Where it is and what it carries
The Tonga Plate covers about 254,000 km² of the southwest Pacific and carries all of Tonga. It is a long, narrow plate made of two parallel ridges: the Tonga Ridge in the east, which holds most of Tonga’s populated islands, and the Tofua Arc of active volcanoes just to the west. Behind it lies the Lau Basin, and in front of it the Tonga Trench.
Older maps often included this area in the Australian or Pacific plates. Geologists treat Tonga as its own plate because the Lau Basin is opening behind it, so the arc moves independently of both its neighbors.
The Tonga Trench
The 1,172 km boundary with the Pacific Plate is the Tonga Trench, where Pacific seafloor sinks west beneath Tonga. Relative speeds average about 19.3 cm/yr and range from about 10.4 cm/yr in the south to about 26.3 cm/yr in the north. No other boundary in the PB2002 model moves this fast. The trench’s deepest point, Horizon Deep, is the second deepest spot in the ocean.
The sinking slab descends steeply to about 700 km beneath Tonga and Fiji. This slab produces a large share of Earth’s deep earthquakes.
The Lau Basin and other edges
West of Tonga, the Lau Basin is a back-arc basin where the Tonga Plate pulls away from the Australian Plate, which carries Fiji. Along a 558 km boundary of spreading ridges and rifts, relative speeds average about 5.5 cm/yr and rise from south to north. In the northern Lau Basin, the plate separates from the Niuafo’ou Plate at about 6.2 cm/yr. The faster opening in the north is one reason the trench moves fastest there.
At its southern end, the Tonga Plate meets the Kermadec Plate along a short transform boundary, at about 3.9 cm/yr. This lies near where the Louisville Seamount Chain on the Pacific Plate runs into the trench.
Earthquakes and volcanoes
The dataset lists 62 earthquakes of magnitude 6.5 or larger since 1900, three of them magnitude 8. A deep M8.0 quake struck about 400 km beneath Tonga in 1903. The 1917 Samoa-Tonga earthquake (M8.0) hit near the northern end of the trench, close to Samoa, and the 2006 Tonga earthquake (M8.0) struck at 55 km depth. Many more deep quakes from the same slab are recorded under the neighboring Australian Plate near Fiji.
The plate holds 17 volcanoes in the dataset, most of them in the Tofua Arc and many underwater. Hunga Tonga-Hunga Ha’apai produced the most powerful volcanic explosion recorded by modern instruments in January 2022. Tofua and Home Reef erupted in 2026, Lateiki in 2019, and West Mata, a deep submarine volcano, in 2009.