Abstract
Pyroclastic fall deposits, their eruptions and the physical controls on their formation are now considered in detail. The classification scheme of G. P. L. Walker (1973b) characterises fall deposits by their dispersal and degree of fragmentation, and this approach is used as a framework for our description. Important volcanological assessments can also be made, because these two parameters are related to the height of an eruption column and the nature of the fragmentation process. In this chapter we also consider some surprising features of the Mt St Helens ash-fall deposits and the implications of these for plinian eruptions. We also focus on the characteristics of distal silicic ash-fall layers. Finally, the properties of welded air-fall tuffs which occur near the vent on a number of volcanoes are described. Criteria for distinguishing these from welded ignimbrites (Ch. 8) and a thermal facies model for pyroclastic fall deposits are also presented.
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