Description: From Waitt and Beget (1996): "Between about 700 and 400 yr B.P. (between tephra layers M and B) a debris avalanche swept to the sea on the west and a small one on the south-southeast. Large lithic pyroclastic flows shed to the southeast; smaller ones descended exiting swales on the southwest and south."
"Lagoon debris-avalanche deposit has a hummocky topography with local relief of 10 m and contains angular andesite boulders at least as large as 3 m. In coastal exposures it is overlain by about 60cm of organic debris and tephra, at whose contact a discontinuous pumice lapilli, apparently tephra B. The deposit has a conspicuous left-lateral levee built over a lithic pyroclastic-flow deposit (unit Mbp) to the south, which is capped by tephra B and underlain by tephra M. Part of the hummocky Lagoon debris-avalanche deposit, though, lies outside (south of) this levee, which suggests that the avalanche arrived in at least two closely spaced pulses."
"The West Island debris-avalanche deposit partly buries the north side of Lagoon debris-avalanche deposit, banked against an apparent sea cliff eroded into the Lagoon deposit. A nearly continuous sea cliff inside of Northwest Lagoon, overridden and modified by the West Island debris avalanche, is perhaps a segment of these same sea cliffs."
"During this prehistoric period numerous domes must have been emplaced at the summit, repeatedly renewing the source for catastrophic debris avalanches. Remnants of these older domes form the east and south sides of the present summit-dome complex. Below the summit area at least three domes were emplaced on the upper flanks, one on the south (Karnishak dome), two on the northwest (domes "I' and 'H"). Another undated and nearly buried dome or lava flow diversifies the upper south flank."