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
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
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
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.