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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-Pascalsecond (Pas). 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 Pas, water~10-3, motor oil~1, glycerine & corn syrup~10 Pas [all at ~25C].
Magma & Rock Compositions
Extrusive (volcanic)
Intrusive (plutonic)
Color
SiO2 (wt%)
Eruptive Temp. (C)
Dry Viscosity (Pas)
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 25C (at sea level pressures) expands to 4500 cm3 at 1000 C.