A field of volcanoes on the Manihiki Plateau: Mud or lava?

A field of about 100 cones surrounding a 1900 m high seamount was discovered on the northeastern edge of Manihiki Plateau (10° 18.5′S, 161°27.5′W) during a February 1987 cruise of the R.V. Moana Wave in the territorial waters of the Cook Islands. The cones appear as acoustically transparent piercements on single-channel seismic reflection profiles collected during this and earlier expeditions to the area. SeaMARC II sidescan images typically reveal nearly conical, steep-sided peaks of about 1 km diameter rising several hundred meters above the smooth and featureless plateau surface. Many of the cones appear to be formed by multiple flow units. A few have summit depressions, and moats partially surround some of the cones. Distributed like holes on a dart board, individual cones become more densely spaced about a bulls eye located near the eastern edge of the Manihiki Plateau, 50 km southwest of Rakahanga Island. At that location, about 40 cones coalesce to form the seamount, which is about 25 km in diameter and rises from a plateau depth of 3200 m to a summit at a depth of 1310 m. Sidescan images show that incised channels trend away from the seamount toward the east, where the irregular seafloor in the area truncates tilted, E-dipping sub-bottom reflectors. To the south and west, the seafloor and sub-bottom reflectors are smooth and dip gently to the southwest. Recent foraminiferal ooze was cored from a satellite cone. Dredging of the seamount summit recovered middle Eocene planktonic foraminifera embedded in burrowed limestone. Some of the limestone cobbles had manganese crusts; coralline fragments and Recent foraminifera were also recovered in the same dredge. Despite the apparent absence of igneous rocks, geopotential modelling indicates a basement high at this location. If fluid escape rather than magma flow is involved in the genesis of these cones, it is localized by faulting associated with basement uplift or the presence of an older igneous intrusion. At this time we cannot say with certainty whether the cones are igneous or sedimentary features.