Tuesday, June 18, 2013

Researchers map 12 new Alaskan volcanoes

How do you tell when you’re sitting on a volcano? Ok, in some cases, it’s fairly obvious; when the mountain actually explodes, or a lava flow pours past you, it becomes pretty clear. Same thing if there’s a large cone constructed, pretty obvious. But those aren’t the only types of volcanoes.

When a volcano becomes dormant, the rocks are often fairly easy to erode. Glaciers can scour the slopes of the volcano, ash can be washed away, and plant life can be all too happy to take root in fresh, fertile volcanic soils.

So, imagine a terrain that is heavily forested, partially glaciated, and not heavily inhabited by man so that nothing has stripped away the soil. You could have ancient volcanoes sitting there, covered by trees, looking like nothing other than hills on the terrain.

Turns out, this was exactly the case in the panhandle area of Alaska, south of the capitol city of Juneau. The area is made up of a variety of islands, heavily forested, and covered by hills and mountains generated by the active subduction in the area. There were a few known, recently-active volcanoes in this area of Alaska, and there is a volcanic province across the border in Canada, but much of the land was not well mapped.

In 2009, the US Geological Survey and the US Forest Service began investigating a volcanic pile near one of the known volcanoes in the area known as Edgecumbe. The researchers thought it would be related to the known volcano, but when they analyzed the chemistry of the rocks, they found it to be highly distinct from Edgecumbe, but instead very similar to a series of volcanoes in Canada.

As the research continued, the scientists kept finding other volcanoes that hadn’t been seen before. Effectively, they were filling out the details of an entire volcanic province, made up of smaller volcanoes related to the volcanism in Canada. Geologists working in the Canadian volcanic province had suspected some volcanoes might exist to the west, but a dozen additional eruptive centers seems to have caught them off guard as well.

One interesting feature of these volcanoes is illustrated in this image, which shows columnar jointing in a lava flow. Columnar jointing forms as lava flows cool; as the flow cools, the volume decreases due to a property known as thermal contraction, causing the part of the flow that cooled off to begin to crack, and pillars like these will form perpendicular to the cooling direction. Vertical columns mean that the flow cooled from the top down, which is the normal behavior for most lava flows; they cool at the surface first.

But here, you also see some of the columns bending over to the side; they’re not vertical any more. Something else must have cooled off the lava at that point, and maybe helped generate the shape of the cliff face. That something else was ice. Ice can make a nice big, solid wall that can stand in the way of a lava flow. When the lava flow runs into it, it melts some of the ice, cooling the lava at the side. The ice shapes the lava flow and also starts cooling it from the side, causing the columns to point sideways as seen here.

Lava flows can be really useful for glacial science because minerals in lava flows are quite easy to date. By identifying lava flows like this one which have run into glaciers, scientists will be better able to date the advance and retreat of glaciers in this part of Alaska, which will give information on how these glaciers have responded to climate changes and what might happen in the future. This lava flow is about 700,000 years old, and tells anyone who looks at it that this area was glaciated 700,000 years ago.

Image credit, Susan Karl, U.S. Geological Survey:

Details and discussion at Weather Underground:

Alaska volcano discoveries signal Canada’s need for closer monitoring

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