San Andreas fault in San Francisco. The zone is roughly 1,300km long and in places is tens of kilometre wide. It slices through two thirds of the length of California.
Diagram This is exactly what I have just explained but it diagram form .
c o m p r e s s i o n a l p l a t e b o u n d a r i e s
The compressional plate boundary is where the plates are forced towards each other.
The continental plate which is heavier and carries land subducts the lighter plate which carries ocean hence the name the oceanic crust.
As it is forced below in the subduction zone it melts and this creates magma. The magma starts to collect in the magma chamber and eventually starts to rise through the cracks in the continental crust.
This causes lots of pressure to build up over time. As a result of a compressional plate boundary fold mountains, earthquakes and volcanoes are often formed.