Volcanoes form at both convergent and divergent boundaries.
The formation of volcanoes requires a specific set of conditions. These conditions are only present in certain areas of the world, and thus volcanoes only form in these areas. Volcanoes occur at both convergent (or destructive) and divergent (or constructive) plate boundaries, as well as over geologic hot spots where lava emerges in the middle of a plate.
Constructive (Divergent) Boundaries
Divergent boundaries are areas where two tectonic plates are moving apart from one another. New crustal material is formed in the interior of the earth, and it is pushed up to the surface as magma. The magma cools and forms new crust. The mid-Atlantic ridge is one such diverging plate boundary, where the North American and Eurasian plates diverge. While most of this ridge is underwater, it does cut through Iceland, forming a line of volcanoes that are still active. Magma that forms at divergent boundaries is usually made of basaltic (from basalt) or rhyolitic (from granite) rock.
Hot Spots
The Hawaiian Island chain is an example of volcanoes formed over a hot spot in the mantle. This hot spot is a place in the Earth's crust where a plume of hot mantle rises to the surface. As it rises, it turns to magma, which is known as lava when it erupts. Hot spots are stationary, but the plates above them move. As a plate moves over a hot spot, volcanic islands form as a chain. In the Hawaiian Islands, the Big Island is the most recently formed, and it is still volcanically active. The other islands in the chain were formed as the Pacific plate moved over the hot spot. Thus, each island as you move away from the Big Island is progressively older.
Destructive (Convergent) Boundaries
Most volcanoes you see on land masses form due to the action of destructive, or convergent, boundaries between two or more plates. At convergent boundaries, one plate dives underneath the other, called subduction. As a plate subducts back into the earth, it passes through the outer layer, or lithosphere, into the athenosphere. As it is pushed deeper into the earth, it heats up, and the crust turns to magma. This magma then moves back up through the crust and emerges on the surface as a volcano. The Pacific "Ring of Fire" is a ring of volcanoes found all along the Pacific rim due to the action of convergent boundaries. Island arcs, such as the Aleutian Islands, are formed in this fashion. Magma formed at convergent boundaries is most often made of andesitic (igneous) rock.
Types of Volcanoes
There are many different types of volcanoes that form. Shield volcanoes have a gentle slope of 5 to 10 degrees, are circular in shape, and have nonexplosive eruptions. Stratovolcanoes are steeper, 6 to 10 degrees at the base and up to 30 degrees at the apex. These sometimes have a crater that formed due to explosive activity of a central vent. Lava in these volcanoes is more viscous (thicker) than in shield volcanoes. Tephra Cones are steepest, at 25 to 30 degrees but have the smallest volume. These often occur in large groups of up to 30 separate cones in one area.
Tags: convergent boundaries, over spot, both convergent, into earth, turns magma
