Charcoal (lower right), found under a lava flow, provides valuable clues to the past eruptive history of a volcano. (Photo courtesy of HVO)

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

The key to unlocking the geologic secrets of a volcano’s future is its past.  The further back we can peer and delve into Pele’s secrets, the better we can understand eruptive behaviors of Hawaii’s volcanoes.  

Examining data from long spans of time paints a clearer picture of a volcano’s eruptive history which, in turn, allows us to better appraise future volcanic activity. 

But how can we be sure of a Hawaiian volcano’s eruptive history when written records began only 170 years ago?  

In terms of geologic time, 170 years is a mere blink of an eye, so we must use unwritten stories — those recorded by volcanoes themselves — to delve deeper into the past. 

In 1980, geologists Jack Lockwood and Peter Lipman discovered that dating charcoal collected from beneath lava flows is a viable way to determine the ages of past eruptions.  The technique, radiocarbon dating, was also used by archeologists to date cultural sites. 

So, what’s unique about charcoal and how is it used to date volcanic events?  Charcoal can be created when lava flows incinerate a forest.  As lava buries vegetation, some of the plant material is reduced to ashes like those produced in your fireplace.  Vegetation that is not completely burned up is preserved in the form of charcoal. 

Carbon is commonplace in nature.  

In the atmosphere, the ratio of radioactive carbon-14 (C-14) to nonradioactive carbon-13 (C-13) and carbon-12 (C-12) is constant.  During photosynthesis, a plant assimilates carbon (this particular ratio of C-14, C-13, and C-12) into the basic building blocks for leaves, branches, stem, and roots.  As soon as the plant dies, however, the ratio begins to change as C-14 decays to C-13 or C-12.   

The rate at which C-14 decays is well known, which enables scientists to use the ratio of radioactive to nonradioactive carbon to determine how much time has passed since the charcoal was created-a radiocarbon age.  These ages are reported as years before present where “present” is 1950, the year when hydrogen bomb tests artificially produced C-14 and altered atmospheric carbon ratios. 

Radiocarbon dating of charcoal can help determine the ages of lava flows up to about 50,000 years ago, which is the upper limit of this technique.  Even with this limit, 50,000 years of eruptive history provides a much better indication of a volcano’s future behavior than 170 years of written records.  The ages of older lava flows, like those on Kohala and most of Mauna Kea, must be determined using methods other than radiocarbon dating.   

Collecting charcoal requires skill — and some luck — so we would like to solicit the help of Hawaii island residents.  We ask that you watch for charcoal, usually in the form of a dark black, sooty layer beneath lava flows, anytime you dig a septic system, excavate for cesspool, or grade a foundation.   

If you find charcoal, the first thing to do is call HVO (967-7328) and let us collect a sample. If that’s not possible, you can collect it yourself. For the charcoal to be useful in revealing a volcano’s past, we need the following additional information with each sample. 

We must know where you found the charcoal as accurately as possible. Note the location with GPS, mark it on a map, or include an address.  In addition, take a photo or make a sketch of the charcoal in place, including the overlying lava flow.  Put the charcoal in a sealed plastic bag, and collect a fist-sized piece of the overlying rock.  Finally, call HVO for mailing or drop-off instructions or to arrange for pick-up. 

The more we know about your charcoal sample, the more valuable it is and the greater help it provides. Proper documentation is the only difference between a valuable radiocarbon age and hibachi fuel. 

By sleuthing out the eruptive history of Hawaii’s active volcanoes, HVO can better understand and forecast future volcanic activity.  Lend us hand by looking for charcoal to date our volcanoes.