The Rocky Mountains were formed aproximately about 2 billion years ago. Sediments like clays, silts, and sands were deposited in an ancient sea and mixed with lava flows. Large amounts of pressure and heat caused these sedimentary rocks to change into gneiss and schist. Then, between 1.4 and 1.7 billion years ago, magma moved its way through the cracks of the metamorphic rock and dissolved some of the gneiss and schist. However, unable to reach the surface due to being blocked by several layers of sediments, the magma slowly cooled, becoming granite and pegmatite (Smithson 6).
Over the next billions of years, during the Proterozoic Era, the North American Plate began sliding over the Pacific Plate. Enormous pressure between the two plates pushed them together, causing the rocks of Rocky Mountain National Park to be caught in a vise and smashed together in folds. Pieces of the Pacific Plate were far beneath the North American Plate and were melted, causing the whole region to rise.
The original mountains were only small islands in the ancient sea. They were slowly eroded away and over time, were once again submerged underwater. From there, the present day Rocky Mountains emerged from the sea 75 million years ago, during the Cretaceous Period. The layers of sediments deposited during the Paleozoic Period were eroded away, exposing the schist, gneiss, and granite. Once again, they were eroded down to hills. About 37 million years ago, during the Tertiary Period, pressures caused the mountains to rise once more, shifting the land so some areas rose and others fell, helping to give the Rockies their present characteristics today.
