How do convergent boundary happen?
Two tectonic plates are moving toward each other and collide.
According to the theory of plate tectonics pieces of the crust called plates are carried across the mantle on convection currents inside the mantle.
In convection current there are always parts of the current where the heat rises causing the liquid to separate which makes it necessary for there to be places where the cooler liquid drops down creating places where the currents come together.
A simple experiment is to take a pyrex container fill it with cold water. Put pyrex on a stove and place drops of f ood coloring red in the middle and blue on the edges. As the water heat up the hot water in the middle will rise and the red color will separate. The blue food coloring will move together in the middle of bowl. This example of convection currents illustrates what plate tectonics theory hypothesis happens inside the earth.
There are three types of convergent boundaries.
The most common is the subduction zones where an ocean plate runs head on with a continental plate and is forced under the continental plate with the sedimentary layers of the ocean plate being melted under the continental plate forming earthquakes and volcanos.
Another types is when two continental plates collide. An example of this is where India and Asia collide forming the Himalayas.
The third types is a transverse fault where two plates run into each other at an angle or almost side by side. The San Andreas fault in Southern California is an example of this type of convergent fault.