Sedimentologic and dynamic differences between hot and cold laharic debris flows of Mayon Volcano, Philippines

Abstract

Numerous hot lahars produced during and immediately after the 1984 eruption of Mayon Volcano, along with frequent cold lahars of the succeeding wet-monsoon seasons, have left widespread debris-flow deposits. Fresh deposits of hot debris flows have thin crusts that are remarkably resistant to erosion and to vegetal growth; these crusts contain clasts with uniform thermoremanent magnetization directions, rare soft-sediment deformation structures, and gas-escape tubes. Deposits of unchannelized hot and cold debris flows both have sheetlike or lobate geometry, non-erosional bases, poor sorting, relatively minor silt and only traces of clay-sized ash, either matrix or clast support, and common out-sized clasts at their tops. It is difficult to distinguish between the deposits after burial, but they have subtle, significant sedimentologic differences. The inversely graded bases of hot debris-flow deposits are thick, suggesting a lack of cohesion and considerable dispersive pressure; they have long clast axes aligned parallel to bedding due to strongly sheared laminar flow, and generally they are succeeded by gradual normal grading from waning-flow conditions. In cold debris-flow beds, only the basal few centimeters may be inversely graded, the bulk being commonly ungraded, indicating the active role of cohesive forces. In the hot flows, vaporization of water by heat from large clasts may have facilitated mobility by decreasing internal friction. Indirect velocity calculations indicate that they moved faster and traveled farther than did cold flows. Proximal hot debris flows were all supercritical, with Froude numbers decreasing enroute. The most distal hot debris flows and all of the cold ones were subcritical. Cold debris flows show no correlation between Froude number and distance of transport.