What is the name of the tectonic plate that underlies the Pacific Ocean?

The tectonic plate underlying the Pacific Ocean is the Pacific Plate.

The Pacific Plate is an oceanic tectonic plate covering most of the Pacific Ocean basin. With an area of roughly 103 million square kilometres, it is the largest of Earth’s major tectonic plates. It formed about 190 million years ago near a triple junction involving the Farallon, Phoenix and Izanagi plates.

The plate is surrounded by a complex network of divergent, transform and convergent boundaries. Subduction around much of its western and northern edges helps create the Pacific Ring of Fire, a zone famous for earthquakes, volcanoes and deep ocean trenches. The Mariana Trench, the deepest known ocean trench, lies along a boundary where the Pacific Plate is subducted beneath the Philippine Sea Plate.

A common mix-up is assuming the plate includes every island or coastline bordering the Pacific. Most Pacific islands sit on or near the plate, but several areas—including parts of New Zealand and the western Americas—belong to neighbouring plates. The Hawaiian Islands formed as the Pacific Plate moved over a relatively stationary mantle hotspot.