Volcanoes are often dramatic sources of new crust being formed. They occur both underwater and on land, and are most often along the boundaries of tectonic plates, although they can form in random hot spots; places where the Earth's crust is thin and lava periodically leaks out. The Pacific Rim, spanning from Indonesia north through Japan, and then down from Alaska through to Chili, earned the name "Ring of Fire" from the chains of volcanoes that trace its boundaries. While most people imagine volcanoes as essentially exploding mountains with lava, volcanoes actually come in several different forms, and are often calmer than the explosive eruptions that are hyped in the news.
Pu'u 'O'o, a Volcano cone on Kilauea, Hawaii. Click here for original source URL
The simplest type of volcano is the cinder cone volcano. These structures consist of a central vent that feeds magma up through a crater. The most famous of this type of volcano may be Paricutin in Mexico. This volcano emerged from a fissure in a corn field on February 20, 1943 and was witnessed by the farm owners. During that first year it grew more than 1000 feet (more than 300 meters), replacing the farm and neighboring villages with a volcano that would continue to erupt and grow for 9 years. As the lava rose to the surface, it simply cascaded out of the central vent and formed the cone for which this class of volcano is named.
The tallest volcanoes are composite volcanoes (or stratovolcanos). These systems have of a shell of lava, ash, and debris that form repeating layers. While they have a central crater (or in some cases multiple craters), their eruptions can come out from the top, or ooze out through ducts in their sides. Mount Fuji in Japan is one example of a stratovolcano. These systems are able to grow so tall because the layering of lava can form strong ribs that support the growing mountain. The volcanoes can also have some of the most explosive eruptions, as was seen in Mount St Helens (Washington, US) and Mount Penatuba (Philippines). During an eruption they may simply spew gas, or the pressure of built up hot gases may throw car-sized boulders and lava into the air as they explosively release. Associated pyroclastic flows — mixes of water, lava and mud — can travel at speeds of 100 miles per hour down the sides of these mountains and are often responsible for the bulk of eruption associated deaths. It was a pyroclastic flow that entombed the inhabitants of the Roman town of Pompei when Mt. Vesuvius erupted explosively in 79 A.D.
The third primary type of volcano is the shield volcano. These systems have a magma reservoir that is able to escape through a number of vents. Mauna Loa Volcano in Hawaii is one of the largest shield volcanoes, and many of Iceland's volcanoes are also shield volcanoes. Most eruptions from this type of volcano consist of the slow extrusion of lava through the vents and associated fissures. Explosive ash is typically associated with water getting mixed into the lava, such as occurred when Iceland's Eyjafjallajökull erupted and melted the glacier above it. As water sank through the volcanoes fissures, steam built up to explosive pressures and blasted ash thousands of feet into the atmosphere.