Italy's Volcanoes: The Cradle of Volcanology

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Frequently asked questions about Etna
How fast do Etna's lavas move?


Eruptions of Etna are often seen in television news services to emit broad streams of brightly incandescent, fluid lava which spill down its flanks to distances of many kilometers. Yet the lavas of this volcano are much more viscous than those of the Hawaiian volcanoes or of Piton de la Fournaise volcano on Réunion island in the Indian Ocean.

The velocity at which lavas move depends on several factors, such as (1) the distance from the eruptive vent(s), (2) the effusion rate, (3) the steepness of the slope on which the lava is flowing, (4) the temperature of the lava, (5) the crystal content of the lava, (6) the presence (or absence) of lava flow channels and/or tubes. Near its source the lava generally flows at a speed of up to several meters per second but rapidly slows as it extends further. Flow within channels or tubes (which generally develop within days to weeks if supply of lava from the eruptive vents continues at a more or less regular rate) can be nearly as fast as at the source, even at a distance of hundreds of meters or even kilometers.

The advance of a lava front, on the other hand, shows an exponential decrease in velocity with increasing distance from the source (or the outlet of a lava tube). At a distance of hundreds of meters or kilometers from the source, a lava front generally advances at a few meters or a few tens of meters per hour, which is slow enough to permit observations at close range and, if a lava flow is encroaching on buildings or population centers, to save human lives and often also property such as households and parts of the threatened buildings. For this reason lava flows at Etna almost never cause deaths and rarely cause injuries. When a lava flow approaches and consumes man-made structures, this process is often painfully slow, and owners of homes or cultivated terrains often stay until the very last moment to watch how the lava does its work of destruction, hoping that a miracle will stop the advance of the lava.

Next Question: How do the people living near Etna feel about "their" mountain?

Copyright © Boris Behncke, "Italy's Volcanoes: The Cradle of Volcanology"

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