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Different Ways Volcanoes Erupt

The Earth's crust is surprisingly thin, considering the size of the planet. Just a few miles below the surface, a mantle of liquid magma hundreds of miles thick seethes in convection currents that carry heat up from the deeper molten nickel-iron core. Sometimes the magma actually reaches the surface, emerging as lava in a volcanic eruption. Exactly how it does this can vary greatly depending on local conditions and circumstances.

Hawaiian Eruptions
- Where the fluid lava simply oozes or, if the pressure is high enough, squirts through a hole in the Earth's crust, the result is a shield volcano like those that make up the Hawaiian Islands. These are among the least violent sorts of volcanic eruptions, and can go on for months or years. While the most famous examples are in Hawaii, they do happen elsewhere, such as an eruption at Oshima, Japan, in 1986. Interestingly, the largest known volcano in the solar system, Olympus Mons on Mars, is a shield volcano, likely built up by Hawaiian-style eruptions.
Strombolian Eruptions
- Named for Mount Stromboli in the Mediterranean, Strombolian eruptions are similar to Hawaiian eruptions, but somewhat more explosive as there is more high-pressure gas contained in the magma feeding them. The gas drives impressive geysers of lava, but takes a while to build up between spurts, making Strombolian eruptions more episodic. Even so, the fountains of lava are not especially violent, and such volcanoes can remain active for centuries, throwing up a spray of lava a few times every hour.
Vulcanian Eruptions
- While Hawaiian and Strombolian eruptions involve very fluid magma with relatively little gas trapped in it, volcanoes like Sakurajima in Japan and that on the island Vulcano (from which the very word volcano originates) are driven by a thicker magma which makes it difficult for trapped gases to escape to the surface before the magma itself gets there. When it reaches the relatively lower pressure near the surface, the expansion of this gas blasts rock and lava high into the air, to rain down as volcanic ash.
Plinian Eruptions
- The famous Mt. Vesuvius, which buried the Roman towns of Herculaneum and Pompeii in A.D. 79, was described in detail by Pliny the Younger, whose uncle Pliny the Elder was one of the eruption's many victims. Combining a high gas content with rapid upward magma flows, Plinian eruptions -- named for Pliny the Younger -- are particularly violent, blasting ash high into the stratosphere but also shattering the solid rock of the volcano cone as well. Mt. St. Helens in 1980 was a Plinian eruption.

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