The Tongan island arc is a series of volcanoes west of the main islands in the Kingdom of Tonga. They form because one crustal tectonic plate (the Pacific plate) subducts - is forced or pushed - beneath the other plate (the Australian). The subducted Pacific plate then sinks into the earth (into a layer called the mantle). The more populated islands of Tonga, to the east are not volcanically active. The Lau Basin is directly to the west of the Tongan island arc, over a feature called a back-arc basin. In this area, the crust splits apart and forms new rock when material deeper down flows upward, melts and erupts at the surface.
This is the region where we are spending ~45 days dropping seismometers to the seafloor and sailing back and forth in a grid pattern to collect sonar, seismic, gravity, and magnetic data. All of these geophysical techniques will help to map both the surface of the seafloor (the bathymetry) and the structure of the crust and upper mantle (the top 10-20 km of the Earth).