Conical seamounts and flat tablemounts or Guyots are related to hot spots and mid-ocean ridges. They are active or inactive volcanoes that have built themselves up high above the seafloor, but are still not above the ocean's surface. As soon as either feature breaks the ocean surface, it becomes an island.
There are two causes of formation. One way seamounts are formed is by Mid-Ocean Ridge activity. When the Earth's tectonic plates are slowly moving apart, magma rises up to fill the gap, sometimes bursting onto the seafloor as volcanic eruptions. There are linear chains of seamounts that originate near mid-ocean ridges and are especially common near fast spreading ridge segments, and seem to be preferentially located near bends or offsets in the ridge crest. They are often asymmetric, with many more seamounts located on one side of a ridge than the other.
Another way seamounts are formed is by Hot Spots. Hot Spots are plumes of molten rock rising from the deep within Earth's mantle, and supply magma to form seamounts. Hot spot plumes are long lived. As the tectonic plate passes over it, you get a chain of seamounts and/ or islands.
When a hot spot interacts with a mid-ocean ridge, the affected segments tend to receive a greater than normal supply of magma from the mantle, leading to more frequent eruptions, and formation of volcanic edifices right on the ridge. Sometimes these formations become so large they rise above the ocean's surface, and an island is born. The erosion of the waves and the wind flatten the top of the island, and, as the plate moves, it no longer has that volcanic activity, preventing it from growing further. As the plate continues to move, the island sometimes erodes and submerges, and reverts back to a seamount or tablemount, depending on how much erosion it experienced. The Emperor Seamount Chain probably looked much like the Hawaiian Islands once, but the volcanoes have since submerged.
Three examples of the Mid-Ocean Ridge Type of seamounts contiguous to North West America are the Vance, the President Jackson, and the Taney Seamounts.
The Vance Seamounts comprise the southernmost of several near-ridge chains located on the Pacific Plate near the Juan de Fuca Ridge. No symmetrical seamount chain occurs on the Juan de Fuca Plate. The nearly flat tops of several of the volcanoes slope gently down to the southeast and several of the calderas are breached on the southeast side. Small cones and shields dot the seafloor around some of the volcanoes.
The only near-ridge seamount chain off the Gorda Ridge is the President Jackson Seamounts, located on the Pacific Plate. The chain consists of eight volcanoes, of which four are isolated volcanoes, less than 10 kilometers in diameter and nearly circular in plan view. The other four form two morphologically complex, coalesced flat-topped structures. The volcanoes form a linear chain and are all relatively small.
The Taney Seamounts are located on the Pacific Plate, west of San Francisco. The Taney Seamounts consist of five aligned volcanoes. The volcano has been so severely modified by three successive caldera collapses that most of the summit platform has been destroyed.
There are a couple of well-known examples of the Hot Spot type of seamounts, such as The Emperor Seamount chain, which sits on the Pacific Plate and is carried in a direction to where it will eventually subduct in the Aleutian Trench. The youngest volcano of the Hawaiian Island chain, the Lo’ihi Seamount, is forming right how, and scientist estimate that it will become an island in about 200,000 years.
The first "undersea island" to be called a seamount, back in 1933, is Davidson Seamount, named after scientist George Davidson of the Coast and Geodetic Survey. It is located 120 km southwest of Monterey, California. It is one of the largest seamounts in U.S. waters. It is an inactive volcano, about 2300 m high and 40 km wide, 1300 m below the ocean surface.
Why is it so important to study Seamounts? Fishermen often find their
best catches above seamounts and often go to those areas first. Volcanologists
want to understand how this unique geologic feature was formed, deep-sea
biologists want to know what lives on it, and, because seamounts affect
ocean currents around it, other biologists wonder what’s happening above
it. Taking a habitat-specific approach by studying seamounts is a useful
contribution to the larger project of building an overall picture of marine
biodiversity. Scientists suspect that the water above Davidson may be a
"hot spot" for sperm whales, and it has been suggested that they may be
feeding on giant squid far below the surface. As we study seamounts, we
might find new species and develop new hypotheses on how communities of
species interact on a seamount. We will better understand the geologic
processes that form seamounts. We can better practice conservation and
protect the species and habitats there. Researching and studying seamounts
helps us to gain a better understanding of the world in which we live.
Bibliography
http://ace.acadiau.ca/science/geol/rraeside/quizzes/worldmap5pacif.htm
http://www.mbari.org/volcanism/Seamounts/SeamountsResearchTop.htm
http://www.npaci.edu/online/v5.15/seamounts.html
http://oceanexplorer.noaa.gov/explorations/02davidson/background/missionplan/plan.html#top
Thurman, Harold V. and Alan P. Trujillo. Essentials of Oceanography.
Prentice
Hall: New Jersey. 6th edition.