There are four different classifications for volcanoes: The Composite, Shield, Cinder Cone, and Lava Cone.
Shield volcanoes are broad and gently sloping. They can come in many different sizes. They are composed of layer upon layer of solidified, basaltic lava. Their eruptions are quiet in nature.
Created by hotspots, like in the case of Mauna Loa above, as well as rift and subduction zones. They are formed by constant, slow moving lava that solidifies.
Composite volcanoes are steep sided and symmetrical. They are the tallest volcanoes at over 12,000 feet. They are formed from alternating layers of lava, pyroclastics and hardened mudflow deposits.
Composite volcanoes are formed at subduction zones where magma is driven up through the vent bringing silica-rich crustal material with it. The volcano is formed from alternating layers of pyroclastics and lava.
Lava domes are small, standing at less than 2000 feet. They have explosive eruptions of rhyolitic material. The thick, viscous lava solidifies often causing irregular shapes.
When lava bubbles up inside the crater left when a composite volcano blows its top.
Cinder cones are small, steep-sided volcanoes with a maximum height of 1500 feet. They are made up of loose pyroclastics and will ocassionaly produce lava flows.
When gas rapidly expands inside the cinder cone the top blows in an explosive eruptions leaving a crater behind. Lava flows up out of the crater and down the sides solidifying as it goes to create its conical shape.
When an explosive eruption occurs it ejects a number of different gases into the atmosphere. These can be as harmless as water vapor, but also contain carbon dioxide, sulfur dioxide, hydrogen sulfide, and flourine. This can cause a number of reactions including turning the sulfuric gases into an acidic mist that turns into acid rain. Excess carbon dioxide would also likely exacerbate the greenhouse effect.
There have been three super eruptions from the Yellowstone area, the last being 640,000 years ago. The area sit on a hotspot under North America. Since the last super eruption there have been 80 or so smaller scale eruptions. Scientist believe that there is a relatively small chance that we will see another super eruption in the next thousand years; the chance being about one in 730,000. If there were to be an eruption their would be death and destruction within a five mile radius, and in Wisconsin we would have to wear gas masks to survive.
Molten rock was thrust upward during the Cretaceous period under what is now South Dakota. Sedimentary rocks collected around the peaks because the area was a seaway at the time. Erosion has removed the sedimentary rocks leaving the granite and other igneous rock behind.
The basaltic lava that comes from the hotspot under Hawaii is slow moving, whereas the lava the comes from the Yellowstone hotspot mixes with silica from the Earth’s crust. This causes the lava to become more viscous, and causes gases to form. Pressure builds to form explosive eruptions.