above sea level - The most productive volcanic systems on Earth are
hidden underan average of 8,500 feet (2,600 m) of water. Beneath the
oceans a global system of mid-ocean ridgesproduces an estimated 75% of
theannual output of magma . An estimated 0.7 cubic miles (3 cubic
kilometers) of lava is erupted. The magma and lava create the edges of
new oceanic plates and supply heat and chemicals to some of theEarth's
most unusual and rare ecosystems.
If an estimate of 4,000 volcanoes per million square kilometers on the
floor of the Pacific Ocean is extrapolated for all the oceans than
there are more than a million submarine (underwater) volcanoes.Perhaps
as many as 75,000 of these volcanoes rise over half a mile (1
kilometer) above the oceanfloor. Technology and hard work by a group
of tenacious explorers/geologists have allowed us our first detailed
glimpses of submarine volcanoes. The following pages outline some of
the basic characteristics and features of submarine volcanoes.
Plate Tectonic Setting
Like continental volcanoes, submarine volcanoes are most common where
tectonic plates move towards or away from each other. In the case of
divergent plateboundaries , where plates are spreading away from each
other, the rate of plate movement plays an important role in
determining the type of volcano that forms andthe rate of eruptive
activity. Submarine volcanoes at convergentplate boundaries
(subduction zones) are much like their subaerial("under air" or
continental) counterparts except that the weightof the overlying water
modifies their eruption style. Hot spots leave linear "tracks" of
seamounts acrossthe ocean basins and build some of Earth's largest
volcanoes.
Hot Spot Volcanism
Hot Spot Volcanism
Hot Spot volcanoes are recognizedby an age progression from one end of
the chain to the other. An active volcano commonly serves asan
"anchor" at one end of the chain. The most studied and best well-known
hot spot volcanoes andseamounts define the Hawaii-Emperor volcanic
chain. The origin and evolution of Hawiian volcanoes, seamounts, and
guyots are described in the Hawaiian Volcano Lessons
Eventually the movement of the tectonic plate carries the volcano off
of its magma supply. The volcano becomes extinct and cools. The plate
beneath the volcano (and above the hot spot) also cools. The rocks
that make thevolcano and plate become more dense. The volcano and the
plate gradually subside as they move away from the hot spot. Even
giantvolcanoes, like Mauna Loa on Hawaii, will eventually disappear
into the ocean. As the volcano subsides below sea-level the top
iseroded flat by waves. This series ofsteps leads to a series of
evolutionary stages that are well illustrated by the Hawaii-Emperor
volcanic chain. This series of steps explains the formation of most
guyots, seamounts with flat tops. Other explanations proposed for the
formation of guyots include extrusion of lava from
ring-shapedconduits.
Another very active submarine volcano is Monowai seamount , about
midway between the Tonga and Kermadec island groups. The volcano is
about 3,000 feet (1,000 m) high and lessthan 600 feet (200 m) below
sea-level. Monowai has erupted at least eight times since 1977. An
eruption was suspected in 1944. Most eruptions were detected using
acoustics but during the 1977-1979 eruption upwelling and discolored
seawater was observed.Monowai erupted most recently in September of
1996 and in April of 1997.
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