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September 17, 2009

Dynamic Earth: Volcanoes, Part 2

Volcanic eruptions are what make volcanoes exciting, and dangerous. There are a multitude of different types of eruptions, but I'll just cover some of the basic ones here. But before I can go into types, I should discuss why volcanoes erupt.

As I mentioned in Part 1, volcanoes form where molten rock from the mantle has bubbled up into the crust. This usually happens at subduction zones, but can occur elsewhere. For instance, the Hawaiian islands are located over a "hot spot" in the center of the Pacific Plate. There is a lot of pressure built up in these magma chambers, between the intense heat and the steam created by any water in the chamber. When this pressure reaches a critical point, the rock sealing the volcano breaks, causing an eruption.

There are a number of different types of volcanic eruptions, though they all occur this basic way. In no particular order, they are:
  • Hawaiian: This type of eruption occurs on relatively small cracks or at a main crater. They shoot jets of incandescent lava into the air. They are very impressive, but less dangerous than other types of eruptions.
  • Strombolian: This type of eruption, like the Hawaiian type, is very impressive. Clods of molten lava burst into the air, arcing down to the ground, where they run as fiery streams. These are less dangerous than other types of eruptions.
  • Phreatic: Also known as "steam-blast" eruptions. They occur when water and a magma chamber meet. The superheated water shoots out of the ground as steam, breaking off bits of rock and shooting them into the air. There is no lava involved in this type of eruption. These tend to be weak, though are occasionally quite explosive.
  • Peléan: Also known as Nuée Ardente or glowing cloud eruptions. These occur when a a plume of gas, dust, rock, and bits of molten lava shoot into the air, collapse back down, and roll down the side of the volcano in a fiery avalanche, known as a pyroclastic flow. These can be very devastating if they hit a populated area.
  • Vulcanian: In this types, a tall cloud of white ash and gas forms above the cone of the volcano. Little magma is released.
  • Vesuvian: This types of eruption is similar to the the Vulcanian types. A large amount of gas and ash is released, forming a massive, cauliflower shaped cloud. They can also have pyroclastic flows.
  • Plinian: These are the most powerful volcanic eruptions. They erupt violently, shooting gas, ash, and molten rock high into the air. The ash fallout can travel hundreds of miles from the volcano, and pyroclastic flows often occur as well.
As I mentioned earlier, there are also the eruptions of calderas (supervolcanoes), which do not fall into any of these categories. So far, there are no eye witness accounts of a caldera eruption, as one has not occured in human history. It would be similar to a plinian eruption, shooting debris, ash, and molten rock high into the atmosphere. It would be on a massive scale, though, easily 2500 times larger than the Mt. St. Helens eruption (also a plinian eruption). According to research, the last time the Yellowstone caldera erupted, it sent 600 cubic miles of material into the air, having world-wide effects. The ash in the atmosphere would temporarily cool global temperatures. Most of the Western United States would be severely impacted, and much of it totally destroyed. Fortunately, scientists believe that the Yellowstone caldera is currently at an equilibrium, and so probably will not go off in our lifetimes. Nevertheless, it is an area under constant monitor. For, while volcanic eruptions are spectacular, they are also deadly. There is still a lot to learn about their effects on life and on the Earth as a whole.

For more information on eruptions, visit Windows to the Universe, the USGS, and the Extreme Science/Extreme Earth webpage.