ABSTRACT
Lava flows across the Earth’s surface when fluid magma erupts and runs down slope under the influence of gravity. Some flows form ribbons occupying narrow valley floors; others may gush out in such volume that they bury whole landscapes. A single lava-flow may erupt for days, as is common in Iceland, or weeks, as on Etna, where half the emissions, nevertheless, last less than 25 days. Eruptions lasting for several months are rare. Movement, cooling and solidification occur simultaneously on different parts of a flow, and the rate of discharge and the speed of cooling largely determine its length and surface appearance. Thus, the longest flows are emitted in great volumes at such speed and at such high temperatures that they can travel long distances in a fluid state before cooling and solidification induce deceleration. The shortest flows are extruded slowly, and relatively coolly, and are soon brought to a halt by rapid solidification. Generalizations are hard to make, however, even among similar basalts. Etna, the most diligent purveyor of basaltic flows of any volcano in Europe during historic time, erupts its lavas, on average, at rates of 25m3 per second, forming flows reaching 7.5km in length and 12m in thickness. However, several Icelandic flows, and many on the Columbia plateaux, are over 100km long, but the longest yet discovered in Auvergne travelled only 50km down the River Alagnon valley. Basaltic flows rarely exceed 1 km3 in volume, but the Roza basalts on the Columbia plateaux cover 40000km2. On the other hand, some lavas are so viscous that they pile up near their vent and cover less than 1 km2. The rhyolites of the Pietre Cotte flow, for example, solidified in a chunky tongue, 1 km long, on the 20° slopes of the Fossa cone of Vulcano in 1739.
