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USGS CMG InfoBank: Two Types of Lava

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Comment: 09:51 - 11:10 (01:19)
Source: Annenberg/CPB Resources - Earth Revealed - 13. Volcanism
Keywords: viscosity, "lava flow", eruption, Hawaii, gas, pahoehoe, aa, "lava tube", temperature, "shield volcano", "composite volcano", vent

Our transcription: Viscosity not only influences the explosiveness of an eruption but the character of its lava as well.
The Hawaiians have a word for each of the two main types of lava surface that are influenced by viscosity.
Runny lava, which contains dissolved gases, forms a smooth billowy surface called "pahoehoe", literally "ropy".
Degassed lava is stiffer, tearing apart its crust through its own forward motion.
This forms a rough jagged surface the Hawaiians called "aa", which translated means "a painful surface for walking".
Active pahoehoe flows develop lava tubes.
As the lava slowly cools along its margins it seals itself in a narrow channel, which may crust over. Insulated in a tube the molten lava loses little heat.
This allows it to travel further than it could otherwise, which is one reason shield volcanoes are so broad and gently sloping.
More explosive volcanoes build steep cones because the lava is stiffer and cannot spread as far and because explosive debris simply piles up around the central vent.

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