Submarine silicic caldera at the front of the Izu-Bonin arc, Japan: Voluminous seafloor eruptions of rhyolite pumice

Myojin Knell caldera, a submarine rhyolitic center 400 km south of Tokyo, i s one of nine silicic calderas along the northern 600 km of the Izu-Bonin(- Ogasawara) are and the first anywhere to receive detailed, submersible-base d study. The caldera, slightly smaller than the Crater Lake structure in Or egon, is 6 x 7 km in diameter; its inner walls are 500-900 m high, and it h as a remarkably Rat Boor at 1400 m below sea level (mbsl), The caldera coll apse volume is similar to 18 km(3), suggesting that more than 40 km(3) of p umiceous tephra may have been erupted at the time the caldera formed. Precaldera seafloor eruptions built a broad volcanic edifice consisting of overlapping composite volcanoes made of rhyolitic lavas, shallow intrusions , and a variety of volcaniclastic deposits-including thick accumulations of rhyolitic pumice erupted at 900-500 mbsl, The caldera-forming eruption pro duced a 150-200 m deposit of nonwelded, fines-depleted pumice that resemble s a colossal layer of popcorn at the top of the caldera wall. Freshly erupted pumice behaved as "sinkers" or "floaters," depending on the environment in which it cooled, The pumice clasts deposited proximally and exposed in the caldera wall were likely quenched in eruption columns that remained below sea level. This pumice ingested seawater and sank as gases f illing its vesicles cooled, particularly as steam in its vesicles condensed to liquid water. Some eruption columns may have broken through the sea sur face and entered the air, especially during vigorous phases of the caldera- forming eruption. These pumices had the opportunity to ingest air as they c ooled, becoming Boaters as they fell back to the sea; these could have been carried distally on the sea surface by the combined effects of ocean curre nts and wind. The age of the caldera is unknown, but it may be as young as several thousa nd years. Its magmatic system at depth retains sufficient heat to sustain a n actively growing intracaldera Kuroko-type polymetallic sulfide deposit, r ich in gold and silver and topped by chimneys emitting fluids as hot as 278 OC, Sufficient time has elapsed, however, for a 250-m-high postcaldera dom e to grow on the caldera floor and for the caldera rim to be deeply scallop ed by slumping.