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February 16, 2001
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Volcanoes beneath the Sea
| A Fiery Birth |
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"Fiery birth of a new Pacific island!" read the May 24 announcement from the Australian Commonwealth Scientific and Industrial Organization (CSIRO). CSIRO had sent a team of researchers to study the dormant undersea Kavachi volcano in the Solomon Islands east of Australia. They discovered that the volcano was not dormant at all. Molten magma and ash were shooting more than 200 feet into the air, while plumes of steam and smoke rose thousands of feet above the ocean's surface. Over the years, the material coming out of Kavachi had built up from the ocean floor, eventually rising over 3,000 feet until it broke through the water's surface and formed a small island. "We arrived to find waves breaking on the volcanic peak. Violent eruptions were taking place very five minutes," reported Brent McInnes, the expedition's chief scientist. Kavachi belongs to the world of submarine volcanoes, which mostly do their erupting out of sight and occasionally form islands above the surface of the water. In the case of Kavachi, these "ephemeral islands" have formed at least 8 times in the past 60 years, only to recede under the water. These islands have reached more than 500 feet in length. |
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Last May, amazed scientists watched as lava and ash blasted through the surface of the Pacific Ocean and high into the air. They were getting a rare glimpse of a live eruption from an undersea volcano. How do these submarine volcanoes work?
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| Underwater Eruptions |
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For instance, the undersea Loihi volcano in the Hawaiian Islands may be invisible at sea level, but it towers almost 10,000 feet above the ocean floor. That's taller than Mount St. Helens. During a 1996 eruption of Loihi, researchers recorded the temperature of the surrounding water at almost 400°F. Some submarine volcanoes form where Earth's tectonic plates meet. For instance, Kavachi and the other Solomon Islands form about 30 kilometers north of where the Indo-Australian and Pacific plates meet in a jumble of smaller, "microplates." Where one plate is pushed—or subducted—beneath another, the bottom plate begins to melt and magma starts to rise through the overlying plate. This results in eruptions along rifts in the ocean floor. Other submarine volcanoes form when tectonic plates move over a "hotspot" where magma punches through from below. This is what formed—and is still forming—the Hawaiian Islands. Since these eruptions are surrounded by water, they behave differently than their land-based relatives. The lava that pours out onto the sea floor cools rapidly in the water. As a result it tends to shatter into sand and debris that begin to pile up in a sloping pattern. Unlike land-based lava flows, which bind to the mountains from which they come, submarine volcanoes build themselves up particle by particle. With each new eruption, more of the debris accumulates on the sides of the underwater mountain. Those volcanoes—which are also called "seamounts"—may exist so far undersea that the weight of the water keeps their steam and gasses from escaping. In many cases, their volcanic activity is invisible at the ocean surface. |
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Kavachi's dimensions may pale next to famous above-ground volcanoes—from Mount St. Helens in Washington state to Mount Etna in Italy. But submarine volcanoes can become just as large and violent.
| Around the World |
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The Caribbean also provides an underwater home to volcanic residents, one of which is named Kick-'em-Jenny. This submarine volcano, located just north of Grenada, has a base about 3 miles wide, it rises 4,300 feet from the ocean floor, and its top stands 525 feet below the ocean surface. It last erupted in 1990. Of course, not all islands formed by underwater volcanoes remain underwater. Volcanic islands, most notably, the major islands in Hawaii have been naturally built around the active volcanoes at their centers. Hawaii's Big Island was constructed by five large volcanoes, including Mauna Loa, the largest active volcano on Earth. Much of Japan and its surrounding islands emerged volcanically from the sea. Similarly, the West Indies islands in the Caribbean, the Azores in the Atlantic, and hundreds of islands in the Pacific have their origins in undersea eruptions. |
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Neither are they confined to the seismically active area covered by the Pacific Ocean. Researchers are monitoring submarine volcanoes from the coast of Oregon to the Mediterranean Sea.
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