GIANT PUMICE DEPOSIT FROM TWAIN ISLET: EAST OF JAVA; MONO LAKE, CALIFORNIA

Layers of pumice blocks that floated across lakes from intralacustrine eruption sites are known at La Primavera, Aniakchak, Taupo, and Mono Lake. On the NW shore of Mono Lake, half a million rhyodacite pumice blocks were wind-driven and grounded on a gently sloping, muddy NW littoral. The beached accumulation is 30–200 m wide, almost continuous for 19 km, and in most places only one block thick. Blocks are 1–12 m across but few are >1 m tall. Eruptive age is unknown, probably less than a few thousand years. Restriction to the NW shore suggests a short-lived eruption when wind direction changed little. Limit of the beached pumice at elevation ~6381 ft (1945 m) is close to lake level in 2022. Because the level was 6417 ft (1956 m) in 1941, when stream diversions from Mono Basin began, the deposit was at least 36 ft (11 m) underwater for an unknown duration. Eruption and grounding evidently took place during a lowstand when lake level was close to what it is today. NE of the main islands in Mono Lake (Paoha and Negit), there are ~12 (depending on lake level) small volcanic islets. Others have suggested that Java Islet was the pumice source, but its dense lavas and evolved compositions do not match the beached pumice. Outcrops on Twain Islet, <200 m east of Java, are phenocryst-poor biotite rhyodacite that is compositionally similar and micropumiceous like the lakeshore pumice blocks. Rhyodacite exposure spans only ~12% of Twain Islet, the rest being reworked sand and pumice. A bedded pile (10 m thick) of coarse rhyodacitic ash dips 10–20º NW or SW toward the lake. The section is mostly plane-bedded and fines-poor. Most beds are 0.3–4 cm thick; a few are lapilli-bearing and 5–20 cm thick, rarely 50 cm. Most granules and lapilli (rarely >5 cm) are micropumiceous, but ~1% are dense black glassy rhyodacite. Nonjuvenile lithics are absent. Neither the base of the ash section nor pre-eruption lake silts are exposed. Enclosed in the bedded ash are free blocks (1–5 m), as well as dikes (1–3 m) and irregular intrusions (<7 m), all of micropumiceous rhyodacite. Absence of bomb sags and oxidation suggests subaqueous eruption. Shoreline of Twain in 2022 was 6380 ft and its high point is ~6420 ft. Dikes intruding the growing pile of bedded ejecta fed pumice blocks into the lake, whence they were wind-driven 5–8 km to the north and west.