10F_h_Volcanoes - Volcanism the nature of volcanic...

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Unformatted text preview: Volcanism the nature of volcanic eruptions Nature of volcanic eruptions controlled by two factors: (1) viscosity- resistance of a material to flow (2) amount of dissolved gas At great depth- gas dissolved (no bubbles) As magma moves up Gas forms bubbles (exsolved) As magma continues to rise - more bubbles form & bubbles expand Bubble expansion & rise are resisted by viscosity of magma Low viscosity magma- bubbles rise, expand & escape easily High viscosity magmas- expansion resisted until expanding force overcomes strength of material - expands suddenly & violently Decreasing pressure Viscosity controlled by: Composition- Silica (SiO 2 ) content (dominant effect) Low silica magmas- Mafic- low viscosity High silica magmas- Felsic- high viscosity- also, usually have more volatiles - more explosive Temperature- higher T lowers viscosity (lesser effect) Mafic lavas (basaltic) >90% of all lava flows- relatively low viscosity- low silica content- low gas content- high temperature (1000 - 1200C) Viscosity and the behavior of lava flows aa pahoehoe Pillow lavas...
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10F_h_Volcanoes - Volcanism the nature of volcanic...

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