Plate tectonics is the theory that the Earth's crust is made up of several large pieces, called tectonic plates. It was the movement of these plates that eventually caused the formation of the Himalayas. Though plates move incredibly slowly, only 1 to 20 cm per year, over millions of years the collision of two or more tectonic plates can cause mountain ranges to occur. Factors such as weather erosion also determine the shape of these mountain ranges, though not their height.
Continental drift is a term used to explain the constant movement of entire continents. All of Earth's continents were once joined in a single landmass called Pangaea. Slowly, continents began to break apart and move due to Earth's shifting tectonic plates. The movement of India, then a continent similar to Australia, caused the Himalayas to form when India crashed into the continent of Asia.
India traveled north to collide with Asia, which was drifting south. In doing so, the shallow sea known as Tethys, which had been separating the two continents, disappeared. The soft sea bed was thrust upward by the collision, and today forms the peaks of the Himalayas. This kind of plate collision is called a convergent boundary because both plates are colliding head-on. Because the convergent boundary is between two continental plates, it is known as a orogenic belt.
The Himalayas are still rising, because the collision between India and Asia is still occurring. The Himalayas continue to grow at a rate of about 5 mm per year. The convergent boundary between Asia and India remains unstable, causing the growth. It also causes the earthquakes, which remain relatively common in and around the Himalayas.
