Although fast flowing in their upper reaches, the distal snouts of aa flows often advance slowly, like shuffling slag heaps. This 1971 Etna lava illustrates the height of the flow front (5-7 meters) and its coarse rubbly surface, with blocks over a meter in diameter. These conceal and insulate the hot core of the flow, which can just be glimpsed glowing red-hot in one or two places, for example in the crevice between the lady in a red poncho and her companion in a brown jacket. When photographed, the flow was creeping forward at a few meters an hour. Flows from this eruption ruined the skiing on Etna, which had previously boasted many smooth ash-covered slopes (left and foreground). Some black mud flows triggered by the eruption can be seen in the middle distance at left of the flow. (Fig. 6.18, 7.10).

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