Volcanoes Notes

Ch 4 Volcanoes Volcano - a vent through which magma, fragments of rock and ash, and gases erupt, or the structure built around the vent by such eruption. Vary depending on magma composition and plate-tectonic setting magma generated: (magma sources) 1) divergent plate boundaries 2) subduction zones (intra plate volcanism) 3) hot spots. 1) Divergent Plate Boundaries - mostly spreading seafloor ridges (rifts) - magma from partial melting of mantle - melt maybe due to reduction in pressure as plates - upper mantle ultramafic so magma is mafic, - basalt forms sea floor - continental rifts less common - mafic magma may melt granitic crust to - make magma more silicic - mafic volcanism more prevalent though - East Africa rift (Mt. Kilimanjaro) 2) Subduction Zones - magma more basaltic (still mafic) if oceanic plate subducted. - magma more silicic (mafic too) if continental - plate subducted. - dewatering -subducted plate contains water, it is - released and rises into spot between overriding - and subducted plates: H₂O reduces melting temp - & mantle material: H₂O reduces magma formation - magma is andesitic to basaltic (depending on extent of 2 - commonly volcanoes on continental edges - above subduction zones are andesitic - since granitic crustal rocks are assimilated - by mafic magma producing intermediate-comp. 3) Hot Spots - mantle plumes - columns of warm material - rising in the upper mantle; if overlying plate - is weak enough magma breaks through - comp: mafic magma from mantle melting - mafic sea floor = mafic (basaltic) eruption - plumes, if mafic magma goes thru continental - crust, mafic may be assim., so more silicic - ? high concentrations of radioactive elements - ? anomalous regions of outer core * Fissure Eruption - the out pouring of magma forming - volcanic rocks at rifts on the sea floor - (magma from a long crack rather than a single - pipe or vent) on continent: Columbia Plateau, US - (50,000 km²) WA, OR, ID - India, Brazil - 750,000 km² Flood Basalts * most volcanoes over subduction zones - Ring of Fire - volcanoes rimming Pacific Ocean Volcanic Structures * Shield Volcano - very flat & low in relation to diameter - basaltic lavas flow freely - high in Mg (manesium) & iron - are fluid - so low in silica - ex. Hawaii flows, thin - hot spot volcanoes are often shield volcanoes (Hawaii) Ch 4 cont... * pahoehoe - from fluid lavas - basaltic - quenched - hardened skin - as they cool develop a - ropy appearance as they flow * aa - from lavas that flow less readily - still basaltic though, - jumbled, blocky flows (hurt to walk on) - produce * pillow lava - when lavas are extruded under water, - rapid quenching of hot flow surface leads to - bulbous flow forms - resembling pillows - glassy exterior; coarse-grained, slowly - cooled interior * columnar jointing - lava contracts as it cools, can - fracture the flow into a mass of polygonal columns - common in basaltic flows (because start - at higher temp - more contraction?) * volcanic dome - compact, steep-sided - more silicic lavas (andesitic & rhyolitic), - are more viscous, flow less readily; ooze out of - tube & pile up close to vent (Mt. St. Helens) Some volcanoes erupt violently due to water & gas trapped in the magma under pressure pyroclastics - "fire", "broken" erupted volcanic material "bits of violently erupted material most energetic pyroclastic erups. more typical of andesitic & rhyolitic lava because thicker lavas tend to trap more gases