ADAM HART-DAVIS / SCIENCE PHOTO LIBRARY ADAM HART-DAVIS / SCIENCE PHOTO LIBRARY
Patterns in cooled pahoehoe lava from a Hawaiian volcano. This type of lava is smooth and can flow at 35 mph down a steep slope, while the other common form, known as a'a in Hawaiian, is rough and clinker-like and cannot move more than 30 yards in an hour. Pahoehoe may harden in smooth sheets or may ooze out and solidify in what look like piles of ropes, as seen here. Both kinds of lava have the same chemistry; their silica content is below 50% which means they have low viscosity at temperatures above 1200'C. This allows gases to escape easily which limits the explosiveness of the eruption. This photograph shows lava of the 1974 flow from Kilauea volcano.
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