Reprinted from Scientific American.com
Expeditions

Mar 30, 2010 04:33 PM in Basic Science | 1 comments

Seamounts: Exploring underwater mountains

By Kathryn Eident

 

pillow-lava

Editor's Note: Journalist and crew member Kathryn Eident is traveling on board the RV Atlantis on a monthlong voyage to explore undersea volcanism in the eastern tropical Pacific Ocean, among other research projects. This is the second blog post detailing this voyage of discovery for ScientificAmerican.com

The water beneath the hull of the RV Atlantis is smooth and glassy as the ship floats on the first science station of this month-long leg. The submarine Alvin has already been launched and is making the hour-and-a-half-long descent to the seafloor thousands of feet below.

Inside the sub, Chief Scientist John Sinton (University of Hawaii) and Dr. Julie Bowles (University of Minnesota) prepare for the volcanic features they’ll see once they near the bottom.

They’re using the sub, guided by a pilot, to explore the spine of the Galapagos Spreading Center (GSC), a mid-ocean ridge about 120 miles north of the Galapagos Islands.

The two scientists have spent the last few days poring over maps and deciding which features they want to explore along the GSC. The bathymetric maps hint at a section of the ridge filled with seamounts of varying widths and heights, some with depressions called calderas, others flat-topped with plateaus. Deep troughs, called grabens, lie in between some of the seamounts.

This area is considered a "normal" mid-ocean ridge, meaning it exhibits many volcanic features, like seamounts, scientists would expect to find at places like the GSC.

Seamounts form from magmatic eruptions, which occur when lava seeps up to the crust through the partially-melted sub-layer, known as the asthenosphere, in the mantle. Here, extreme temperatures and pressures force lava up through the layers of the Earth in tubes criss-crossing the mantle like a sort of chaotic plumbing for lava.

As the plates spread apart, the lithosphere, or crust, is moved aside like a lid, allowing molten rock from the liquid asthenosphere to seep through.

The result: underwater mountains and hills all along the axis of the mid-ocean ridge.

In general, seamounts have wider bottoms and steep sides with flat or collapsed tops. They can rise a few feet from the ocean bottom, or thrust thousands of feet high into the water column.

On this trip, Sinton and Bowles explore a seamount that reminds Sinton of a penguin, prompting him to christen it the Spanish name "Pinguino." Peering through the porthole stationed on the port (left) side of the sub, it appears to Sinton that the 350-foot-high seamount is covered in pillow lavas with very little sediment cover.

Pillow lavas form when molten rock is pushed up onto the seafloor at a very slow rate. As the molten rock reaches the crust, it quickly cools and hardens. The cooled lava acts as insulation for the lava coming behind it, and the form slowly grows into a bulbous-like chunk of rock. The outside of the rock is glassy—hard and shiny—while the inside may contain smaller crystals of various minerals, like olivine, a combination of magnesium, iron, oxygen and silicon.

As they glide around "Pinguino," Bowles and Sinton notice a lack of sediment covering the rocks. This observation leads Sinton and Bowles to think the lava is fairly young—maybe a few hundred years old.

There are lots of pillow lavas here too, prompting Sinton to form a hypothesis: perhaps the seamounts in this region formed from the same volcanic eruption, made up of many flows? He will have to make more observations and wait for geochemical analysis of the rocks Alvin has picked up to learn the answer to this question, but it's a start and Sinton is excited.

Though Sinton and Bowles have only explored a small portion of the GSC, they already have data that will help them eventually understand how the ridge formed. Their observations will help other scientists onboard design their Alvin dives, too. In all, it's been a successful dive, Sinton said. Twenty-six more dives to go—with a last minute addition of an extra dive—and countless questions remain to be answered.

Image: Lights from the submarine Alvin illuminate pillow lavas strewn about the seafloor. The photo was taken by a camera mounted on the sub. (Photo courtesy of John Sinton)

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