The formation of volcanic islands occurs as tectonic plates move over fixed "hot spots" in the ocean, where underground rock melts to form magma. This magma rises to the surface to become lava that cools and builds up mounds, which eventually break through the surface of the water to form islands. The process takes thousands of years and involves hundreds of eruptions, explains Live Science.
The movement of tectonic plates means the islands that form eventually move away from the hot spot and become extinct volcanoes. As they move away, new islands begin forming over the same hot spot. The cycle of volcano birth and extinction leads to the formation of island chains, as seen with Hawaii. The island of Hawaii sits atop the same hot spot responsible for the formation of Kunai 4.5 million years ago.
The Hawaiian Islands were formed by such a hot spot occurring in the middle of the Pacific Plate. While the hot spot 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. This hot spot is responsible for a chain more than 1,500 miles long that stretches from the Big Island of Hawaii to Kure Atoll and consists of islands, reefs, shoals and seamounts.