Stratovolcanoes are constructed from multiple eruptions of this nature, creating a layered and very high structure. Lava domes are formed by the slow extrusion of highly viscous magma. They are usually formed by a large explosion caused by the built-up pressure of the gases trapped in the thick molten rock, but after that, when the gas pressure is decreased, the magma oozes out slowly. Before 1980, Mount Saint Helens was an example of a stratovolcanoe. Since its massive explosion in that year, a lava dome has grown up inside the crater that that event left.
Volcanoes can occur solitarily or in belts and chains.
The location of these volcanoes splits them into three groups. Rift volcanoes, subduction volcanoes, and hot spot volcanoes. This has a lot to do with plate tectonics, or moving of the segments of the earth's lithosphere. They fill the separations with lava flows, creating new sea floor and mountain ridges beneath the waves. They create ridges and peaks.