Underwater volcano mapped in Southeast Alaska
Sean Doogan
The yet-to-be-named volcano in Southeast Alaska differs from many on the ocean floor. Its top was likely above the water when it last erupted some 10,000 years ago. Courtesy: NOAA
Jim Baichtal has a habit of cruising the website of the National Oceanographic and Atmospheric Administration looking for new hydrographic surveys of Alaska.
“I was just checking the website before I headed out one day,” he said, “and when I saw the survey results of an area near Ketchikan, all I can remember saying is, ‘Oh, my gosh!’” Buried inside the NOAA data were 3D renderings of a previously unknown volcano, beneath the depths of Behm Canal inside Misty Fjords National Monument.
The yet-to-be-named volcano differs from many on the ocean floor. Baichtal, a U.S. Forest Service geologist, says its top was likely above the water when it last erupted some 10,000 years ago. NOAA imaging and new underwater video of the area shows what appears to be leftovers of ash or cinder near the volcano crater.
“If it erupted entirely underwater, we would see quenched magma, a completely different type of rock,” Baichtal said. Upcoming tests on rocks taken during a recent dive there should tell the ultimate tale.
New mapping technology
Before NOAA began its hydrographic survey, using the latest 3D multi-beam sonar, the only maps of sea floor in the vicinity were created in the early 1900s using a technique called “lead lining.” Until the 1940s scientists used ropes with lead weights attached to them to determine the sea floor’s depth in any given location. With enough readings, a crude hydrographic map could be created.
But the maps were not detailed enough to show the features of the volcano under Behm Canal. That changed when NOAA began its survey of Southeast waters. The agency’s ships now bounce sound waves off the ocean floor and measure the time it takes for them to return to the surface.
The technique gives a detailed 3D view of the seafloor. NOAA says the vastness of Alaska means it will likely have plenty of new areas to survey in the coming decades. “We follow the weather,” said Commander David Zezula, with NOAA’s Office of Coast Survey. Typically, in the fall at least one NOAA survey ship plies the waters of Southeast Alaska. They move north with better summer weather. The Raineer, a NOAA survey ship is expected to head to the Bering Sea later this year.
After seeing the outline of the volcano, near Eddy Rock in Behm Canal, on the new NOAA maps, Baichtal decided to take a look under water for himself. He joined Gary Freitag, a Ketchikan-based professor of oceanography with the University of Alaska. Freitag has a remotely operated vehicle (ROV) capable of diving to 800 feet. Together they took video and rock samples from the volcano’s slope as well as its crest. But the trip was cut short.
“We just had a little bit of time for the ROV dive because when we went to the area, we had a group of students from Ketchikan with us,” said Freitag. Another trip is planned for the summer, and Freitag hopes to get more video of the structure of the newly discovered volcano.
“It is a very interesting feature,” said Freitag. “When I took some samples, I had to look hard for a clean place to use the ROV’s pinchers because the volcano is covered in sponges.”
Ice makes fire?
Baichtal says the volcano, like many in southeast Alaska was once pushed above the surface during an eruption. But its climb from the depths may have been the result of crustal response – when the ground “bounces back” after a large weight is lifted. If you go back 10,000 years, much of Alaska was covered by ice up to 4 miles thick. That is a lot of weight on the land beneath – pushing it down as much as 400 feet, Baichtal said. When the ice melted, the land rebounded, actually rising because the great weight of the ice had been removed. “The new volcano is in an area where these de-glaciation events often precede an eruption,” Baichtal says. When the ground rebounds, it opens up vents along fault lines, releasing magma. That is likely what happened at Behm Canal, and Baichtal said it is probably the cause of a lot of eruptions in Southeast Alaska just after the last great ice age.
New studies are beginning to show the effects of ice-age climates and volcanoes. “We frequently see eruptive episodes about 3,600 to 4,000 years after de-glaciation,” Baichtal said. The newly discovered volcano seems to follow that pattern, last erupting a few thousand years after a massive glacier that once covered its peak, melted away.
The underwater Southeast volcanoes can also tell scientists a lot about the glaciers that once covered them. Many glaciers form, melt and reform over thousands or millions of years, but for the most part geologists can only see the evidence of the last glacier in any area; it’s usually stripped the land of any clues about its predecessors.
That’s where volcanoes come into play. By looking at the shape, size and orientation of the vent from an old eruption, as well as examining the cooling pattern and quenched area around the volcano, geologists can learn a lot about ancient glaciers.
Baichtal says he plans to study the new volcano as much as he can, and he’ll examine new NOAA underwater maps to see if he can find others. Despite containing half the US coastline, much of Alaska is still unmapped by modern techniques. Baichtal is confident he’ll find something that piques his interest, whether a volcano, new shoreline, or glacial moraine.
from: http://www.alaskadispatch.com/article/20130526/underwater-volcano-mapped-southeast-alaska
