pillow lavas

Submarine eruptions at mid-ocean ridges produce fresh lava flows like these "pillow" lavas that form as lava slowly oozes out of a fissure on the seafloor. Click image for larger view and more information.

Submarine Volcanism

Bill Chadwick
Oregon State University

The Ring of Fire

The "Ring of Fire" is a circular arc of active volcanoes that surrounds the Pacific Ocean basin. Much is known about the volcanoes on land within the Ring of Fire (for example, in the Aleutians, the Cascades, the Andes, etc.), but comparatively little is known about the submarine volcanoes, simply because they are underwater and more difficult to observe. This multi-year project aims to explore submarine volcanoes within the Ring of Fire in two very different tectonic settings on either side of the Pacific—one in which new seafloor is created and the other in which old seafloor is destroyed.

Volcanoes and Plate Tectonics

The Earth is covered with "tectonic plates" that are in constant motion over the partially molten interior. Almost all the volcanoes on Earth occur where tectonic plates are either moving apart or are moving together. Where tectonic plates are moving apart, molten rock—magma—rises up from deep within the Earth to fill the gap, and in doing so, creates new ocean floor. The boundaries between separating plates are called "spreading centers" or "mid-ocean ridges." They are the sites of frequent submarine volcanic eruptions. Explorer Ridge in the northeast Pacific, which was the focus of the first year of this project in 2002, is an example of a mid-ocean ridge.

Where tectonic plates are moving toward each other or colliding, one plate is usually forced under the other in a process called subduction. At subduction zones, the plate that is forced downward dives back into the Earth and is eventually melted and recycled. This process also causes melting above subduction zones. This molten rock rises back up to the surface to feed chains of active volcanoes called volcanic arcs. Volcanic arcs can be on land or underwater, or can be a combination of islands and submarine volcanoes. The Marianas arc in the western Pacific is an example of a volcanic arc with many volcanic islands and underwater volcanoes. It will be the focus of this project in the second and third years of this project, 2003 and 2004.

Shallow submarine eruption at Kavachi Seamount

In contrast to the relatively gentle eruption of basaltic submarine volcanoes, island arc volcanoes often erupt lavas with a higher silica content and more dissolved gases. Photo courtesy of Australian Commonwealth Scientific and Industrial Corporation. Click image for larger view and more information.

Volcanoes in these two different settings have very different characteristics. Mid-ocean ridge volcanoes tend to be linear and look like long low ridges, whereas arc volcanoes are cone-shaped, centralized, and isolated. Mid-ocean ridge volcanoes also have more primitive lava compositions because the lava comes more or less directly from the Earth's interior without changing much.

On the other hand, the lava that reaches arc volcanoes rises through a longer path from its source region to the surface. So its composition can be significantly changed along the way. This difference in compositions is reflected in the fact that mid-ocean ridge eruptions are generally non-explosive, whereas arc eruptions often are explosive.

Volcanoes, Hydrothermal Vents, and Life

Seafloor hot springs called "hydrothermal vents" are common in areas where submarine volcanoes bring hot magma near the seafloor or erupt lava at the surface. Seawater that circulates deep within a submarine volcano gets heated before it returns to the seafloor. Hydrothermal vent fluid is rich in dissolved chemicals, and these hot springs support a unique ecosystem of microorganisms and animals that is independent of sunlight. Biotech companies are particularly interested in the heat-tolerant microbes that live on and below the seafloor at submarine volcanoes because they have novel enzymes that may be useful for creating new medicines. Because the DNA of organisms living at seafloor hot springs is very primitive, some scientist believe that life on Earth may have originated at deep-sea hydrothermal vents on submarine volcanoes.