Geography for one thing. Volcanoes are found all over the world, under the ocean and on land. They erupt because of pressure from the gases that have been produced through the melting of rock (magma). As these gases build up, they have to be released and how they are released determines the type of volcano. There are shield volcanoes like the ones found in Hawaii. The lava (magma on the surface) flows slowly out of these vents and build up into mountains and sometimes eventually islands. On the Big Island, Kilauea has been active since 1984 and there are webcams located on the Pu'u O'o vent and Halema'uma' u vent http://hvo.wr.usgs.gov/cams/POcam/. This is a relatively "safe" volcano because you have time to leave the area to get to safety as opposed to volcanoes like Mt. St. Helens. That is a stratovolcano. When they erupt, it is usually very violent and destructive. There is also a webcam on Mt. St. Helens, but I do not have that website, but should be easy to find from the Hawaii Volcanoes Observatory site, as they list other volcanoes also the border. Stratovolcanoes can have lava flows, but usually when they erupt, it consists of a pyroclastic flow which is hot ash, steam, rocks, that swiftly move out from the volcano and destroy anything in its path. Mudflows called lahars also may result especially if the mountain had a glacier or snow near its summit and these are very dangerous and destructive also. You can go to About.com and visit the Geology newsletter. There is some information on volcanoes there along with great info on other geologic formations. I think you will enjoying visiting the Hawaii Volcanoes site and check out Mauna Loa, which is not only the largest volcano in the world, it is largest mountain in the world...taller than Mt. Everest by several thousand feet because the bottom of the mountain is on the ocean floor and Mt. Everest start at sea level. I hope this helps a little.
