A plate caught in a squeeze
The Molucca Sea Plate is a microplate of about 418,000 square kilometers, or 0.08% of Earth’s surface. In PB2002 it covers the Molucca Sea, the Gulf of Tomini and a large part of northern and eastern Sulawesi, about 3.8% of the land of Indonesia. Its crust is mixed: oceanic floor under the seas and volcanic arc crust on Sulawesi.
The Molucca Sea is a classic example of arc-arc collision. Two volcanic arcs, the Sangihe arc on the west and the Halmahera arc on the east, are converging. The oceanic plate between them dips beneath both arcs, and much of it has already disappeared into the mantle.
What happens at its edges
The plate’s longest boundary, 1,462 km, is with the Banda Sea Plate to the south. PB2002 classifies it as a mix of oceanic convergence, rift and transform segments, with relative speeds of about 0.9-5.5 cm/yr.
To the west and north, a 1,327 km boundary with the Sunda Plate is mostly subduction, including the North Sulawesi Trench where the Celebes Sea floor dips south beneath Sulawesi’s northern arm. Relative speeds range from about 0.5 to 4.7 cm/yr.
To the east, a 479 km boundary with the Bird’s Head Plate is the fastest, at about 8.6-10.1 cm/yr. It combines subduction and strike-slip faulting where the Halmahera region on the Bird’s Head Plate presses westward.
Earthquakes and volcanoes
The dataset lists 48 earthquakes of magnitude 6.5 or larger on the plate since 1900, two of them magnitude 8 or more: the 1965 Ceram Sea earthquake (M8.2) and the 1939 Sulawesi earthquake (M8.1), which struck about 150 km deep. Other large events include the 1996 earthquake north of Palu (M7.9), the 1990 earthquake near Gorontalo (M7.8) and the 1998 earthquake off Luwuk (M7.7).
The plate holds one volcano in the dataset, Colo on Una-Una Island in the Gulf of Tomini. Its 1983 eruption forced the evacuation of the whole island. The active volcanoes of the Sangihe and Halmahera arcs lie just outside the plate, on the Sunda and Bird’s Head plates.
Why the plate is debated
Geologists agree that the Molucca Sea region is a collision zone, but not on how to divide it into plates. The original Molucca Sea plate is mostly subducted, and the PB2002 polygon is a practical way to describe the remaining block and its boundaries. Other studies split Sulawesi into several smaller blocks or treat the area as broad deformation. Treat the boundaries on this map as approximate.