Direct temperature measurements of deposits, Mount St. Helens, Washington, 1980-1981

Abstract

A program of temperature studies of the eruptive products of Mount St. Helens was established May 20, 1980, 2 days after the catastrophic eruption of May 18. Temperature-depth profiles were measured by thermocouple to determine the emplacement temperatures of deposits of the debris avalanche and blast of May 18 and of deposits of the pyroclastic flows of May 18, May 25, June 12, July 22, August 7, and October 17, 1980. At some of the localities where deposits had cooled appreciably before measurement, emplacement temperatures were recovered mathematically from data gathered from observations of the cooling histories of the deposits. In addition, methodologies and specialized temperature-measuring equipment were developed to maximize efficiency in data collection and to minimize risk to personnel. In general, the more recent eruptive deposits were emplaced at higher temperatures than the earlier ones. Emplacement temperatures of deposits of the debris avalanche ranged from about 70 to 100°C. Temperatures of deposits of the blast ranged from about 100 to 325°C and varied with the azimuth of the measurement site from the vent; the higher temperatures were measured in the northeast sector. Emplacement temperatures of the later pumiceous pyroclastic-flow deposits ranged from about 300 to 680°C, and temperatures in nearvent facies were about 750 to 850°C. The most important features shown by the data obtained from the deposits produced by the blast are (1) their generally lower temperature relative to deposits emplaced by the subsequent pumiceous pyroclastic flows, attributed to relatively low temperatures of the cryptodome and admixing of significant amounts of accidental material and air; (2) an azimuthal variation of emplacement temperatures, hotter toward the east, attributed to thermal inhomogeneities in the source material and differential cooling associated with differing grain sizes and terrains over which the blast traveled; (3) emplacement temperatures that exhibited little change with distance from the vent to distances as great as 20 km, attributed to little cooling along the path, preferential elutriation of the cooler, finer particles, preferential interaction between air and the blast material at the head of the flow, and production of heat during the flow, possibly by friction; (4) thermal stratigraphy observed in some of the ponded pyroclastic-flow deposits in major valleys, attributed in part to a grain-size effect (finer upward) and in part to longer residence of the younger deposits as veneer deposits on the adjacent ridges before their secondary mobilization into the valleys; and (5) temperatures in the veneer deposits and moving blast cloud that were possibly higher than emplacement temperatures of the ponded pyroclastic-flow deposits. The most important features shown by the data obtained from the postblast, pumiceous pyroclastic-flow deposits are (1) generally hotter deposits in the later eruptions than in the earlier ones, attributed to less admixing of the pyroclastic material with air because of the observed progressive decrease in vigor of the successive eruptions; (2) little heat loss (100-200°C) of near-vent material during eruption, as predicted by simple adiabatic expansion; (3) little downflow cooling in the deposits after the first several hundred meters of travel, attributed to the same factors proposed for the blast deposits; (4) multiple flow units that were emplaced at different temperatures by most of the eruptions, attributed to differential cooling through differing amounts of admixed air in accordance with the vigor of the individual eruptive pulses; and (5) cooler material in the ash-cloud deposits than in the source pyroclastic flows because of more effective cooling of the elutriated finer material by the admixed air. Descriptions of the materials and methods we used, and the tabular data and temperature-depth profiles that support our findings, are included as appendices.