Mountains build up when a continent pushes against another plate.
The tectonic plates on the earth's crust can contact one another in three possible ways. Convergent plate boundaries are where two plates are directly pushing and colliding into one another. Convergent plates are what form mountains and islands; the exact type of land masses they form depends on where the plates are converging.
South America
The South American plate and the Nazca plate are converging near South America's west coast. This is an oceanic-continental plate convergence, where the plates are colliding close to a continental boundary. This type of convergence typically builds up mountains near the edge of the continent. The convergence of these two plates forms the Andes Mountains that run through Peru and Argentina.
Northwest North America
Most of the area along North America's West Coast is along transform plates that slide alongside each other, like the San Andreas Fault in California. There is, however, a convergent plate boundary in the Northwest along the states of Washington and Oregon. This is where the North American plate and the smaller Juan de Fuca plate converge, and it is another oceanic-continental plate boundary. The convergence of the two plates forms the Cascade mountain range, which runs from northern California up into British Columbia. Mount St. Helens and Mount Adams are formed from this boundary.
Alaska
Just south of Alaska is the convergence of the Pacific plate and the North American plate. This is an oceanic-oceanic convergence, when the convergence is completely under the water. The convergence of these plates has pushed land up above the water, forming the Aleutian Island chain -- an arc of more than 300 islands formed from active volcanoes.
Himalayas
The Himalayan mountain range is an example of plates directly converging above water. The Eurasian plate and the Australian-Indian plate are converging right along India's northern border. These two plates were separated by a water mass called the Tethys Sea until India collided with the rest of Asia roughly 55 million years ago. This convergence has resulted in the world's tallest mountain, Mount Everest.
Japan
The island nation of Japan was formed by the convergence of the Pacific plate and the Eurasian plate. This is another example of a dual-oceanic plate convergence, which forces land up above the surface in the form of volcanic islands. This creates the Japanese chain of mountains that includes Mount Fuji.
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