Mountain Building

The theory of plate tectonics helped geologists understand why mountains form in some regions but not in others. Geologists found that most mountains form along plate boundaries.

Some mountains form when two plates with continental crust at their edges collide along a convergent boundary. Neither plate is subducted during such collisions. Instead, the crust buckles, folds, and thickens, pushing up tall mountains. The collision of the Indo-Australian and Eurasian plates produced Earth's highest mountain range, the Himalayas. Mountains may also form when an oceanic plate collides with a continental plate. The collision of the Nazca and South American plates produced the Andes, shown in Figure 31.

Mountains can also form along diverging plate boundaries. The mid-ocean ridge system forms one long chain of mountains on the ocean floor. In places, the mountains of the mid-ocean ridge rise above sea level. One example is the island of Iceland in the North Atlantic Ocean. As you will read in the next two sections, plate tectonics explains not only mountain building, but many other changes on Earth's surface as well.

Figure 31 The Andes, which ex tend along the western side of the South American plate, have risen as a result of a collision between that plate and the Nazca Plate. Inferring What type of plate boundary exists between the South American and Nazca plates?

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