The Hawaiian Islands are the tops of volcanic mountains rising above the ocean floor. These volcanoes formed over a localized hot spot of magma generation. As the older volcanoes formed great shields and died, the movement of the ocean floor and crust shifted them to the northwest.
A higher percentage of sand shores are found on the older islands. Beaches on the Hawaiian Islands are smaller than those on the continental shores because of the young age of the islands, the absence of large rivers to supply sediment, and the shape and exposure of the island beaches to the wave systems that affect the islands.
The sand on the beaches is also different in that it is primarily calcareous and of biologic origin. The calcareous sand originates as shells and test of animals or algae that live on the fringing reefs or shallow waters adjacent to the islands. Two exceptions are some beaches near stream mouths that are detritus basalt sand, and a few beaches on the island of Hawaii are black volcanic glass sand generated by the steam explosions that occur when hot lava flows into the ocean.