Why do earth scientists think that rigid slabs oflithosphereare descending intothe mantle at theTongaIslands?

Answer 1

It's just a theory and it is called Seafloor Spreading and Plate Tectonics. Since it is just a theory some people don't buy into it, but it does explain things seen in nature, including the Tonga Islands. If you have a better theory then you need to write up an abstract and submit it to GSA. But if you don't have some letters after your name; such as BS, MS or PhD; then don't expect to get a good reception, or even a chance to speak.

Answer 2

Because it could explain the seismic activity there. Polynesia has volcanic and plate tectonics that may be driving seismic activity and friction on the lithosphere and mantle.