A volcano forms over a magma chamber deep in the Earth's crust. These magma chambers are connected the mantle, a molten layer of the planet, beneath the crust. Often, these pools of molten rock form along active zones of plate tectonics. The best example of this is the Ring of Fire. All along the Pacific Plate is a rim of active volcanoes that form as the plate is melting as it subducts beneath continental plates.
There are really several distinct kinds of volcanoes. The most well-known, of course, is the classic cone-shaped volcano, called a strato-volcano or a composite volcano. These are the majestic, towering peaks, rising as much as 10,000 feet into the air. They are made up of alternative layers of cooled lava, ash, and rocks thrown out of the volcano in an eruption. Mt. St. Helens was a strato-volcano, as is Mt. Fuji.
Shield volcanoes are also relatively common. Unlike the mountain-like strato-volcanoes, shield volcanoes form a broad dome, composed primarily of lava flows. Shield volcanoes are not particularly explosive, making them the easiest, and safest, to study. The Hawaiian Islands are shield volcanoes, as is Olympus Mons on Mars.
Another type of volcano is the cinder cone. These are simple, small volcanoes, composed mostly of lava ejected in an eruption. A cinder cone has formed in the middle of Crater Lake.
Finally, there are supervolcanoes: calderas. At first glance, it is hard to see a caldera. This is because they are gigantic, covering hundreds to thousands of square miles. They form when a massive magma chamber forms in the Earth's crust. Pressure builds in it, eventually buckling the crust above it. This collapses in on itself, ejecting lava and debris across as much as half a continent. The best example of a caldera is the Yellowstone supervolcano, which covers 1500 square miles. If it were to collapse, the resulting eruption would devastate the western United States and Canada, and have effects world-wide. It would be about 8000 times bigger than Mt. St. Helens. Fortunately, scientists are monitoring it closely, and believe that the Yellowstone caldera has reached an equilibrium point, and isn't likely to erupt in our lifetime.In the next part, I'll discuss more about volcanic eruptions, and the impacts those have on the planet.
For more information of volcanoes, check out Windows to the Universe. For more on supervolcanoes, look at the Discovery Channel website.