How fast does lava flow?

The answer to your question depends on the composition of the lava and the slope it is going down.   Pahoehoe, the smooth form of basalt usually travels very slowly–at speeds of only a few meters per hour, averaged over the whole flow. ‘A’a, the rough clinkery form of basalt travels faster, ranging from a few hundred meters per hour up to 10 km per hour on steep slopes. When you get to lava compositions such as andesite, dacite, and rhyolite, where the viscosities are really high, the lava flows become very slow–perhaps moving at average speeds of only a few meters per day. In Hawai’i the fastest flows we’ve recorded were those of the 1950 Mauna Loa eruption. These were going about 6 miles (10 kilometers) per hour through thick forest. That was the velocity of the flow front. Once the lava flows became established and good channels developed, the lava in the channels was going at more like 60 km/hour! On January 10,1977, a lava lake at Nyiragongo, a stratovolcano in Zaire, drained in less than one hour. The lava erupted from fissures on the flank of the volcano and moved at speeds up to 40 miles per hour (60 km/hr).


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