When two continental plates collide that’s when Mountains form.
At these boundaries the solid rock splits and huge pieces of rock pile on top of each other.
Where these boundaries occur are the highest mountains on the earth since neither plate sink but get crushed upwards.
A great example is the Himalayan Mountain Range, the highest mountain range in the world.
For more info, please click:http://www.pbs.org/wgbh/aso/tryit/tectonics/crush.html