Why exactly do the tectonic plates move in different directions?

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I fully understand that tectonic plates are just pieces of crust that move because of the convection cells of lava and rock beneath the surface moving them towards a certain direction. But my question is *why* do the different convection cells all move in different directions, what determines that? So for example, the Indo-Australian and Pacific plates have a convergent border, so my question is why does the Indo-Australian plate’s convection cell move east and the Pacific Plate’s convection cell move west rather than both of them just moving in the same direction?

In: Earth Science
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For the same reason that the atmosphere of the Earth – which is governed by the same fluid dynamics, albeit at much higher speeds – doesn’t just smoothly flow in one direction: there are asymmetries that deflect currents in different directions, and once that deflection starts you get things like the Coriolis effect that cause them to curl and form vortices with complicated behavior. As just *one* example, there’s a giant slab that used to be a whole tectonic plate currently floating in the mantle under North America.

The interior of the Earth is basically a giant lava lamp, with the core as the heating element and giant chunks of molten mantle playing the role of the squishy things inside the lava lamp.