describe what happens to plate a as it collides with plate b why

When plate A collides with plate B, plate A is usually forced downward into the mantle if it is denser (typically an oceanic plate) than plate B.

What happens to plate A?

In a typical school diagram, plate A is the oceanic plate and plate B is the continental plate.
As they move toward each other, the denser oceanic plate A bends and sinks beneath plate B, forming a subduction zone.

As plate A descends:

• It slides into the mantle, where temperature and pressure increase.

• Parts of plate A start to melt, forming magma.

• This process can generate earthquakes along the boundary where the plates grind past each other.

Why does this happen?

The key reason is density.
Oceanic crust is thinner and made of heavier (mafic) rocks, so it is denser than continental crust and is pulled down by gravity when the plates collide.

Continental crust is thicker and less dense, so it “floats” higher and does not usually sink; instead, it stays on top while plate A dives underneath.

If both plates were continental, neither would easily sink, and instead they would crumple and thicken to form high mountains, like in a collision zone.

Simple mental picture

Imagine pushing a thin metal tray (plate A) under a thick foam board (plate B).
The tray easily slides underneath because it is thinner and heavier, just like the denser oceanic plate subducting under a more buoyant continental plate.

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Learn to describe what happens to plate A as it collides with plate B and why, including subduction, density differences, and how these collisions create earthquakes, volcanoes, and mountains.

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