Seamounts are submarine mountains, often volcanic cones, that project 150-3,000 ft (50-1,000 m) or more above the ocean floor. They are formed primarily by rapid undersea buildups of basalt, a dark, fine-grained rock that is the main component of the ocean's crust.
Seamounts form by submarine volcanism. After repeated eruptions, the volcano builds upwards into shallower water. If a seamount eventually breaches the water's surface, it becomes an island. Wave action can then erode the exposed rock, and the peak may be flattened or leveled off. Flat-topped, submerged seamounts, called guyots or tablemounts, are seamounts that once breached the ocean's surface, but later subsided.
Sometimes seamounts occur as matching pairs located on opposite sides of an oceanic ridge. Speculation on the origins of these features led to the idea that such pairs were once part of a single volcanic complex that had split and separated. This helped support the concept that there are spreading centers along the ocean ridges where slabs or plates of the earth's lithosphere are moving away from each other. Volcanic eruptions form new seafloor and seamounts in the gap, or rift, that develops. This spreading, an integral part of the theory of plate tectonics (which explains the motion of the earth's plates) has been measured to occur at a rate of between 0.8-4.0 in (2-10 cm) per year.
Seamounts are more numerous than terrestrial volcanoes and reach greater heights. They may form in groups or clusters, or can be found aligned in submarine volcanic mountain chains known as oceanic ridges. As seamounts slowly move away from the oceanic ridge due to seafloor spreading, their tremendous mass causes them to subside. At the same time, sediment "rains" down from above, slowly burying them over millions of years. As a result, especially tall seamounts may occur as isolated features rising from the abyssal plain. This is the deep, flat section of the ocean floor far removed from an oceanic ridge, where sediments are often thousands of feet thick. Somewhat closer to an oceanic ridge, where sediments are not so thick, the tops of partially buried seamounts form what are called abyssal hills.
Some seamounts are very tall, broad volcanic features with gentle slopes, known as shield volcanoes. Mauna Kea on the island of Hawaii is a good example. It rises over 32,810 ft (10,000 m) above the ocean floor, making Mauna Kea—not Mt. Everest in the Himalayas—the world's highest mountain. These massive volcanic structures form when isolated hot plumes of molten rock rise from the Earth's mantle, forming what is called a hot spot. Iceland is another example of an island formed by hot spot activity.