Question

What is a collisional boundary?

Answer 1

If an ocean floor pushes toward a land mass then it will slide under the land mass, because the latter is lighter and less dense than the ocean floor.

But if two land masses meet, the two will crush together at what is called a collisional boundary. They crumple and fold. The result is a mountain range.

Impressive example: the Himalaya formed when India crushed into Asia.