Submarine Volcanoes
One of the most exciting dimensions of the Earth-building process is volcanism .
The power and destructive capability of volcanoes is well known, but there are many
more volcanoes underwater, termed submarine volcanoes, than there are on land. Some
are quite active with recent eruptions, whereas many others lie quietly dormant.
During eruptions, molten earth, or magma, comes close to the ocean floor and
escapes through fractures in the Earth's crust. Now termed lava, the molten rock flows
across the seabed, solidifying as it comes in contact with the cold ocean water.
Submarine volcanoes are not found
everywhere under the sea, but only in
places where magma rises close to the
ocean floor. Most submarine volcanoes are
directly connected with the movements of
the Earth's tectonic plates . The outer crust
of the Earth is covered with rigid plates that
gradually move over the partially molten
interior.
(image from nsf.gov)
There are three circumstances where tectonic plates can interact with each other
and the Earth's molten interior to form submarine volcanoes. One form of interaction
occurs when a tectonic plate slides over a "hot spot" of magma under the seafloor. A
second circumstance occurs where tectonic plates are spreading apart at the mid-ocean
ridges, allowing magma to rise from deep within the Earth. A third type is where plates
are moving toward each other, which often forces one plate under the other, in a process
called subduction.
Hot Spots
In certain areas of the ocean, hot magma chambers lie below the moving tectonic
plates. Cracks and fissures penetrate into the magma chamber, allowing pressure to be
released and magma to flow out onto the seafloor. Over time, underwater mountains,
called seamounts, develop, and in some cases they may reach the surface of the ocean or
above, creating volcanic islands. The tectonic plates are continually moving, but the
magma chamber remains stationary. Eventually, the magma breaks through the ocean
floor in a new location and another seamount begins to form.
Over geologic time, many volcanic seamounts may form in a row as the tectonic
plate continues its advancement. This can create an underwater mountain range. If the
volcanoes rise above the ocean's surface, they will form a chain of islands. Hawaii is an
example of such an island chain. Kauai, the westernmost island, is the oldest, and most
eroded by wind and water. Its volcanic activity has long since become dormant, whereas
the youngest and easternmost island, the big island of Hawaii, is still volcanically active.*
Because the Pacific tectonic plate is still moving west, a new volcanic seamount is
forming east of Hawaii. Oceanographers have named the 3,000meter (9,840-foot)
seamount Loihi. It may reach the surface in the distant future.
Mid-Ocean Ridges
Most of the volcanic eruptions on Earth occur
on the mid-ocean ridges, but oceanographers have
only recently been able to detect them. Mid-ocean
ridges spread out like the seams on a baseball, across
the floor of the world's oceans. They form extensive
volcanic mountain ranges, averaging 1,000
kilometers (620 miles) wide, 1,000 to 2,100 meters
(3,280 to 6,888 feet) high, and extend for 65,000
kilometers (40,300 miles) around Earth.
(image from noaa.gov)
The Ring of Fire is the name given to the margin of the Pacific Basin where active
volcanoes are common. Also known for its tectonic activity, the Ring of Fire is a zone of
frequent earthquakes. These phenomena are manifestations of basin-encircling
subduction zones where the Pacific Plate slides under another plate. The volcanic arcs are
on the overriding plate and typically are parallel to the ocean trench that marks the
downturning of the subducting Pacific Plate.
The Atlantic Mid-ocean Ridge is the most continuous and longest, extending from
the Arctic Ocean to beyond the southern tip of Africa. Many of these ridges have
spawned submarine volcanoes that lay astride the spreading zone. The Axial Seamount is
an example of a submarine volcano rising from a mid-ocean ridge. It is located on the
Juan de Fuca Ridge, a prominent feature on the seafloor of the northeast Pacific Ocean,
about 484 kilometers (300 miles) west of Oregon and Washington.
The Juan de Fuca Ridge is a mid-ocean ridge, a place where two of the Earth's
tectonic plates are slowly moving apart, or diverging from each other. Along the plate
boundary, magma rises up to fill the gap and creates a new ocean floor. This is why Axial
is so volcanically active and why oceanographers are using it as an underwater research
observatory. Axial Seamount rises 1,000 meters (3,280 feet) above the surrounding
seafloor and yet is still 1,400 meters (4,590 feet) below sea level. A large volcanic crater,
or caldera, 3 kilometers (1.9 miles) wide, 8 kilometers (5 miles) long, and 100 meters
(328 feet) deep, marks its summit where the seamount has collapsed inward. The caldera
is studded with recent lava flows and hydrothermal vent fields.
Subduction Zones
When tectonic plates converge, they meet at an area termed a subduction zone.
One plate is pulled, or subducted, under an advancing plate. This can create a variety of
submarine activity. The farther down the plate becomes subducted, the greater the
pressure. The area of the Earth's crust between the two plates becomes liquefied and
escapes upward towards the surface, erupting as a volcano. The active and dormant
volcanoes that created the Aleutian Islands of Alaska were formed in this manner.
These types of submarine volcanoes also form chains, called island arcs, that
typically parallel the subduction zones. In reality, the entire Pacific Ocean basin is
fringed with zones of subduction. This "Ring of Fire" (see figure on page 205) fuels
many active volcanoes on land and underwater.
Subduction earthquakes can be the most dangerous to human life because they are
often located along coastlines. Exploding mountains and lava flows are not the only
danger from a subduction volcano. Earthquakes are always associated with submarine
volcanoes and are usually small and numerous. Most of them are too small to be detected
by seismometers on land.
Researchers use underwater microphones, called hydrophones, to detect
submarine earthquakes. However, earthquakes associated with a subduction zone can be
quite large. As the ocean floor subducts, the leading edge of the overriding plate gets
pulled along under itself. Eventually the pressure releases and the overriding plate snaps
back up. This causes fractures that can produce volcanoes as well as significant
earthquakes of magnitude 9.0 or higher. The land movement, in turn, can trigger massive
tsunamis with the capability of widespread devastation along populated coastlines.
Crouse, R. (2003, January 1). Volcanoes, Submarine. Encyclopedia.com. Retrieved July
30, 2014, from http://www.encyclopedia.com/doc/1G2-3409400337.html