ABSTRACT

Three lahars that resulted from the flow transformation of an milled pyroclastic surge caused by ejected lithic debris and hydrothermal water during the cataclysmic Mount St. Helena eruption of 18 May 1980 deposited about 1 10⁶ m³ of massive, poorly sorted, poorly graded volcaniclastic sediment on the southwest flank (SWF). Mean peak flow velocities over a 5-km radius ranged from more than 30 m/s in the proximal area (mean slope angle of 13°) to about 5 m/s in the distal area (mean slope angle of 4° 9); the lahars rapidly decelerated as deposition occurred on forested slopes. Average yield strength of the SWF lahars ranged from 9,000 to 12,000 dn/cm²; Bingham viscosity ranged from 2.8 10³ to 50 10³ poises. Average flow density of the SWF lahars was about 2.0 g/cm³.

To determine the grain-size distribution of the SWF lahars, spot sampling of deposit matrix from vertical exposures was supplemented with point-count sampling of larger particles. Results from these analyses indicate that downflow changes in mean gram size and sorting occur only in the coarse fraction of the deposits larger than a critical diameter of about 4 mm, and occur only in the basal portion of the deposits; both mean grain size and sorting coefficient increase slightly with distance. The deposits show weak inverse grading with respect to mean grain size and exhibit a weak trend of upwardly poorer sorting

Fabric analyses of clast long axes indicate strong planar orientations parallel to flow surfaces. An eigenvalue analysis demonstrates the occurrence of both flow-parallel long-axis orientations with upstream dips and flow-transverse orientations. Similar clast fabric results were obtained for deposits of the North Fork Toutle River lahar. However, the mean clast long-axis dip in the SWF lahars is significantly shallower than the mean clast long-axis dip in the North Fork Toutle lahar, and is more like clast long-axis dips reported for tills and other sediment flows. The data from the SWF lahars indicate that lahar clast fabric may be more complex and variable than previously suggested.