A mid ocean ridge is an area where new sea floor is created throught the divergent (speading) motion of the tectonmic plates in that area. It comes down to density. The center of the ridge is high, because it it new (just formed) and hot. The edges are low because it is old and cold. There are two main theories on the cause: slab pull and ridge push.
Ridge push is the classic idea that the ridge is a high point of a convection current, pushing the plates aside and allowing new mantle material to rise up and fill the hole.
Slab pull is the new idea that the subducting end of the plate, that is really old and dense, pulls on the entire plate as it subducts, and thus pulls apart itself in places, causing a hole which is then filled by rising material, forming the ridge.
It's kinda a chicken/egg thing, so we may never be able to rule one or the other out.
Geochemical evidence (isotopes showing mantle source in MORBs) and geophysical evidence (deepest earthquakes at subduction zones) supports the fact that this occurs, but does not specifically address the cause.What drives the motion of the plates on the earth?,geophysical and geochemical evidence for mantle convection?The mantle is a chemically distinguished layer and consists mainly of the rock peridotite. Mechanical layers divide the mantle into 3 parts: the Lithosphere, the Aesthenosphere, and the Mesosphere. The lithosphere (10 to 200 kilometers thick) is rocky and fairly well attached to the crust of the Earth, though it has a different chemical composition than the crust. It is the presence of the lithosphere that gives the tectonic plates strength enough to not break up.
Immediately below the lithosphere is the aesthenosphere. This layer is what scientists term "plastic." Basically it is hot enough that it is not rocky, but it is not a fluid either. The best comparison for it is Silly Putty. This is the layer that convects heat.
Below the aesthenosphere, at a depth of 660 kilometers, the mesosphere is a part of the mantle that is hot, but is under so much pressure that it is solid and rocky.
It is important to note that scientists are still not sure what drives continental drift and convection is only one theory and likely only one of many components that cause drift.
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