Where it is
The Caroline Plate covers about 1.5 million square kilometers, or 0.3% of Earth’s surface. It lies in the western Pacific, north of New Guinea, between the Philippine Sea Plate to the west and the main Pacific Plate to the north and east. Almost all of it is ocean floor. The dataset lists no country land on the plate, even though it shares its name with the Caroline Islands.
The seafloor includes the West Caroline Basin and the East Caroline Basin, separated by the Eauripik Rise, a broad underwater ridge.
What happens at its edges
The longest boundary, 1,809 km, is with the Pacific Plate. PB2002 classifies most of it as oceanic transform, with stretches of oceanic convergence and a short spreading segment. The relative speed is only about 1.0-1.2 cm/yr. Features along this boundary include the Sorol Trough in the north and the Mussau Trench in the east.
To the west, a 1,403 km boundary with the Philippine Sea Plate near Palau moves extremely slowly, from almost zero to about 0.8 cm/yr. It includes the Ayu Trough, a slow spreading center, and stretches of oceanic convergence.
The southern edge is the most active. Along 765 km, the Caroline Plate subducts beneath the Bird’s Head Plate at the New Guinea Trench at about 2.2-2.8 cm/yr. Farther east, it converges with the Woodlark Plate along a 426 km subduction zone at about 6.7-7.4 cm/yr, the fastest motion on any of its boundaries. In the southeast, a 900 km boundary with the North Bismarck Plate is mostly oceanic convergence at about 0.9-1.7 cm/yr.
Earthquakes
The Caroline Plate is one of the quietest plates in the region. The dataset lists only 7 earthquakes of magnitude 6.5 or larger on the plate since 1900, and none reached magnitude 8. The largest was a 1911 earthquake near Yap (M7.8). A 1912 earthquake east of Palau reached M7.6, and a 1907 earthquake near Vanimo on the north coast of New Guinea reached M7.0. In 2014, an M6.9 earthquake struck in the Micronesia region.
The dataset lists no volcanoes on the plate. That fits its setting: it is the plate that sinks at its main trench, not the overriding plate where subduction volcanoes form.
Why it is a separate plate
Many simple world maps include the Caroline Plate within the Pacific Plate. Detailed studies of seafloor structure, earthquake slip directions and plate motions show that it moves slightly differently, enough for PB2002 and several other models to treat it as its own plate. Because the motion is slow and the region is remote, the plate’s boundaries are less sharply defined than those of the Philippine Sea or Pacific plates. Future GPS data from island stations around its edges may sharpen or redraw them.