Alaska - a charcoal tree will allow the dating of Crater Ridge's eruption.

A group of scientists , led by a resident of Sitkan , found in the bank of a creek in Alaska an unusual relic of an eruption : a charcoal tree.

Samples made ​​by Jim Baichtal , Forest Service geologist on Kruzof island - Photo Kitty LaBounty / in Alaska Public Media

According to the botanist K. LaBounty, University of Alaska, " The tree was standing , largely intact , carbonized but not petrified. If we look closer, we could see its whole structure, see the place where the roots , some branches, nodes always are present. "
On examination, the tree is embedded in a layer of volcanic pumice that have completely buried, so hot that it was carbonized in place and remained frozen , standing and maintained by the pumice so far .

Jim Baichtal , a geologist from the Forest Service , indicates that these pumice come from an eruption of Crater Ridge, one of the structures of the Edgecumbe volcanic field, on Kruzof island. This field covers 260 km² and includes the andesitic stratovolcano Edgecumbe , domes and craters of Crater Ridge and Shell Mountain, aligned on a SW-NE line marking a regional fissure.

SE Alaska - the Edgecumbe volcanic field and its major volcanoes aligned SW-NE - from a topographical relief map of the AVO - USGS

SE Alaska  -  On the foreground, the top of the Edgecumbe and his colored dacitic pumice slopes of the post-glacial pyroclastic episode - in the background , domes of Crater ridge , wooded - Photo Jim Riehle - 05.2002 (U.S. Geological Survey , Alaska Volcano Observatory) .

Crater Ridge is truncated by a caldera of 1600 meters wide and 240 meters deep . The latest dated eruptions are phreatomagmatic explosions that took place in the middle of the Holocene, the last activity being dated by the GVP of  2220 BC ( radiocarbon dating uncorrected to this day) .
The post- glacial activity has produced voluminous pyroclastic deposits , equivalent to 7.6 cubic kilometers of dense rock , and found to Juneau and Lituya Bay, 200 km. further north.

Pumice and charcoal wood samples and a piece of obsidian were collected to precisely date the original explosive eruption and its chemistry.

Carbonization or petrification :

 

 

At left , detail of the carbonized tree / Kruzof island / photo Kitty LaBounty / in Alaska Public Media.

Right, a petrified tree / Yellowstone NP - Tower Roosevelt / photo Bernard Duyck

Carbonization, is the transformation of an organic substance into carbon under the effect of heat . Carbonization of wood gives charcoal.

The process of petrification occurs when wood is buried under a layer of sediment where it keeps first due to a lack of oxygen, before a mineral rich water is flowing through the sediment and replaces some just the wood cells with minerals (mostly silicates, such as quartz ). Less than a hundred years are probably necessary for some pieces of wood to be petrified.

Sources:

- Alaska Public Media - Tree buried by volcanic eruption could reveal seismic secrets - by E.Schoenfeld

- Global Volcanism Program - Edgecumbe