Seamounts are submarine volcanoes that rise more than 1,000 m above the seafloor; they occur individually, in clusters, or in lines. They are most abundant in the Pacific Ocean basin where more than 10,000 have been counted. Some of them form islands when large volumes of molten material produce volcanoes that grow above the surface of the sea. Mauna Loa on the island of Hawaii has an elevation above the seafloor of about 10,000 m and a volume of 4,000 cubic kilometers, one of the largest mountains on earth. Seamounts with flat tops are called guyots, named after the Swiss-American oceanographer Arnold Guyot. Their flat tops look like they were eroded by waves, but they are generally found 1,500 m below the sea surface, making such a theory seem implausible. However, submarine exploration of guy-ots has shown that many have dead coral reefs on their surfaces and have features that look like they were created by wave erosion.

A good way to explain their flat tops is to examine the Hawaiian Island chain, which contains a long string of volcanoes, many below the ocean's surface. As we trace the line of volcanoes westward from Hawaii, we find that they get smaller and older. Hawaii is active, but Kauai, 500 km to the west, is not; it is about 5 million years old. Midway Island is 2,600 km away and is 28 million years old. In between are other islands, seamounts, and guyots. At the Yuryaku seamount—3,500 km from Hawaii and 43 million years old—the chain turns northward and is now called the Emperor Seamount chain; it terminates near the Aleutian Islands, where the oldest seamount is 65 million years old. The Hawaiian-Emperor Seamount Chain is 5,500 km long.

How can such a long string of volcanoes be produced, volcanoes that are within plates, not along plate boundaries? Geologists suggest that below the lithospheric plates, "hot spots" or "plumes" are generated within the mantle and stay fixed in place for a long time. About forty plumes have been identified on the earth.

The molten materials from these plumes pierce the plate and build volcanic edifices. As the plate moves, the volcano is carried away from the source of volcanic material and becomes dormant and then extinct. Today, even though Mauna Loa is still active, Hawaii is moving away from its hot spot, and a new volcano named Loihi is being formed on the seafloor to the east. Erosion eventually destroys volcanoes that are no longer being renewed by volcanism, bringing them down to sea level.

However, other factors are also involved. Over the hot spot, the plate bulges upward. As the plate moves past, not only does it move downward but, in addition, the hot rock cools, shrinks, and becomes denser and sinks. As a result, the farther that volcanoes move away from the hot spot, the lower they get, until they eventually sink into the sea; they erode and develop a growth of corals when their surface is in shallow water. Changes in the direction of the line of seamounts can show geologists how plates move over the course of time.

—Sidney Horenstein

See also: Geology, Geomorphology, and Geography; Plate Tectonics


Hamblin, W. Kenneth, and Eric H. Christiansen. 2000. The Earth's Dynamic Systems. Upper Saddle River, NJ: Prentice Hall; Montgomery, Carla. 1996. Fundamentals of Geology, 3rd ed. New York: McGraw Hill Professional Publishing; Nunn, Patrick D. 1994. Oceanic Islands. Oxford, UK: Blackwell; Siebold, E., and W. H. Beyer. 1993. The Sea Floor: An Introduction to Marine Geology. Berlin: Springer Verlag.

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