DESCRIPTION:
Submarine Volcanoes, Vents, Ridges, and Eruptions

Submarine volcanoes and volcanic vents are common features on certain zones of the ocean floor. Some are active at the present time and, in shallow water, disclose their presence by blasting steam and rock-debris high above the surface of the sea. Many others lie at such great depths that the tremendous weight of the water above them results in high, confining pressure and prevents the formation and explosive release of steam and gases. Even very large, deepwater eruptions may not disturb the ocean surface.

The unlimited supply of water surrounding submarine volcanoes can cause them to behave differently from volcanoes on land. Violent, steam-blast eruptions take place when sea water pours into active shallow submarine vents. Lava, erupting onto a shallow sea floor or flowing into the sea from land, may cool so rapidly that it shatters into sand and rubble. The result is the production of huge amounts of fragmental volcanic debris. The famous "black sand" beaches of Hawaii were created virtually instantaneously by the violent interaction between hot lava and sea water. On the other hand, recent observations made from deep-diving submersibles have shown that some submarine eruptions produce flows and other volcanic structures remarkable similar to those formed on land.

During an explosive submarine eruption in the shallow open ocean, enormous piles of debris are built up around the active volcanic vent. Ocean currents rework the debris in shallow water, while other debris slumps from the upper part of the cone and flows into deep water along the sea floor. Fine debris and ash in the eruptive plume are scattered over a wide area in airborne clouds. Coarse debris in the same eruptive plume rains into the sea and settles on the flanks of the cone. Pumice from the eruption floats on the water and drifts with the ocean currents over a large area.

Volcanoes are not randomly distributed over the Earth's surface. Most are concentrated on the edges of continents, along island chains, or beneath the sea forming long mountain ranges. ... In the past 25 years, scientists have developed a theory -- called plate tectonics -- that explains the locations of volcanoes and their relationship to other large-scale geologic features.

According to this theory, the Earth's surface is made up of a patchwork of about a dozen large plates that move relative to one another at speeds from less than one centimeter to about ten centimeters per year (about the speed at which fingernails grow). These rigid plates, whose average thickness is about 80 kilometers, are spreading apart, sliding past each other, or colliding with each other in slow motion on top of the Earth's hot, pliable interior. Volcanoes tend to form where plates collide or spread apart, but they can also grow in the middle of a plate, as for example the Hawaiian volcanoes.

Plate Tectonics Menu

The boundary between the Pacific and Juan de Fuca Plates is marked by a broad submarine mountain chain about 500 kilometers long, known as the Juan de Fuca Ridge. Young volcanoes, lava flows, and hot springs were discovered in a broad valley less than 8 kilometers wide along the crest of the ridge in the 1970's. The ocean floor is spreading apart and forming new ocean crust along this valley or "rift" as hot magma from the Earth's interior is injected into the ridge and erupted at its top.

[Map,11K,InlineGIF]
Juan de Fuca Ridge - Gorda Ridge - Axial Seamount -- showing features of Pacific/Juan de Fuca/North American subduction system relative to Western United States. Open blue arrows, ridge-spreading directions; solid blue arrow, convergence direction.

Juan De Fuca - Gorda Ridge - Axial Seamount Menu

Axial rises 700 meters above the mean level of the ridge crest and is the most magmatically robust and seismically active site on the Juan de Fuca Ridge between the Blanco Fracture Zone and the Cobb offset. The summit is marked by an unusual rectangular-shaped caldera (3 x 8 kilometers) that lies between the two rift zones. The caldera is defined on three sides by a boundary fault of up to 150 meters relief. Hydrothermal vents colonized with biological communities are located near the caldera fault or along the rift zones. Following the discovery of hydrothermal venting north of the caldera in 1983, a concentrated mapping and sampling effort was made in the mid-late 1980s.

Juan De Fuca - Gorda Ridge - Axial Seamount Menu

The volcanoes of this region are unusually explosive, and include Kikai, which produced one of the earth's largest Holocene explosive eruptions about 6,300 radiocarbon years ago. ...

Most volcanoes in this region result from subduction of westward-moving oceanic crust under the Asian Plate. In the Izu-Marianas chain, however, the crust to the west is also oceanic, forming island arcs where volcanoes are largely basaltic but far more explosive than oceanic hotspot volcanoes.

(This region) ... has the largest number of submarine volcanoes, mostly extending down the Izu-Marianas arc, and the largest number of reported submarine eruptions. The many reports of water discoloration over submarine vents have also contributed to this region's record number (180) of eruptions preceded by a question mark, indicating uncertainty that the eruption actually took place. ...

Japan Volcanoes and Volcanics Menu

Kavachi, one of the most active submarine volcanoes in the SW Pacific, occupies an isolated position in the Solomon Islands far from major aircraft and shipping lanes. Kavachi, sometimes referred to as Rejo te Kvachi ("Kavachi's oven"), is located south of Vangunu Island only 30 kilometers north of the site of subduction of the Indo-Australian plate beneath the Pacific plate. The shallow submarine basaltic-to-andesitic volcano has produced ephemeral islands up to 150 meters long at least eight times since its first recorded eruption during 1939. The roughly conical volcano rises from water depths of 1.1 - 1.2 kilometers on the north and greater depths to the south.

Solomon Islands Volcanoes and Volcanics Menu

Kick-'em-Jenny, an actively growing submarine volcano 8 kilometers (5 miles) off the north shore of Grenada, rises 1,300 meters (4,300 feet) from the sea floor. Its summit has grown from 235 meters (770 feet) below the sea surface in 1962 to 160 meters (525 feet) twenty years later. Numerous historical eruptions, mostly documented by acoustic signals, have occurred since 1939, when an eruption cloud rose 275 meters (900 feet) above the sea surface.

West Indies Volcanoes and Volcanics Menu

Since the beginning of a historical records early in the 19th century, eruptions have occurred frequently at Mauna Loa and Kilauea. These two volcanoes on the Big Island are among the most active in the world. Nearby Loihi Seamount, off the Big Island's south coast, is the newest Hawaiian volcano, not yet visible above the ocean surface. ...

If the hot-spot theory is correct, the next volcano in the Hawaiian chain should form east or south of the big Island. Abundant evidence indicates that such a new volcano exists at Loihi, a seamount (or submarine peak) located about 20 miles off the south coast of the Big Island. Loihi rises 10,100 feet above the ocean floor to within 3,100 feet of the water surface. Recent detailed mapping shows Loihi to be similar in form to Kilauea and Mauna Loa. Its relatively flat summit apparently contains a caldera about 3 miles across; two distinct ridges radiating from the summit are probably rift zones.

Photographs taken by deep-sea camera show that Loihi's summit area has fresh-appearing, coherent pillow-lava flows and talus blocks. Examination of samples dredged from Loihi indicates that the pillow-lava fragments have fresh glassy crusts, indicative of their recent formation. The exact ages of the sampled Loihi flows are not yet known, but certainly some cannot be more than a few hundred years old. In fact, the occurrence of earthquake swarms at Loihi during 1971-1972, 1975, and 1984-1985 suggests major submarine eruptions or magma intrusions into the upper part of Loihi. Thus, Loihi appears to be a historically active, but as yet submarine, volcano. ...

Link to: Hawaiian Volcano Observatory Website for MORE Loihi Information

[Map,27K,InlineGIF]
Map, Major Volcanoes of the Mariana Islands
-- includes location of a section of the Marianas Trench

As with oceanic-continental convergence, when two oceanic plates converge, one is usually subducted under the other, and in the process a trench is formed. The Marianas Trench (paralleling the Mariana Islands), for example, marks where the fast-moving Pacific Plate converges against the slower moving Philippine Plate. The Challenger Deep, at the southern end of the Marianas Trench, plunges deeper into the Earth's interior (nearly 11,000 meters) than Mount Everest, the world's tallest mountain, rises above sea level (about 8,854 meters).

Subduction processes in oceanic-oceanic plate convergence also result in the formation of volcanoes. Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano. Such volcanoes are typically strung out in chains called island arcs. As the name implies, volcanic island arcs, which closely parallel the trenches, are generally curved. The trenches are the key to understanding how island arcs such as the Marianas and the Aleutian Islands have formed and why they experience numerous strong earthquakes. Magmas that form island arcs are produced by the partial melting of the descending plate and/or the overlying oceanic lithosphere. The descending plate also provides a source of stress as the two plates interact, leading to frequent moderate to strong earthquakes.

North Pacific Volcanic Islands Menu

Ruby (-230 meters), a submarine volcano that rises to within 230 meters of the sea surface northwest of Saipan, was detected in eruption in 1966 by sonar signals (Norris and Johnson, 1969). In 1995 submarine explosions were heard, accompanied by a fish kill, sulfurous odors, water bubbling, and the detection of volcanic tremor.

North Pacific Volcanic Islands Menu

The mostly submarine Vestmannaeyjar volcanic system is the southernmost volcanic center of the eastern volcanic zone that cuts across east-central Iceland. It consists of a roughly 30 x 40 kilometer group of volcanic islands and submarine cones occupying a shallow shelf off the south coast of Iceland. Vestmannaeyjar was the site of two of Iceland's most noted 20th-century eruptions. The new island of Surtsey grew from the ocean floor during 1963-67. The island of Heimaey, 20 kilometers to the north, was the site of dramatic eruptions in 1973 during which lava flows partially overran the town of Vestmannaeyjar. Several other volcanoes have erupted during historical time. Twenty-two postglacial eruption sites are known in the Vestmannaeyjar system.

Iceland Volcanoes and Volcanics Menu

The uninhabited 2 x 2.4 kilometer White Island (321 meters) is the emergent summit of a 16 x 18 kilometer submarine volcano. The island consists of two overlapping stratovolcanoes; the summit crater appears to be breached to the SE because the shoreline corresponds to the level of several notches in the SE crater wall. Intermittent steam and tephra eruptions have occurred throughout the short historical period, but its activity also forms a prominent part of Maori legends.

New Zealand Volcanoes and Volcanics Menu

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