A plate inside a spreading basin
The Manus Plate covers only about 8,200 km² of seafloor in the Bismarck Sea, northeast of mainland Papua New Guinea. It carries no land. It lies within the Manus Basin, a back-arc basin that opens behind the volcanic arc of New Britain as the Solomon Sea Plate sinks to the south.
Geologists split it off because the Manus Basin does not open along one simple ridge. Its spreading centers and transform faults outline a small block that moves independently of the larger plates around it. The PB2002 model calls it the Manus Microplate. Because the plate is so small, its rotation changes the direction of motion noticeably from one end of a boundary to the other.
Its two boundaries
The Manus Plate has only two neighbors. Its 191 km boundary with the South Bismarck Plate is mostly a spreading ridge, where the plates move apart at about 8.5 cm/yr on average and up to about 15.6 cm/yr. That top speed is one of the highest relative motions in the PB2002 model.
Its 223 km boundary with the North Bismarck Plate is more mixed, with spreading, transform and convergent segments. Relative speeds there vary widely, from about 0.3 to 9.5 cm/yr. That wide range shows how quickly the motion changes over short distances around such a small, rotating plate.
Earthquakes and seafloor vents
Large earthquakes are uncommon here, partly because the plate’s edges are young, hot seafloor that breaks before large stresses build up. The dataset lists only two quakes of magnitude 6.5 or larger since 1900: an M6.7 quake in 1988 about 129 km southwest of Kavieng, and an M6.6 quake in 1953 about 118 km south of Kavieng. Both were shallow, at 33 km and 20 km depth.
The dataset lists no volcanoes on the plate, though volcanic and hydrothermal activity continues along the spreading centers of the wider Manus Basin. The basin hosts some of the best-studied hot springs on the western Pacific seafloor. Hot, metal-rich fluids rise through the young crust and build chimneys and mounds of copper, zinc and gold sulfides. These deposits made the Manus Basin the target of one of the first proposed deep-sea mining projects.