Chapter 9 Intrusive igneous activity Plutons form below surface from magma May take millions of years to cool Named by shape and size of the igneous body Dike—Thin, tabular fine-grained layers cutting across sedimentary beds Sill—Thin, tabular fine-grained layers parallel to sedimentary beds Laccolith—bulged body parallel to sedimentary beds Batholith—large intrusive body perhaps hundreds of km wide Origin of magma—formed by melting solid rock Heat—geothermal gradient 20-30oC / 1 km depth 100 km down—1200-1400oC—near melting temperature of granitic rock Rising basaltic magma is hotter—heats almost melted granitic rock Pressure—melting increases volume—confining pressure lessens melting Release of pressure on hot solid rock can trigger melting Can occur when rock is brought to higher levels in crust by convection This happens at mid-ocean ridges where lithosphere diverges Volatiles—water content affects melting temperature (other volatiles too) Wetting a dry, hot rock can trigger melting Subduction draws water-laden sediments into asthenosphere Water is driven off Causes overlying crust to melt Partial melting Since minerals melt in a specific order (see Bowen’s reaction series, pg 60), triggers of melting likely result in only part of solid rock melting Result is magma with higher silica content than rock from which it melted Leaving behind a silica-depleted solid rock The magma is nearer to granite than the rock it comes from Plate tectonics and igneous activity Ring of fire—composite volcanoes surrounding the Pacific basin Andesite magma—explosive eruptions Hot-spot and oceanic ridge volcanoes—fluid basalts of Hawaii and Iceland Most submarine volcanoes are also are of this type Continental interior volcanoes—diverse in type Yellowstone—explosive supervolcano/caldera giant Columbia Plateau basalt Plate motion provides the mechanisms to melt rocks into magmaConvergent plate boundaries—volatiles drive melting From sinking subducted slab—creates basaltic magma Migrates upward and creates volcanic peaks parallel to trench of subduction Continental volcanic arcs form andesitic magma because the basaltic magma melts and assimilates some of the underlying continental crust Divergent plate boundaries Release of pressure allows partial melting of mantle by decompression melting Basaltic magma is generated Either cools in place as gabbro, or erupts to ridge as basalt Lava erupting below water forms ‘pillow lava’ Some divergent plate boundaries occur with continents also—East African Rift zone Intraplate volcanism Mantle plumes Yellowstone Canary Islands Hawaiian Islands Over 100 hot-spots across Earth have been
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