Geologically, the origin of the Himalayas is the impact of the Indian tectonic plate. This travels northward at 15 cm per year, and joined the Eurasian continent about 40-50 million years ago. The formation of the Himalayas resulted in the lighter rock of the seabeds of that time being lifted up into mountains. An often-cited fact used to illustrate this process is that the summit of Mount Everest is made of marine limestone.