. Divergent Boundaries Subaqueous Eruptions at Mid-Ocean Ridges • • • Generally non-explosive Generally (runny, low-viscosity lava and constrained by water pressure at ocean depths) at Extruded lava produces Extruded “pillows” of basalt with of quenched, glassy outer rind and aphanitic interior. and Minor explosions of pillows Minor (due to gas escape) can produce rock containing glassy fragments (hyaloclastite). (hyaloclastite). Pillow basalt Hyaloclastite Divergent Boundaries Subaerial Eruptions on Land in Continental Rifts Lava flows Again, fluid mafic lava quietly extruded from fissures. Can form laterally extensive, sheet-like layers of basalt Volcanism at Convergent Plate Boundaries Volcanism (at Subduction Zones) More significant “wow” factor (more explosive potential). Lower-temperature, high-viscosity, volatile-rich (intermediate to felsic) magma produced by hydration melting. High gas pressure + viscous magma = explosive behavior Lava flows + lots of pyroclastic material. Volcanism at Convergent Plate Boundaries Volcanism (at Subduction Zones) Lava Flows Andesite lava flow Rhyolite lava flow Andesite lava flows (from intermediate magma) and rhyolite lava flows (from felsic magma) less laterally extensive than mafic (basaltic) flows due to higher viscosity In turn, viscosity can influence the buildup of pressure in magma magma The buildup of pressure tends to be higher in high-viscosity magmas than low-viscosity magma. This is because high-viscosity magmas resist flow (so gas does not escape as freely as in low-viscosity magmas). More pressure buildup = more explosive eruptions Volcanism at Convergent Plate Boundaries Volcanism (at Subduction Zones) Products of Explosive Volcanism • • Gas expansion – Vesiculation – formation of bubbles (forms vesicular texture Vesiculation (if infilled, called amygdoidal texture) (if Explosions – Rip lava into pieces – Pyroclasts...
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This note was uploaded on 02/13/2014 for the course EARTH SCI ES2200 taught by Professor Camtsujita during the Fall '13 term at Western University (Ontario) - Also known as University of Western Ontario.

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