Where it is and why it is separate
The Sandwich Plate is a microplate in the far South Atlantic, at the eastern end of the Scotia Sea. It carries the South Sandwich Islands, part of the British overseas territory of South Georgia and the South Sandwich Islands. The plate is almost entirely ocean, and the islands are small and uninhabited. They form an intra-oceanic arc, built on oceanic crust rather than on the edge of a continent.
Geologists separate it because it is bounded by a subduction zone on one side and an active spreading ridge on the other. That arrangement lets it move independently of the larger Scotia Plate to the west, and earthquakes outline its edges clearly.
What happens at its edges
The eastern edge is the South Sandwich Trench, where the South American Plate sinks westward beneath the plate. This 845 km boundary has a relative speed of about 7.4 cm/yr, reaching 8.4 cm/yr, one of the fastest convergence rates in the Atlantic. The slab is old Atlantic seafloor, dense enough to sink steeply and pull the trench back toward the east.
The western edge is the East Scotia Ridge, a back-arc spreading center that separates the plate from the Scotia Plate at about 7.3 cm/yr. As the trench retreats east, this ridge fills the gap with new seafloor. A short 93 km transform boundary with the Antarctic Plate closes the plate at its southern end.
Earthquakes and volcanoes
For its size, the plate is very active. The dataset lists 38 earthquakes of magnitude 6.5 or larger since 1900, and 2 reached magnitude 8. A M8.1 struck in 1929, and another M8.1 in 2021, as part of a sequence that also included a M7.5. Several large quakes occurred well below the surface inside the sinking slab, including a M7.8 in 1964 at about 125 km depth. These deeper events trace the slab as it descends westward beneath the islands.
The plate holds 9 volcanoes in the dataset. Mount Michael on Saunders Island has a persistent lava lake, and Montagu Island is the largest island in the chain. Zavodovski, Bristol Island, Candlemas Island and Southern Thule are other active centers, and the Protector Seamounts rise from the seafloor at the northern end of the arc.