Event Name : Makushin Lava Ramp
|Start: 54000 || Years BP Ar/Ar || |Description:
|Eruption Product: || andesite ||
From Bean (1999): "The feature to the east of Makushin known as the "Lava Ramp" consists of a massive andesite lava flow which fills a small portion of upper Makushin Valley and spills over the divide into the upper portions of Driftwood Valley (Fig. 4.14 [in original text]). Total volume is estimated to be 5 km3 (Nye and others 1986). 40Ar/39Ar age dates of <13 ka and <54 ka as well as the presence of glacial striations indicate that it is late Pleistocene in age (McConnell and others, 1997). The "Lava Ramp" creates a broad flat divide between Makushin and Driftwood valleys. Tephras are generally well preserved here and are often intercalated with peat layers. Gullies, which form a parallel drainage pattern, provide excellent exposures. Site locations on "Lava Ramp" are indicated on Fig. 4.12 [in original text]."
From McConnell and others (1998): "Approximately 5 km3 (Nye and others, 1986) of massive andesite lava flows which fill the glacially carved Driftwood Valley. The upper reaches of the Lava Ramp flows are obscured by a glacier, but it is believed that the flows were erupted from a flank vent on the eastern side of Makushin volcano (Nye and others, 1984). The andesite are porphyritic containing 30 vol.% plagioclase, 5 vol.% clinopyroxene, 2 vol.% reversely zoned orthopyroxene and minor amounts of normally zoned olivine and opaques in a brown, microlitic groundmass. Flows exposed in upper Makushin Valley exhibit contorted columnar jointing suggestive of contact with glacial ice. At least some fraction of Makushin Valley was still choked with ice at the time of eruption, blocking the flow from inundating that valley. The surface of the Lava Ramp flow has been modified by glacial scouring. 40Ar/39Ar age determinations from two samples yielded poorly constrained ages of late Pleistocene (table 2 [in original text])."