Craters, calderas, and hyaloclastites on young Pacific seamounts

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Abstract
Craters, calderas, and bedded hyaloclastites are commonly associated with seamounts. New Sea MARC 1 side‐looking sonar data for the summit of MOK seamount located near the East Pacific Rise (EPR) at 10°N show that MOK has a large caldera consisting of three coalesced circular depressions. These data also reveal many features such as stepped crater walls, talus deposits, ring faults, intracaldera flows, small cones, lava tubes and channels, and other features. Results of an ALVIN submersible dive in the crater of seamount “F” near the EPR at 21°22′N, 108°37′W revealed the presence of bedded hyaloclastite deposits; bedded hyaloclastites were also recovered by dredging of six other volcanoes all with summit craters. The bedded hyaloclastites are thin blankets of several types of basalt glass shards in a matrix of bottom sediment, clay, and ferromanganese minerals. The glass shards are chemically homogeneous and similar to midocean ridge basalts and transitional basalt lavas typical of young seamounts. Hyaloclastite deposits are crudely inverse graded with tabular, platy shards parallel to bedding. The characteristics of bedded hyaloclastites on seamount suggest an origin by rapid eruption rate, explosive mixing of magma and seawater, followed by rapid transport and deposition, such as may occur during submarine lava fountaining.