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View submission: Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science
First, two big misconceptions/myths:
1. While Earth's mantle flows slowly, it is overwhelmingly solid (think putty). The outer core is liquid, but that is not directly related to plate tectonics. The mantle and crust do contain some areas of partial melt (up to a few percent).
(1a. Volcanoes aren't supplied from some ocean of magma, but rather localized magma chambers of partially solid 'crystal mush'. The melting point decreases with pressure. Magma is generated in the mantle and crust by hot rock flowing upward, which partially melts as it decompresses.)
2. The plates aren't just passively being driven along by convection in the underlying mantle. The plates are an integral part of the convection.
(Edit: 3. Not all, or even most, of Earth's heat is from radioactive decay. Roughly half of Earth's internal heat flow is from heat leftover from Earth's formation: the kinetic energy of the large objects that collided to form it, and the friction form dense iron sinking to form the core.)
Plates are made of relatively rigid rock (as opposed to the flowing rock below), a layer called the lithosphere, which comprises both the crust and the uppermost part of the mantle. Immediately below the lithosphere is the asthenosphere, the relatively weak and low-viscosity layer that makes up the rest of the upper mantle.
The largest driving force in plate tectonics is slab pull. The subducting lithosphere (slab) pulls the plate along. The slab sinks because it is (a) compositionally denser than the continental crust it may be subducting under and (b) cooled so that it is denser than the surrounding/overlying rock of more similar composition. (Oceanic crust can subduct under younger, more buoyant oceanic crust, not just continents, and in any case has to sink through the weak asthenosphere.) When oceanic crust first forms from cooling magma at mid-ocean ridges, it is still relativley hot. It cools and contracts (becoming denser) as it age and spreads away from the ridge. The flow induced in the asthenosphere by the subducting slab exerts another driving force called slab suction, which accelerates the subducting and overriding plates.
Another, weaker, driving force in plate tectonics is ridge-push, but this is a misnomer. Magma upwelling at the ridge does not actively push the plate along. The young, hot lithosphere is buoyant, rising to form the ridge, but the seafloor gets deeper with the cooler lithosphere away from the ridge. Ridge-push is the lithosphere sliding downhill because of gravity. The buoyant magma passively upwells from the mantle as the newly made oceanic lithosphere moves away on either side.
There is viscous drag from the underlying flow acting along the entire base of the lithospheric plates. But this is much weaker than slab pull, which it tends to oppose.
There's nothing here!