Exceptions hotspot volcanoes associated with isolated

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Unformatted text preview: ce running down the _______ centre of the Atlantic Ocean. Exceptions: “Hotspot volcanoes” (associated with isolated heat plumes from mantle) 2 Magma composition primarily related to degree of partial melting of asthenosphere rock Intraplate hotspot (mafic) Convergent boundary (intermediate-felsic) Divergent boundary Convergent boundary (intermediate-felsic) (mafic) Intraplate hotspot (mafic) (or felsic if melting involves cont. crust)) hydration Convergent boundaries: Magma produced by “_________ melting” (water driven from subducting oceanic plate lowers melting point of asthenosphere rock). The melted part is enriched in silica, producing magma of intermediate to felsic composition) decompression Divergent boundaries: Magma produced by “______________ melting” (melting of hightemperature asthenosphere under reduced pressure close to surface). More complete melting of silica-poor asthenosphere rock produces a magma of mafic composition. Intraplate hotspots (independent of plate boundaries): Discrete heat plumes that melt high-temperature asthenosphere rock to produce a magma with a mafic composition. But..in continental areas, contamination of magma by molten continental crust can lead to a more felsic composition. Eruption style: Parent Materials Basalt • Fluid (low viscosity) basaltic (mafic) (mafic) effusive lavas generally produce quiet “______” eruptions (associated with divergent boundaries and hotspots) • More viscous intermediate and felsic violent lavas produce more explosive (“______”) eruptions (generally associated with convergent boundaries, but sometimes with intra-continental hotspots) Rhyolite (felsic) • Viscosities of intermediate lavas are sort of in-between mafic and felsic but viscous enough to be explosive Explosive tendency also related to gas content Higher volatile (gas) content also increases tendency for magma to be explosive (producing more pyroclastic debris (fragmental material deposited as tephra)) Mafic composition: low silica, high temp. (thus lower viscosity) + low gas content = least explosive _lower tendency to produce pyroclastics _____ Felsic (and intermediate) composition: high silica, low temp (thus high viscosity) + high gas content = most explosive higher _______ tendency to produce pyroclastics 3 Eruption style and volcanic landforms The characteristics of volcanic landforms (including shape and size) reflect the eruption style, characteristics of the volcanic materials produced, and...
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