Seamounts

Seamounts, formed by undersea volcanoes, are far more numerous and much larger than their volcanic counterparts on land. Some may briefly become large enough to rise above sea level as islands, but almost all eventually resubmerge as guyots or atolls. Radiometric dating of seamounts has helped to verify the theory of continental drift.

Origin and Growth

Seamounts are volcanoes that rise from the sea floor but lack sufficient height and land area to be classified as islands. Like their volcanic counterparts on land, seamounts are roughly conical or domed, and many have craters at the top. Compared to volcanoes on the continents, seamounts are much larger and higher and may rise to as much as 10,000 meters above the ocean floor. A seamount that has a flat top is called a guyot. Almost all guyots were islands at one time, and they acquired their flat tops through the erosive action of waves, the growth of corals, or a combination of both processes. The islands also may have undergone resubmergence caused by the subsidence or sinking of the sea floor or by an eustatic rise in sea level. The tops of some guyots can be as much as 3,000 meters below present sea level, but most are about 1,000 meters deep. A guyot that is less than 200 meters deep is called a bank. Seamounts and guyots are a prominent feature of the ocean floor, particularly in the southwestern Pacific Ocean, although many also exist in the Atlantic Ocean. Some, including the Grand Banks, found some distance off the coast of Newfoundland, Canada, are associated with vast midocean ridges, but many rise from the deep, flat sections of the ocean floor known as the abyssal plains. Some seamounts are isolated, others occur in clusters, forming a large volcanic plateau, and others form a long chain.

Little is known regarding the origins of seamounts. It is known, however, that magma (molten rock) rising through the sea floor squeezes its way beneath the saturated but relatively buoyant bottom sediments and then crystallizes. After many of these intrusions, a solid base is built up, and cone development can begin. The magma that makes up seamounts is not of the explosive variety, so seafloor eruptions are relatively quiet. The water pressure in the deep ocean is so great that the gas and steam normally associated with volcanoes on the continents cannot escape. Instead, the lava (magma that has escaped the interior) oozes out quietly in submarine flows. Because deep seawater is quite cold, the lava is chilled very rapidly. It often cools into wrinkled, glassy crusts that enclose round blobs. These structures, common on the sea floor, are known as lava pillows. The rock that makes up the pillows is a dark, fine-grained rock known as basalt, rich in iron and magnesium-bearing minerals.

As the seamount grows, its increasing size brings it nearer the sea’s surface and, consequently, into areas of lower water pressure. At a depth of about 2,000 meters, the gases associated with the lava and the heated seawater itself can begin to expand and explode violently as the lava cools, resulting in a glassy, fragmented structure known as hyaloclastite. It is also at these or shallower depths that the submarine eruption may be noticed by passing ships as an area of dark, turbulent water, floating fragmented and bubbly rocks called pumice, and gas and ash clouds at the surface. During both stages, magma may continue to be injected along cracks and layers inside the growing seamount and crystallize internally. If the magma finds a place to escape along the sides or base of the mountain, a flank eruption may build a parasitic cone.

The next stage in the growth of a seamount, when the eruption is close to sea level, is critical. The effects of the exploding gases, combined with wave erosion on the loose, fragmented debris, prevent the seamount from emerging above the water to become an island. If the volume of lava erupted is sufficient to keep pace with these destructive forces, however, subaerial lava flows may begin to cover the hyaloclastite with an armor plating that is resistant to wave erosion, and a volcanic island is born.

Erosion and Subsidence

During its life as an island (likely to be only a few million years, a relatively brief period compared to its life as a seamount and guyot), the mountain is attacked by wind, running water, and glaciers (depending on its latitude). All these factors tend to reduce its elevation even as subsequent eruptions may build it up. If the conditions are tropical, coral reef growth may encrust the wave-battered shores. However, every island eventually succumbs to the combined effects of erosion and subsidence (sinking) of the sea floor that begin to submerge it again. Coral growth may be able to keep pace with the effects of subsidence for a time. This extensive coral growth sometimes results in a completely coral-capped volcano known as an atoll. Many atolls in the South Pacific have more than 1,000 meters of massive coral growth above their volcanic bases.

If coral growth cannot keep pace with the rate of seafloor subsidence, the island or atoll becomes submerged once again, this time in the form of a bank and then finally as a guyot once it reaches a depth of 200 meters. The mountain erodes gradually on the sea floor: The sides are subject to erosion by currents, crumbling, and slumping, and the rocks undergo chemical reactions with the seawater. The entire mountain is also gradually buried in the ubiquitous “rain” of dust and the tiny remains of microscopic plants and animals, which cap the top and drape the sides in thick layers of fine sediments.

Continental Drift

No guyot lasts for a very long period of geologic time, however, because all are eventually carried by continental drift to subduction zones along the edges of continents, where the sea floor is effectively recycled as it moves downward into the mantle below the continental mass. The study of seamounts and guyots has helped to verify the idea of continental drift (plate tectonics) and to show how fast plate movement occurs. The sea floor consists of one or more slabs (plates) of quasi-rigid rock 50 to 80 kilometers thick. The oceanic lithosphere, as it is called, is created at the great midocean ridges, where two plates are being driven apart from each other by convection currents in the underlying magma. Magma wells up to fill the gap that is created. As the magma cools and crystallizes, it is added to the slab and becomes part of the plates. Thus, new seafloor lithosphere is added to both diverging plates at midocean ridges. Sometimes, a seamount will grow at the midocean ridge (Iceland is an example of a volcanic island in this location) and become split in half as the two plates pull apart.

The slabs of oceanic lithosphere can slide about on a slippery zone in the upper mantle, known as the asthenosphere, just as slabs of ice on a winter pond can drift on the water. Where stresses push two plates together, one is forced to override the other. The plate forced underneath (subducted) plunges deep into the asthenosphere and eventually melts. Because the oceanic lithosphere is thin and dense compared to the continental lithosphere, the sea floor is usually subducted or recycled. Thus, the sea floor, carrying its seamounts, is geologically much younger than the continents.

Hot Spots

Although some seamounts erupt near midocean ridges, many that erupt far from plate boundaries are associated with hot spots. Hot spots, or plumes, are believed to be long, narrow fountains of magma that rise from deep within the mantle, well below the asthenosphere. These magmas have a chemical composition quite different from the midocean ridges' magma. Seamounts are generally made up of alkaline basalts (basalts enriched in the elements sodium and potassium compared tomidocean-ridge basalts).

The upwelling magma plume at the hot spot dramatically affects the seafloor lithosphere passing over it. The rising plume pushes the lithosphere upward as much as 1,000 to 1,500 meters into a broad arch or swell and heats the lithosphere from below, making it thin and stretched. Finally, a seamount is born if magma can break through the weakened lithosphere. Eventually, however, the oceanic lithosphere is carried by plate motion away from the hot spot, which, unlike the lithosphere, has a relatively fixed location. The seamount or volcanic island begins to move, along with the lithosphere in which it is embedded, down the side of the bulge caused by the upwelling magma. In addition, the heavy burden of a large volcano on the thin oceanic lithosphere causes it to sag downward into the soft asthenosphere below. These effects combine to pull a seamount downward or submerge an island into a guyot.

At the same time that the old volcano is moving off the hot spot and subsiding, magma may continue to erupt through the new lithosphere over the hot spot, and a new volcano will emerge next to the old one. Over time, repeated eruptions as the plate slowly moves over a stationary hot spot create a type of submarine volcanic chain called an island arc. The shape of the arc reflects the direction of movement of the plate over the plume below. In this way, over millions of years, a whole chain of seamounts, some of which may have been islands at one time, extend from the location of the hot spot in the direction of plate motion for thousands of kilometers. In this fashion, the Hawaiian Islands and the closely related Emperor Seamounts were formed as the Pacific plate was pulled over a hot spot in a northwesterly direction toward the subduction zone near the Aleutian Islands of Alaska. New seamounts, such as one discovered off the coast of Guatamala in 2023, as well as increased acitivty from known seamounts, such as from the Axial Seamount observed off the coast of Oregon in the early 2020s, continue to be researched.

Principal Terms

atoll: a tropical island on which a massive coral reef, often ringlike, generally rests on a volcanic base

basalt: a rock that results when lava rich in iron and magnesium and low in silica is cooled rapidly; it has a fine-grained, dark-colored appearance

guyot: a drowned volcanic island with a flat top caused by wave erosion or coral growth

hot spot: a column or plume of molten rock that rises from deep within the mantle and can cause volcanic eruptions if it penetrates the lithosphere

hyaloclastite: the rock type that results when lava is chilled rapidly and explosively beneath the sea at shallow depths, resulting in a fragmented, glassy texture

lava: magma that has erupted from a volcano

lithosphere: the rigid outermost layer of Earth that floats on the softer layer (the asthenosphere) beneath; it is thinner under the oceans than on the continents

magma: a general term for molten rock within the mantle layer

midocean ridge: a roughly linear, submarine mountain range where new seafloor lithosphere is created by the process of seafloor spreading

pillow lavas: lavas that have been rapidly cooled by water as they erupt and consequently develop crusted, rounded, or pillow-shaped structures

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