VOLCANIC MIXED AVALANCHES - A DISTINCT ERUPTION-TRIGGERED MASS-FLOW PROCESS AT SNOW-CLAD VOLCANOS

A generally unrecognized type of pyroclastic deposit was produced by r apid avalanches of intimately mixed snow and hot pyroclastic debris du ring eruptions at Mount St. Helens, Nevado del Ruiz, and Redoubt Volca no between 1982 and 1989. These ''mixed avalanches'' traveled as far a s 14 km at velocities up to approximately 27 m/s, involved as much as 10(7) m3 of rock and ice, and left unmelted deposits of single flow un its as thick as 5 m. During flow downslope, heat transfer from hot roc ks to snow produced meltwater that partially saturated the mixtures, a pparently giving these mixed avalanches mobilities equal to or greater than those of ''dry'' debris avalanches of similar volume. The avalan ches studied for this report began as snow avalanches, triggered durin g explosive phases of the eruptions by ballistic bombardment of snow s lopes by hot pyroclastic debris. Erosion by the avalanches incorporate d additional snow, fragments of glacier ice, and other rock debris. Mi xed-avalanche deposits were massive, very poorly sorted, and ungraded to inversely graded prior to melting. Although rock debris composed as much as 70 wt% of the frozen samples, the bulk volumetric content of rock debris was only 9% to 36%. Snow and ice composed between 36% and 72% of the frozen samples, with void space making up the remainder of the volume. Deformation and consolidation after melting of the ice con tent reduced the deposits to loose, porous layers that were only a fra ction of their original thickness and that later gave little hint of t heir origin. After melting and desiccation, the deposits are highly su sceptible to erosion and unlikely to be well preserved in the stratigr aphic record.