How do mountains form?

Fold mountains form where plates push together, crumpling and stacking rock layers; the Himalayas rose when the Indian plate collided with Asia, starting about 50 million years ago. Volcanic mountains such as Mount Fuji and Kilimanjaro build up from layers of lava and ash. Fault-block mountains, like the Sierra Nevada in California, form when blocks of crust are pushed up or drop along faults. Dome mountains form when magma pushes the crust up without erupting. At the same time, erosion by rain, ice and rivers constantly wears mountains down.