Numerous mountain ranges were formed by the collision of two continental tectonic plates, including the Himalayas, the Alps, the Appalachians and the Atlas mountains. It is also thought that the Rocky Mountains formed in part due to small pieces of land on the Pacific oceanic plate colliding with North America.
The Himalayas are the most obvious example of a collision between two continental plates. This range began forming approximately 40 to 50 million years ago, as the Indian plate crashed into the Eurasian plate. The Atlas and Alp ranges in Europe were similarly formed when the African plate collided with the Eurasian plate, while the Appalachians formed more than 300 million years ago when all of Earth's continents came together to form the Pangaea supercontinent.
When two continental plates collide, the rocks forming their crust fold together and pile up to create mountains, as the crusts are both of a similar density and thickness. However, when an ocean plate collides with a continental plate, the lighter oceanic crust slides underneath the continental crust. The crust then melts to create magma as it slides deeper into the Earth's mantle, which eventually leads to the formation of volcanoes at these convergent boundaries. The majority of the world's active volcanoes are found at divergent boundaries.