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ISNS 4359 EARTHQUAKES AND VOLCANOES
Spring 2005
Steve Bergman, Instructor
Lecture 2. Earth's Interior & Plate Tectonics, continued
Plate tectonics is driven by gravity (slab pull & mantle convection) & closely related to Earth’s interior temperature variations.
Heating most materials decreases their rigidity and strength. Temperature (T) is a measure of a material’s kinetic energy. The
surface of the sun is white hot at a T of ~5500C, mainly fueled by fusion of hydrogen in the sun’s core. The Earth’s center is also
white hot and at about the same T, but for very different reasons. Global average land surface T averages ~15C (wide range= -90
to 50C due to climate zones) and has widely varied in the past. Atmospheric T varies from <-90 to >700C in discrete zones.
Ocean T generally decreases with increasing water depth to ~0-5C at >1000 m. Beneath the Earth’s solid surface, T generally
increases with increasing depth except at subduction zones where the downgoing lithosphere cools off the surrounding mantle.
Heat Flows from hot to cold; heat flow (HF) units are Watts/m2 - [1 W=1 Joule/sec]
Earth's surface & atmosphere receive ~200 W/m2 of energy radiated from the sun. On average, the Earth’s surface receives ~0.1
W/m2 conducted and convected from the Earth's interior, mainly due to heat left over from Earth's formation (impact
gravitational energy converted to heat energy), heat of crystallization of the core, and radioactive decay. Radiated solar energy
hitting Earth in 1 day (~1022 Joules) = ~6 times energy stored in global oil & gas reserves. {Audible sound wave energy is also
measured in W/m2; 0 dB (decibel) = 10-12 W/m2; jet take off at 60 m (threshold of pain- ear damage): 120 dB=1 W/m2}
Heat energy flows by three mechanisms: radiation, conduction and convection.
Radiation is the transfer of heat by photons in the visible to infrared electromagnetic spectrum (eg. Sunlight).
Heat escapes from the Earth’s hot core by convection & conduction
Conduction-kinetic energy transferred by atom to atom exchange (no motion involved).
Convection-mechanical process transfers heated material to another place; most materials expand when heated (less dense) and
compact when cooled (denser); buoyancy: hot, low density material rises; whereas cold, high density material sinks.
Radioactive Heat Generation due to 40K, 238U, & 232Th mostly confined to continental crust (accounts for ~10% of
total HF; mantle radioactivity generates ~2% of total HF). Since Earth’s formation (4.5 billion years ago), <25% of
radioactive elements remain. Plot below shows a geotherm (how T varies with depth beneath the surface) compared
with the solidus (melting temperature) of the mantle and core.
Total Earth HF ~41 TW (1 TW=1012 W)
Continental HF averages 0.065 W/m2 & decreases with age:
0.080 W/m2 @ 30 Ma to 0.040 @ 3000 Ma. (Ma=million years)
Oceanic HF averages 0.10 W/m2 & decreases with age:
0.14 W/m2 @ 15 Ma to 0.05 W/m2 @ 150 Ma.
Layer
Crust
thickness (km) 7-38
thermal gradient
(C/km)
20-30
Mantle
2900
Core
3480
0.5-10
0.5-1
temperature (C)
top
base
0-15 ~400-800
~400-800 ~3000
~3600
~5500
The lithosphere-asthenosphere boundary is defined by a
T=1300C (at a depth of ~120 km, shallower at the midocean spreading ridges, generally deeper beneath the
continents).
Lithosphere - Conduction / Insulates interior
Asthenosphere – Convection