Deep Sea Volcanic Action
Submarine volcanoes are underwater fissures in the Earth's surface from which magma can erupt. They are estimated to account for 75% of annual magma output. The vast majority are located near areas of tectonic plate movement, known as ocean ridges. Although most are located in the depths of seas and oceans, some also exist in shallow water, which can spew material into the air during an eruption. Hydrothermal vents, sites of abundant biological activity, are commonly found near submarine volcanoes. The first scientists to witness exploding rock and molten lava from a deep sea volcano, seen during a 2009 expedition, report that the eruption was near a tear in the Earth's crust that is mimicking the birth of a subduction zone. Scientists on the expedition collected boninite, a rare, chemically distinct lava that accompanies the formation of Earth's subduction zones.
Boninite is a mafic extrusive rock high in both magnesium and silica, formed typically during the early stages of subduction. The rock is named for its occurrence in the Izu-Bonin arc south of Japan.
Nobody has ever collected fresh boninite and scientists never had the opportunity to monitor its eruption before, said Joseph Resing, University of Washington oceanographer and lead author of an online article on the findings in Nature Geoscience. Earth’s current subduction zones are continually evolving but most formed 5 million to 200 million years ago. Scientists have only been able to study boninite collected from long-dead, relic volcanoes millions of years old.
Resing was chief scientist on the expedition, funded by the National Oceanic and Atmospheric Administration and the National Science Foundation, that pinpointed the location of the West Mata volcano, erupting 4,000 feet below the surface. West Mata is a submarine volcano located 120 miles southwest of the Samoas. The volcano is currently the deepest erupting submarine volcano in the ocean, and the first to ever be filmed erupting.
At subduction zones the oceanic crust on one tectonic plate slides beneath another, producing abundant volcanism and contributing heat, gases and mineral-laden fluids to ocean waters. Scientists have long studied the impact of subduction zones on geological and geochemical cycles. To puzzle out how subduction zones form and evolve they study inactive contemporary marine volcanos that do not produce boninite and they collect and study boninite lavas collected on land and examine cores collected from the deep sea.
"West Mata lies above the subducting Pacific plate and is part of the rapidly expanding Lau Basin, which is bounded by Samoa, Tonga and Fiji. The large bend at the northern end of the Tonga trench produces a tear in the Pacific plate and creates unusual lavas that usually only form at very young subduction zones." Riesing said.
Conditions are right for boninite to form, there’s lots of seawater released from subducting rock that mixes into relatively shallow mantle that has previously melted, causing the mantle to remelt at high temperatures. Boninite lavas are believed to be among the hottest from volcanoes that erupt on Earth.
"What makes this exciting is how uncommon these eruptions of boninite are, both now and in the past. Locked within the boninite is critical information about the rates and magnitudes of subduction-zone magmatism and global geochemical cycles." Rubin said.
The scientists writing in Nature Geoscience think the release of gaseous water, carbon dioxide and sulfur dioxide from the slab is the reason the eruption was so explosive. No one realized such energetic eruptions happened so deep, Resing says. Streams of red and gold lava 35 feet long shot through the water and lava-skinned bubbles some three feet across emerged.
West Mata, which the scientists estimate has been erupting for at least three years, and eight other elongated volcanoes that overlap each other in the northeast Lau Basin sit within one of the most magmatically active areas on Earth, Resing says.
"Observing the eruption in real time was a rare and special opportunity because we know so little about how submarine volcanic activity behaves. This is one of only a handful of glimpses of the process we’ve had to date and is the first time we’ve actually observed natural submarine earthlight from the glowing magma." Embley said.