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What kinds of volcanic eruption don't produce lava flow?
Answered by Jennifer Horton and Discovery Channel
  •  Jennifer Horton

    Jennifer Horton

  • Discovery Channel

    Discovery Channel

  1. During an episode of activity, volcanoes may erupt in a variety of distinct ways, from a very slight oozing of slow-moving lava to a violent eruption followed by a fast-moving, pyroclastic (comprised of a quickly moving river of hot ash, rock fragments and gas) flow. Some volcanoes may even exhibit a variety of eruption types over the course of a single episode. The primary factor that determines how a volcano erupts is the type of magma that is supplied to it. The three main types of magma, determined by their chemical composition, are rhyolitic, basaltic and andesitic. Of these three types, rhyolitic is the most viscous -- that is, it is the most resistant to flow. Rhyolitic magma, which is high in silica and has the lowest temperature of the three types -- around 650 to 800 degrees Celsius (1200 to 1472 degrees Fahrenheit) -- also tends to have the highest gas content.

    While you might think a magma that's resistant to flow would be a good thing, eruptions of highly viscous magma can actually be the most dangerous because of the amount of gas that builds up. All magmas contain gas, and as the magma moves toward the Earth's surface, the gas bubbles begin to expand and release pressure. But a high viscosity prevents the gas from being able to expand as easily, resulting in a buildup of pressure. When this magma reaches the surface, all of that built-up pressure comes out in force, causing a violently explosive eruption. These violent eruptions can spew pyroclasts (hot fragments) and tephra (volcanic ash) for miles around. The gas and tephra shot out by an explosive eruption of this nature can produce an eruption column rising up to 28 miles (45 kilometers) into the air [source: Nelson]. If, however, the eruption column collapses, a pyroclastic flow may form. The most dangerous type of volcanic eruption, these flows can reach temperatures of 1500 degrees Fahrenheit (816 Celsius) and reach speeds of up to 450 miles (724 kilometers) per hour [source: USGS].

    Effusive eruptions, on the other hand, which involve an outpouring of lava onto the ground, tend to be less explosive and generally move slowly over the Earth's surface, giving people plenty of time to get out of the way. Though they can damage wildlife and other stationary obstacles in their path, they generally don't threaten people's lives. With the explosive eruptions characteristic of more viscous magma, instead of oozing out as a lava flow, the magma shoots into the air, breaking into pieces and cooling as it falls.

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  2. Strombolian volcanic eruptions are characterized by the high viscosity (flow resistance) of their lava. As a result, they don't throw a lot of lava high into the air - - only about 50 to 100 feet (15 to 90 meters) - - nor do these eruptions produce a lava flow. They generally just spit out some ashy tephra (volcanic material). However, because the lava is so thick, the gas pressure needs to build up to a high level before the volcano blows. So when a Strombolian eruption happens, it comes as a series of short bursts, each with a monstrous boom. In short, a Strombolian eruption's bark is worse than its bite.

    Vulcanian volcanic eruptions occur in short bursts and have little lava flow, due to the high viscosity (flow resistance) of their magma. Vulcanian eruptions also often shoot football-sized pyroclastic rocks into the air. Because Vulcanian eruptions contain highly viscous magma, they require a high level of gas pressure before they erupt; the force of the gas pressure being released causes a sustained eruptive column.

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