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Leg 125

Scientists recover 40-million-year volcanic record

May 15, 1989
-- Scientists recently returning from a cruise in the Pacific Ocean reported that they recorded the 40-million-year history of a submarine volcano and recovered rocks that formed in Earth's mantle.

The 27 scientists from nine countries were on board the Ocean Drilling Program's drill ship, JOIDES Resolution, which operated near the arc shaped volcanic chains of the Mariana and Izu-Bonin islands between Guam and Japan.

The discoveries will enable scientists to better understand the nature of volcanic processes in the circum-Pacific's chain of volcanoes, called the Ring of Fire. The scientists--

drilled a hole more than half a mile deep into the roots of a 40-million year volcano--

drilled several holes through a thick sequence of muds near the volcano, enabling them to count and date the layers of volcanic ash, much like a botanist counts a tree's rings--

drilled into a volcanic-like structure where Earth's mantle -- the thick shell between Earth's crust and its core -- is exposed on the seafloor. The scientists found small pockets of natural gas trapped in the mantle rocks.

The hole drilled into the volcano -- the deepest drilled into a submarine volcano -- penetrated a thick pile of lava flows to reach the channels that once fed the magma from depth to the surface. Scientists determined the age of the volcano by dating the microfossils of plants and organisms trapped by successive eruptions of lava.

Lava flow contains history of volcanic eruption

One of the holes drilled near the volcano recovered more than 1200 feet of mud and sand that recorded 100 layers of ash from violent volcanic eruptions. The ash beds show that volcanic activity in the region south of Japan has fluctuated during the last 40 million years; in recent times the area has experienced relatively intense activity.

The mantle rocks recovered from a submarine mountain belt near the deep ocean trench north of Guam will give scientists additional information about a newly discovered seafloor formation. Scientists previously had thought that these conical formations, called seamounts, were dead volcanoes. The drilling results, however, have revealed that these mountains are composed not of lava, but rock that rises in a solid state from many miles beneath the seafloor. This new kind of seamount is a direct result of the process of plate tectonics.

Because of plate tectonics, the Pacific crustal plate is moving westward. At the deep ocean trenches that outline the western Pacific, the Pacific plate buckles and sinks as much as 400 miles into the Earth's interior. Deep within the mantle, increasing heat and pressure distill the fossil bearing sediments' water and gases, causing the material to rise and react with the mantle's rocks.

Scientists find pockets of hydrocarbon gases

This process alters the amalgamation of regenerated sediment and mantle rocks, transforming it into a light, greenish-gray material called serpentine, a type of asbestos. Serpentine usually mixes with natural gases and surrounding water, causing it to rise along deep fractures. When it reaches the seafloor, it oozes out like toothpaste from a tube to form the giant, volcanic-shaped mud domes.

Recovery from these mounds of altered rock and sediment produced additional proof that natural hydrocarbon gases can be trapped in rocks that formed deep in Earth's mantle and carried to the surface of the seafloor.

In addition to the pockets of hydrocarbon gases, scientists recovered other natural resources such as native copper and iron pyrite from the submarine volcano.

During the next two years, the scientists will analyze the material and data obtained from the cruise to learn more about the deep plumbing systems of submarine volcanoes, the processes of mantle alteration occurring near deep-ocean trenches and possible sources of minerals for further exploration and development.

Co-chief scientists for the Leg 125 were Dr. Patricia Fryer of the University of Hawaii at Manoa, Honolulu, and Dr. Julian Pearce of The University, Newcastle upon Tyne, the United Kingdom. Dr. Laura Stokking was ODP staff scientist.

The ship departed Guam Feb. 20 and arrived in Tokyo on March 18. The 27 scientists were from Australia, Canada, Italy, the Federal Republic of Germany, France, Japan, Norway, the United Kingdom and the United States.

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