ISNS 4359 EARTHQUAKES AND VOLCANOES Spring 2005 Steve Bergman, Instructor Lecture 12. Magmas, Lavas, & Plate Tectonics Magma - molten rock beneath the earth’s surface (crystallizes as intrusive rocks-plutonism) [magmas are multiphase=liquid(s)±solid±gas]. A vast majority of melt stays within the Earth’s interior because it lacks the means to erupt at the surface. Upper mantle rocks melt to form most magma. Lava - molten rock that erupts on the Earth's surface, eventually cools and crystallizes as extrusive rocks; (~10% of the melt formed in the interior has enough energy to erupt on the surface). Eruption style depends on a melt’s pressure, temperature, and composition, especially volatiles (all of which control a magma’s physical properties, such as density, viscosity, surface tension, etc.) as well as eruption rate, vent geometry, nature of country rocks, and crustal stress regime. Current Global Magma flux (from the mantle to the crust, including intrusives & extrusions) ~ 30 km3/yr: ~62% spreading ridges, ~26% subduction arcs, ~12% intraplate hotspots For comparison, Global human activity moves ~17 km3/yr (soil/rock), Mountain Building ~12 km3/yr, Sediment Transport by Rivers ~10 km3/yr. 3 ways to melt a rock: • Raise Temperature (T),• Lower Pressure (P), or • Change Rock Composition (mainly add H2O or CO2). Increase T: Conduction (crustal thickening), Impact melting (impactor can melt itself plus 40 times its mass), Underplating of crust by basalt magmas, (Rare) radioactive element deposits (Oklo, Gabon). Decrease P: Convective upwelling in the mantle (decompression melting). Change rock composition: Adding fluids (1-5 wt% H2O or CO2 lowers melting T by ~100-300 C). Partial melting (5-20%, selected minerals) of mantle (peridotite) yields basalt (small amounts [<2%] of basaltic melt occur in the Low Velocity Zone at 100-150 km depths). Partial melting (10->50%) of the crust (gabbro-granite) yields rhyolite. Viscosity (measure of a material's resistance to flow), determines magma transport, eruptive behavior & flow morphology. Units-Pascalsecond (Pas). High viscosity=thick & pasty, Low viscosity=thin & fluid. Melt Viscosity: related to temperature, pressure, composition (SiO2 & dissolved gases), crystals, bubbles. Typical values: air~10-5 Pas, water~10-3, motor oil~1, glycerine & corn syrup~10 Pas [all at ~25C]. Magma & Rock Compositions Extrusive (volcanic) Intrusive (plutonic) Color SiO2 (wt%) Eruptive Temp. (C) Dry Viscosity (Pas) Wet Viscosity (4% H20) Melt Density (gm/cc) Mafic (Mg-Fe) basalt gabbro Dark ~50 ~1100 102.5 [soupy 102 ice cream] ~2.7 Intermediate andesite diorite Grey ~60 ~1000 104 103.5 ~2.5 Felsic (Si-Al) rhyolite granite Light ~70 ~800 1010 [peanut butter] 106 [toothpaste] ~2.2 Melts are less dense than most surrounding rocks in the source regions and rise toward the surface. Fractional crystallization occurs where magmas pond at depth, dense minerals settle, and residual melt floats, progressively changing melt composition from the left to the right above. Magmas can mix and assimilate their country rocks to change composition (usually increasing SiO2). Adding water to the melt, increasing T, or decreasing Silica content of the melt decreases its viscosity. Solubility of H2O or CO2 in the melt increases with increasing P & T; therefore, cooling or decompression causes gases to come out of solution (boiling). Work of Boiling: 1 cm3 of water at 25C (at sea level pressures) expands to 4500 cm3 at 1000 C.