The Hawaiian Islands were formed by volcanic activity

A bathymetric map of Kachemak Bay, Alaska

The Hawaiian Emperor seamount chain is a well-known example of a large seamount and island chain created by hot-spot volcanism. Each island or submerged seamount in the chain is successively older toward the northwest. Near Hawaii, the age progression from island to island can be used to calculate the motion of the Pacific Oceanic plate toward the northwest. The youngest seamount of the Hawaiian chain is Loihi, which presently is erupting from its summit at a depth of 1000 meters. Image courtesy of U.S. Geological Survey.

The Earth’s outer crust is made up of a series of tectonic plates that move over the surface of the planet. In areas where the plates come together, sometimes volcanoes will form. Volcanoes can also form in the middle of a plate, where magma rises upward until it erupts on the sea floor, at what is called a “hot spot.”

The Hawaiian Islands where formed by such a hot spot occurring in the middle of the Pacific Plate. While the hot spot itself is fixed, the plate is moving. So, as the plate moved over the hot spot, the string of islands that make up the Hawaiian Island chain were formed.

The Hawaiian Islands form an archipelago that extends over a vast area of the North Pacific Ocean. The archipelago is made up of 132 islands, atolls, reefs, shallow banks, shoals, and seamounts stretching over 2,400 kilometers (1,500 miles) from the island of Hawaii in the southeast to Kure Atoll in the northwest.

For more information:
Hawaiian Archipelago, Coral Reef Information System
Seamounts and Hot Spots, New Millennium Observatory