| Numerous hot lahars produced during and immediately after the 1984 eruption of Mayon Volcano, and 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, and 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; have long clast axes aligned parallel to bedding due to strongly sheared laminar flow; and generally 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 travelled farther than cold flows. Proximal hot debris flows were all supercritical, with Froude numbers decreasing enroute. The most distal hot, and all the cold debris flows were subcritical. Cold debris flows show no correlation between Froude number and distance of transport. |
