At one time, North America was a mostly flat land, largely covered by a shallow sea. The deposits of that ocean can still be found today, but the continent had few mountains. However, America now has several mountain ranges, including the tall and majestic Rockies. It is only natural that a lot of people wonder how the Rockies were formed.
Uplift and volcanoes
Mountain chains are usually created in one of two ways: Uplift and volcanic activity. In the latter, a string of volcanoes erupt, ejecting millions of tons of rock and lava. This produces typically cone shaped mountains, which can reach thousands of feet in height. One of the longest mountain ranges on Earth has been, and currently is being created under the ocean. This is called the mid-Atlantic ridge. Most people don't know about it, simply because it is normally hidden under thousands of feet of ocean water.
There is indeed volcanic activity near the Rockies as well, most notably around Yellowstone National Park, with its hot springs, geysers and fumaroles or smoke and steam vents all giving proof that there is a magma pocket under the ground in this area. Despite this volcanic evidence, the Rocky Mountains weren't formed by volcanic activity. They were formed primarily by uplift.
Uplift is exactly what it sounds like. The land is raised in some places by extreme forces and pressures. Whereas volcanoes can raise cone shaped mountains, as it did in the Cascade range in North America, uplift is often more dramatic, though also not as fast, normally. The peaks of the mountains tend to be higher and more jagged, further sculpted by erosion.
The underlying cause of both volcanoes and land uplift is mostly, though not always, due to plate tectonics. Greatly simplified, the idea is that the entire world we live in and the crust beneath the sea are all setting on shifting plates. Sometimes these move away from each other, or they may rub against each other and form fault zones, such as the San Andreas fault in California.
However, sometimes the plates collide with each other. If one is lighter than the other, it slips over the heavier one. This is called a subduction zone. However, if both plates are close to the same density, 'wrinkles' in the earth's crust are formed. These are mountain ranges, and include the Rockies. They also include the Himalayas.
The action is slow, but it doesn't stop. The collision that caused the Rocky Mountains is continuing, and the mountains are still growing in places. That should not be thought to mean that it is in all places. While the Himalayas are still growing as a whole, the Rockies aren't. This is a mountain range that is seeped in antiquity, by human terms. The forces that caused the mountain chain still do continue, however they are not face on, like they once might have been.
The Rocky Mountains were cause mostly by continental uplift, caused, in turn, by the collision of two massive continental plates. Wind and water further shaped the spectacular mountains seen there today. Now towering over a mile above sea level in places, it is hard to imagine that this was once an inland ocean at sea level.