• BBC World Service1:00 am to 6:00 am

Menu Schedule Links

Signal Status

There are currently no events to display.

1410_RadioLab

Tree Dates Eruption That Spewed Ash Across Southeast

By | November 12, 2013

We recently told you the story of an ancient, charcoal tree found beneath a pile of ash near Sitka. Scientists hoped tests would help them better understand the area’s rich volcanic history.

Young Blake LaPerriere found this tree during a family outing near a beach on Kruzof Island, west of Sitka.

Structural details can still be seen in the charcoal tree found near Sitka. Photo courtesy of Kitty LaBounty.

Structural details can still be seen in the charcoal tree found near Sitka. Photo courtesy of Kitty LaBounty.

The trunk and branches were charred as they were buried in hot volcanic ash and debris thousands of years ago. But they remained upright and became visible as ocean waves washed away some of the surrounding material.

Download Audio

Forest Service Geologist Jim Baichtal took samples and sent them off to a lab. The results came up with an approximate date: 13,180 years ago.

Baichtal says that generally corresponds to earlier tests on other ancient items found in the ash area. But those results were in question. These aren’t.

“This has low probability of any kind of contamination and it’s definitely contained in the flow. And you can see from the deposit that there were a couple different surges of that eruption. So you know this tree was living on the landscape and got encased in this very, very hot pumice ash at the time of the eruption,” he says.

Hear our first report about this tree.

The tree age provides a firm date for the explosion from Crater Ridge, a cone near Mount Edgecumbe, the region’s most visible volcano.

“There was a relatively large eruption that blanketed a good portion of Southeast Alaska with volcanic ash,” he says.

Baichtal says about 2 ½ inches covered much of Baranof Island, parts of Glacier Bay and as far east as Juneau.

So anytime it’s found, it dates what’s above or below.  (Read a CoastAlaska report on more Southeast volcanos and lava flows.)

The same ash layer has also been found in Sitka Sound, a large ocean bay next to the volcanic area. That’s no surprise. But it was in layers of fresh-water sediments.

“We know that Sitka Sound itself was elevated above its present location and (was) a non-marine environment. It had a fresh-water lake in it,” Baichtal says.

In fact, it was almost 200 feet higher than it is today. Baichtal says that’s due to weight and pressure from advancing and retreating ice-age glaciers.

Tests also established the tree as a Sitka Spruce, which of course, grow in groups.

Does that mean there are more charcoal trees under the ash?

“Without a doubt. There was a forest growing there that just was rapidly buried.”

Baichtal says tests on the tree revealed nothing earth-shattering. But, like so much in science, it added information that will help the experts shape a better picture of Alaska’s past.

You can subscribe to APRN’s newsfeeds via email, podcast and RSS. Follow us on Facebook at alaskapublic.org and on Twitter @aprn.

Comments

Please read our comment guidelines.