What is the average speed of lava?

rocky Ben

Dear Ben,

Lava flows travel at different speeds. 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.

The fastest measured lava flow speed (of the flow front, not of lava flowing in a pre-existing channel) was about 60 kilometers per hour. This occured at Nyiragongo volcano, in Zaire. The lava at Nyiragongo is a very fluid type of basalt, and during that eruption, a lava lake drained out a series of cracks in the flank. This allowed a large amount of lava to be fed directly to the flows, and combined with the very fluid nature of the lava, led to the rapid flows. These flows killed almost a hundred people.

Steve Mattox, University of North Dakota 

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