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How Are Volcanoes & Plate Tectonics Connected?

The outer crust of the Earth's surface is known as the lithosphere. The movement of the lithosphere on the earth's surface is also known as plate tectonics. The surface of the earth is broken up into plates that are responsible for seismic activity, including earthquakes and volcanoes. Volcanoes form into mountains as erupted debris collects over millions of years. The formation of volcanoes and their activity is directly connected to plate tectonics.

Plate Tectonic Basics
- The tectonic plates are composed of the uppermost crust of the continents and the ocean floor, also known as the oceanic crust. These plates are approximately 50 miles thick and sit atop of molten areas. Each plates moves at the rates of a few centimeters per year. The areas at which the tectonic plates meet are known as plate tectonic boundaries. There are three types of plate tectonic boundaries, including divergent, convergent and transform boundaries, which describe how the plates pull apart, collide and slide past one another.
Volcanoes
- Volcanoes are found at both convergent and divergent plate tectonic boundaries. These places are also known as hot spots. Magma forms in these hot spots, where heat and pressure cause it rise up, forming a volcano. Volcanoes are found in the subduction zone of convergent plate boundaries. When tectonic plates converge, it means that they push together, recycling the broken material of the plate's collision back into the earth. The volcanoes themselves do not subduct as the plates collide. Divergent plate boundaries are areas where the tectonic plates pull apart and form volcanoes in ocean basins.
Island Arcs
- Island arcs are chains of volcanoes that form islands in the ocean. Island arcs form as the result of two oceanic plates converging or colliding together underwater. After millions of years of eruptions, the debris that is spewed from the volcano piles up and hardens. The underwater volcano continues to grow in size until it rises above the surface of the water. These volcanoes tend to occur in chain-like sequences underwater, eventually forming arcs of island volcanoes.
Intra-Plate Volcanoes
- Although most volcanoes occur at the boundaries of tectonic plates, there are some exceptions to the rule. Some volcanoes do not occur at plate boundaries, but rather form on the plates themselves. These volcanoes are called "intra-plate" volcanoes. Intra-plate volcanoes are typically found in linear chains on the plates of the oceanic crust. These volcanoes can be active, just like the volcanoes found at the plate tectonic boundaries.

Nature