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Geological Society, London, Special Publications; 1984; v. 16; p. 29-35;
DOI: 10.1144/GSL.SP.1984.016.01.03
© 1984 Geological Society of London

Processes

Subaqueous pyroclastic flows: their development and their deposits

Eizo Yamada

Geological Survey of Japan, 1-1-3 Higashi, Yatabe, Ibaraki, 305 Japan

Subaqueous pyroclastic flows are generated by eruptions in shallow water or close to a shore. Their flow mechanism is similar to that of their subaerial counterparts, except that they incorporate steam and not air. Near the source area, their deposits are non-sorted and resemble subaerial flow deposits. In some cases evidence of high temperature is recognized. Within a short distance (less than a few km) they are sorted and five subdivisions can be determined. In upward succession these are: (i) massive graded pumice tuff; (ii) parallel-laminated pumice tuff; (iii) parallel-laminated sandy tuff; (iv) parallel-laminated fine tuff; and (v) massive very fine tuff. With increasing distance from the source, and laterally from the main direction of the flow, the lower divisions gradually die out. Deposition of the upper three divisions is greatly influenced by sedimentary processes. The thickness and constitution of each division vary according to the volume and nature of the associated eruption.





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