What happens to the leading edges of convergent plates as they collide?

Answer 1

This depends on what material (oceanic crust or continental crust) the leading edges are made of.

When Continental crust hits Continental crust (eg India ploughing into Asia) the crust thickens by thrusting and folding and a mountain belt forms (the Himalayas). There is little or no volcanic activity associated with this sort of collision.

When Oceanic crust meets Continental crust (eg the Pacific plate and the west coast of South America), the oceanic crust sinks below the continental crust forming an oceanic trench on the oceanic side (a subduction zone) and a mountain belt on the continental side (by folding and uplift). This sort of collision produces a large amount of volcanic activity.

When Oceanic Crust meets Oceanic Crust, the actively spreading side sinks below the stationary side to form a back arc basin associated with island arcs and subduction zones (eg the Caribbean or the east of Papua New Guinea). This sort of collision also involves a large amount of volcanic activity.