What are plate tectonics and how do they relate to volcanoes?

Ali

Hi Ali,

Plate tectonics is the idea that the top 100 km or so of the Earth's surface is not a single shell like an egg, but rather it is made up of a number of large pieces (more like a cracked egg). Additionally, these pieces move around. In some places they grind sideways such as where the pacific plate grinds against the north american plate to form the San Andreas fault system. These are called transform plate boundaries and volcanoes are not found along them.

In other places the plates are actually created (at "divergent boundaries") such as mid-ocean ridges and the East African Rift. Here, two plates move away from each other and magma from below wells up to fill in the space. When this magma cools it has become the newest part of the plates. It is then pulled apart and still younger magma wells up to fill in the space. It is not clear whether the rising magma causes the plates to move apart or if the plates first move apart and the magma quietly flows upward to fill in the gap. Perhaps it is a combination of these two. Anywhere that lava is active on the surface of the Earth (even if it is under water) is considered to be a volcano. Some folks consider the 70,000 km-long system of mid-ocean ridges to be a single volcano. The East-AFrican rift is an on-land example of a divergent boundary. In addition to its extensive system of rifts there are actual volcanoes along its length.

The third type of boundary is that most closely related to volcanoes, and these are called convergent boundaries. Most convergent boundaries involve at least one plate being an oceanic plate, and this plate diving under the neighboring plate as they are moved together. You can have an oceanic plate diving under a continental plate (such as where the east-moving Nazca plate is diving under the South American plate), or you can have an oceanic plate diving under another oceanic plate (such as is happening along the Tonga-Kermadec arc). The plate that is forced down starts to heat up, and pretty soon all the fluids (like water) that are part of the sediments that are also carried down, start to boil and move upwards. These upward-migrating fluids cause melting of the overlying rocks, and this melt migrates to the surface to erupt at a volcano. All the volcanoes around the Pacific ocean rim are such subduction-zone volcanoes.

Hopefully this has given you a taste of a fascinating subject, Plate Tectonics.

Sincerely,

Scott Rowland

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