Seamount is a volcano that forms on the ocean floor. There may be as many as 1 million volcanoes on the Pacific Ocean floor alone, roughly 750 times the number on dry land. Seamounts occur throughout the ocean, wherever magma (melted rock inside the earth) rises to the sea floor and erupts. These submerged volcanoes form in deep basins, in areas near the continents, and at the mid-ocean ridges where lava (magma that has erupted) hardens into new sea floor material.
A typical seamount is cone-shaped, with a circular or almost circular base. It may form with a sharp peak or a flat top. Either kind of seamount may have a crater at the top. A crater forms if the hot rock under the top drains away, leaving a large part of the top unsupported. The top then collapses, producing the crater. Some islands sink beneath the oceans as they age. Waves may plane off their tops as they sink, producing flat-topped, submerged volcanoes called guyots.
Seamounts come in all sizes, but most are less than 1,600 feet (500 meters) high. A seamount that grows tall enough to break the surface of the ocean is called an oceanic island or volcanic island. In some regions, seamounts must reach a height of at least 16,000 feet (5,000 meters) before breaking the surface.
The Hawaiian Islands are oceanic islands—and another Hawaiian island is in the making. About 19 miles (30 kilometers) southeast of the island of Hawaii, Loihi << loh EE hee >> Seamount is erupting and increasing in height. Its summit is about 3,300 feet (1,000 meters) below the surface of the ocean. It may break the surface in about 50,000 years.
