Amount of Distortion at Continental Collisions

Something that I've wondered for a while now - When continents collide, how much of the landmass gets distorted by the collision before it settles?

To perhaps give a clearer idea of what I'm talking about, I'll use an example - How much of the Indian Plate is still intact, and how much is now either driven under the Eurasian Plate or contorted into a different shape i.e. the Himalayas?

Any answers would be greatly appreciated

1 Answer

I think of the plates not as icebergs floating around in a sea but as pieces of porridge skin on the surface of gently simmering porridge, moved about by convection forces from below.

The Indian plate became fused with the Australian plate about 43 million years ago, so now there is only one plate, the Indo-Australian plate. Since then, the whole mass has been moving north at about 4 cm per year. A relatively small amount, perhaps a quarter of what was once the Indian plate, has crumpled against the Eurasian plate to form the Himalayas.

I don't know what the total mass of the Indo-Australian plate is, but it must be many trillions of tons. Imagine the momentum of many trillions of tons of rock moving at 3 mm per month, in addition to which there are convection forces from the mantle pushing it along! Bulldozing the Himalayas must have slowed it down a little, so 43 million years ago it was moving even faster.

• Don't mistake 1.5 inches for 15 inches. Mar 3, 2020 at 21:26
• This answer isn't quite correct. It is assuming that plates have momentum in the same way as a car driving into a wall has momentum that keeps pushing it along even though its front is already deforming. But that's not the case for plates -- they're moving so slow that momentum is not an issue: If the forces that move it along (e.g., the friction at its underside from the mantle moving past it) stopped, the plate would instantly come to a stop as well. So the collision itself hasn't slowed down the plate -- it is all an instantaneous force balance at every time instant. Mar 3, 2020 at 22:37
• Nothing is instant. If you bang a nail with a hammer, it takes a finite amount of time for the force to be transmitted from the nail head to the point which penetrates the wood. It is not instant, though it may seem to be. It is impossible for a plate movement to stop instantly. Mar 3, 2020 at 23:17