The rheological behaviour of basaltic lavas

Abstract

Used in the nineteenth century as a cornerstone for the ‘Craters-of-Elevation’ hypothesis (Chapter 1), the fact that the fronts of some of Etna’s solidified lavas may be seen resting upon steep slopes, with dips of up to at least 35°, has since helped to initiate a fundamental advance in the understanding of lava flow evolution. Implicit in the earlier association between lava attitudes and ‘Craters-of-Elevation’ was the assumption that lavas, like large bodies of water, would continue to flow down an incline until being ponded behind an obstacle or within a depression, or until reaching a horizontal surface. In support of this contention, De Beaumont (1834) cited active Etnean lavas as examples of those which had been rarely observed to consolidate upon slopes with angles greater than 5° and which, in passing over steeper gradients, left behind only superficial veneers of solidified lava or coatings of scoriaceous debris. Hence the existence of old lavas lying on steep slopes was presented by De Beaumont as evidence that the underlying surface had been tilted to a higher angle after the emplacement of the flows.

‘This fiery and burning deluge immediately spread itself to above six miles in breadth, seeming to be somewhat of the colour of melted and burning glass; but, as it cools, becomes hard and rocky, and everywhere in its passage leaves hills and pyramids of that matter behind it.’

(The Earl of Winchilsea, 1669)