You asked "How do mountains form?" Answer, in two ways.

The first is fairly simple, the second less so. Both arise from the fact that the inside of the Earth is hot, and that heat energy is released to the surface in various ways. The most simple is a hole in the Earth's surface, allowing lava to pour out. If this occurs for long periods of time, it builds up to form a mountain.

Think here of Mt. Shasta, Mt. Hood or Mt. St. Helens. These mountains often stand apart from each other, but note they form a long line. That is because the lava involved is arising from a linear feature caused by the collision of two great portions or "Plates" of the Earth's Surface. The internal heat of the Earth drives these plates, causing them move at a rate of millimeters to centimeters per year.

Off of California, Oregon and Washington, there is a subduction zone - where one plate dives under another. The Plate on which the Pacific Ocean sits is diving under the one bearing North America. When the descending Pacific plate reaches a certain depth - it melts, sending lava back to the surface causing these volcanoes.

But the motion of these plates also plays another role in mountain building. As I described it above, you might think that, when the Pacific Plate collides with the North American Plate, they just slide by each other. Well, they do to a degree, but there is also an immense amount of friction involved, and as the Pacific Plate descends, it drags against the North American Plate, creating compression (the two plates are "butting heads"). That compression has to be accounted for - and it is by the surface of the North American Plate being raised up. (This, by the way, is also what generates Earthquakes, but that is beside the story).

Perhaps a way of visualizing this compression is to think of what happens when you grab one side of a rug and push it against the wall. As you push, you are compressing the rug. The rug cannot respond by "disappearing" - it has to go somewhere. It rumples. The rug is still the same overall length, but it occupies less space.

Many of the world's mountain chains (the Alps, the Andes, the Sierra, the Appalachians) were formed precisely this way -because the collision of two plates caused compression, and the rocks caught in between were folded and thrust up to "get them out of the way".

Finally, and to close a loop.... In many cases, areas of folding and uplift that form mountains are often involved with faults or breaks in the Earth. If they are associated with an active zone of collision where one plate is diving under the other - then these "rumpled rug" mountains will also include volcanoes, where lava finds its way to the surface - The best example here is the Andes.

Mountains result from the collision and friction between tectonic plants in the Earth's crust, or between sub-plates, components of the plates.

For example, the Transverse Ranges in California are formed by a bend in the San Andreas Fault, which cause southwestern California on the Pacific Plate to collide directly with northeastern California on the North American Plate. The Great Basin ranges of which the Sierra Nevada are really just the most westerly and largest - result from the stretching of Nevada, causing blocks of earth bound by faults to sink and others to rise, making mountains.

The Rocky Mountains are more mysterious, possibly having to do with some interaction between the North American Plate and the Pacific Plate in the past. The Appalachians are what are left over from the collision of what was to become North America with what was to become Europe, just as the Himalaya today are formed by the collision of India with Asia.

There is one note here: volcanoes don't form this way. The Cascades formed out of magma rising up from the melting Juan de Fuca plate that is subducting under North America from northern California on northward. This of course means a continental collision, so the Cascades also have mountains made by faults as well, much like the Andes Mountains in South America, just less vigorous (the Andes are much larger than the Cascades).