Pyroclastic surges are low-concentration turbulent flows that form in at least three ways: (1) eruption column collapse (ground surge, base surge), (2) elutriation from the top of a moving pyroclastic flow (ash cloud), and (3) directly from a crater without an accompanying vertical eruption column.* * Note added in proof: During the April 17, 1979, eruption of Soufriere, St. Vincent, nuées ardentes were observed to form simultaneously with (and without collapse of) a strong vertical eruption column that attained an altitude of about 20,000 m (Richard Fiske, personal communication). Ground surge deposits occur at the base and ash cloud deposits occur at the top of pyroclastic flow sequences. Ground surges and ash clouds may move independently of their associated pyroclastic flow, and, like flows directly from a crater, their deposits may occur alone or within any part of pyroclastic flow sequence. The presence or absence of surge layers, their position with respect to pyroclastic flow deposits and their physical characteristics have significance with respect to the characteristics or eruption columns and flowage mechanisms of pyroclastic flows. © 1979.
