The Colorado River has flowed through the Grand Canyon for about five million years, and it continues today to slowly cut its channel deeper into the rock. In the deepest part of the canyon, the water flows past its oldest rock, known to geologists as the Vishnu Schist. This dark metamorphic layer is 1.7 billion years old and all that remains of a mountain range that scientists estimate had peaks as tall as the Rocky Mountains or the Himalayas, if not taller.
Metamorphic rock forms from any type of parent rock whose texture and chemical makeup is changed due to intense pressure and heat. The pressure comes either from the weight of overlying rock or from compressive forces generated when rocks are deformed during mountain-building episodes. These forces cause the temperature of the rock to verge on its melting point, so that the solid rock "bakes" into another form. Because the size, spacing, and shape of the parent rock all change under heat and pressure, metamorphic rock is extremely hard and resists erosion better than other rock types.
Mountains are built by tectonic processes that cause portions of Earth's crust to rise or collide. These processes are fueled by heat escaping from Earth's interior. Mountain ranges have large roots that counter the enormous mass of the range above Earth's surface. As erosive processes such as wind and water wear down mountain ranges, their roots are buoyed upward by the underlying mantle. Gradually, rocks from deeper and deeper levels inside Earth rise to the surface and are worn away. Over about 450 million years, the range that overlay the Vishnu Schist eroded into the sea to form a level plain. As climatic conditions changed and coastlines advanced and retreated over the area, new rock layers were laid down upon the roots.
Though they may seem like permanent features, mountains do not last forever. Most major mountain ranges both present and past began as sediments -- the by-products of erosion of even older mountain ranges. This is part of the rock cycle in which rocks are uplifted, distorted, and folded, and eventually leveled again by erosion.