Is mantle convection organized?

It's not completely random. The mantle is nearly 3000 miles thick, and its viscosity is enormous. It simply cannot convect in cells much less than 3000 miles across, otherwise the shear stresses would be too great to overcome with the available energy. Apart from that, the convection pattern will look pretty random.

There was an old theory that the core grew quite slowly with liquid metal gravitationally settling out of the mantle. When it was relatively small, the convection pattern would simply rise at one point, radiate all around, and descend at the opposite point. You couldn't get any more organized than that, eh? And it would have created only one stable location for Pangaea . . . It's an attractive theory, but I don't think its timescales match up.

You need to reread your text book the sinking isn't necessarily at plate boundaries. Yes convection occurs under continental plates. It happens the same way it does beneath all plates. The plates are moved by mantle convection, but do not control the convection. Many scientists believe the Eastern Idaho Snake River Plain resulted due to volcanism from the North American Plate moving across a "hot spot". Or a mantle plume. The Hawaii Islands are from the same type of mantle plume rise and are not on the oceanic plate boundary, but are mid-plate.

Where magma sinks creates some plate boundaries, those called subduction zones. The mantle is a major portion of the interior of the Earth and the crust is a thin part of the Earth. When the mantle moves the crust has to follow, which is one of the major aspects of the Theory of Plate Tectonics.

From seismic and different geophysical information and laboratory experiments, scientists frequently accept as true with Harry Hess' theory that the plate-utilising tension is the sluggish stream of warm, softened mantle that lies decrease than the inflexible plates. this theory became first seen interior the Thirties by ability of Arthur Holmes, the English geologist who later stimulated Harry Hess' thinking approximately seafloor spreading. Holmes speculated that the around action of the mantle carried the continents alongside in plenty the comparable way as a conveyor belt. although, on the time that Wegener proposed his theory of continental flow, maximum scientists nonetheless believed the Earth became a sturdy, immobile physique. We now be attentive to extra suitable. As J. Tuzo Wilson eloquently stated in 1968, "The earth, as a replace of appearing as an inert statue, is a living, cellular element." the two the Earth's floor and its indoors are in action. decrease than the lithospheric plates, at some intensity the mantle is in part molten and can flow, albeit slowly, in step with stable forces utilized for long classes of time. in basic terms as a sturdy steel like steel, whilst uncovered to warmth and tension, could be softened and take different shapes, so could additionally sturdy rock interior the mantle whilst subjected to warmth and tension interior the Earth's indoors over thousands and thousands of years.