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My daughter has a writing assignment based on a handout about plate tectonics. One paragraph says:

Molten rock from the earth's mantle pushes up through the surface at the mid-Atlanic ridge. The rock cools and forms new sections of crust on the ocean floor. The ocean floor has to make room for the new crust. To make enough room, the ocean floor must spread out. As a result, the earth's plates shift.

It's been a long time since I was in school and this is far from my field, but this doesn't sound right to me. Isn't the main cause convection currents in the earth's mantle? Is cooling lava pushing the plates apart a cause? I thought it was basically an effect — where the plates diverge, molten rock pushes up and fills in. (Or, is this just two ways of looking at the same thing?)

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  • $\begingroup$ Well technically episodic rifting/diking at spreading center does involves push with pressures (at least) greater than the hydrostatic pressure at that depth. Its not like a void just opens up and gets filled with magma. $\endgroup$ – stali Apr 28 '15 at 16:06
  • $\begingroup$ I read that as meaning that the molten rock (magma) pushes up [against gravity] - which is correct. I don't think it intends to say anything about pushing on the tectonic plate. Because the crustal rock is virtually incompressible, and no significant gaps form in the crust anywhere, except at the ridge, the ocean floor must spread out [i.e. move horizontally] . Perhaps this text could have been explained better, but not incorrect. $\endgroup$ – Mark Rovetta Apr 30 '15 at 1:25
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No, lava does not push the plates apart.

The mid-ocean ridge basalt is passively filling the space left by the plates moving apart. Plate motion is driven by gravity (image by Vic DiVenere at Columbia; see note below):

Forces driving plate tectonics

There are thought to be two main effects at play: ridge push, and slab pull. We don't know much about their relative importance, but it probably varies spatially (e.g. Ribeiro, 2012, Soft Plate and Impact Tectonics, Springer).

Ridge push happens not because the new lava is pushing, but because the ridge is high up. It's so high that in some places it sticks out of the water (e.g. Iceland). It's high because it's hot — but not from deep convection. A geophysical technique called seismic tomography suggests that the hot zone is relatively shallow. As the new plate cools, it thickens and sinks 'downhill', pushing the older plate ahead.

Slab pull is easier to imagine: a subducting plate, sinking into the asthenosphere (uppermost mantle), pulls the younger plate behind it. Not all margins are destructive, however, so slab pull does not operate everywhere. Indeed, the relative important of push and pull is debated.

You will sometimes see figures like this one, strongly implying that convection and plate motion are strongly coupled. The earth is a complex system so the processes are certainly related, but we don't understand the frictional forces (drag) at the lithosphere–asthenosphere boundary well (e.g. read this primer by DiVenere). These images, while prevalent, are an unhelpful over-simplification.


Note: I'm using DiVenere's copyrighted image under 'fair use' terms. Please replace it with an open version if you find one! (But not one with convection cells :)

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To add to a already complete answer by kwinkunks, perhaps one of the most surprising things about plate tectonics to geophysicists was the fact that in some cases extension in the lithosphere (rifting) is a direct cause of subduction zones. When the idea of subduction zones was finally formalized (~1969) most geophysicists expected all subduction zones to be accompanied by an area of compression, but what is later found is that rifting centers opened up nearby. What studies further found is that the motion of the upper plate and the viscous coupling of the subducting plate caused the trench to roll backwards in the opposite direction of subudciton! This trench rollback opens ups an oceanic ridge, what we call back-arc spreading centers!

So the whole point of this small digression is that plate tectonics is a very complex issue, and not all the forces are well understood. Models show that the mantle is convecting near plate margins at orders of magnitude greater than plate motion. Inside the mantle wedge of subduction zones we see rapid and complex mantle flows that react to plate geometry in unpredictable ways.

Melting (lava/magama) certainly plays a apart in our plate motions, but we really have no idea how much it does.

To be honest, if we did, a lot of geophysicists would be looking to do something else.

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