The dark-colored charcoal (left of rock hammer) from a log buried by lava was found at the base of an ‘ā‘a flow in the District of Ka’Å« on the Island of HawaiÊ»i. The age of this charcoal, determined by an accelerator mass spectrometer radiocarbon method, is 2075 plus or minus 36 years before present. USGS photo.

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

One of the fundamental premises of geology is that the key to understanding the future is to look at the past. In order to understand how a volcano will behave, geologists map the deposits of past eruptions.

An important element for characterizing volcanic deposits is to establish if the eruption was predominantly effusive (characterized by lava flows) or explosive. Furthermore, we want to know the spatial distribution of the deposits, and how frequently and where the different types of eruptions occur.

To help determine the timing of eruptive activity, geologists use a radiocarbon age-dating technique. Collecting charcoal is the most common method used in Hawaii, not only by geologists, but also by archaeologists, ecologists, and others.

How does radiocarbon, or carbon-14, dating work?

Carbon-14 is produced in the atmosphere and readily utilized by plants to build tissue, fiber, and wood. Carbon-14 is radioactive and has a half-life of 5,700 years. As long as a plant is alive, the amount of carbon-14 in its tissue remains approximately the same.

Once the plant dies, however, the quantity of carbon-14 in the plant tissue decays, so that after 5,700 years the amount of carbon-14 is 50 percent of the amount present when the plant was alive. After another 5,700 years, the concentration is down to 25 percent of its initial concentration.

Any high temperature volcanic product, such as a lava flow, spatter, and hot ash, can create charcoal when it burns or buries a plant. In Hawaii, geologists dig under lava flows to recover charcoal left from plants.

Scientists use the decay rate of carbon-14 to obtain age-dates from this charcoal. A relatively new accelerator mass spectrometer technique can now provide ages between 80 and 100,000 years.

Geologists often make assumptions about the charcoal they collect. We assume that plants are alive at the time an eruption occurs. In addition, we assume that charcoal is created when a lava flow covers the vegetation.

These assumptions can create problems if the charcoal is created from wood that was already dead when it burned. There are other potential sources for confounding a radiocarbon age, such as dating forest fire charcoal or “old” living wood, for example, the core of a log 1 m (3 ft) in diameter that could be quite a bit older than the exterior of the log.

To minimize these problems, geologists make sure that collection techniques are impeccable to reduce the chance that spurious charcoal is recovered. We also try to minimize infiltration of contaminant charcoal. Given the choice of age-dating a log or a twig, we choose the twig to avoid inadvertently dating old wood in the interior of the log.

Once charcoal is recovered, we dry the sample and pick out small pieces of black shiny charcoal that has a distinctive “snap” when broken. Soft pliable charcoal is discarded. The sample is then sent to a radiocarbon-processing lab, where it is chemically treated to remove modern carbon. The sample is converted to graphite, which is used to determine the radiocarbon age.

Once we get the results from the lab, how do we then decide if the age is “good”?

The radiocarbon age has to fit into the stratigraphic framework based on geologic mapping of the volcanic deposit. For example, if “Flow B”, dated at 550 years old, is bracketed by Flow A, dated at 1,000 years, and Flow C, dated at 1,500 years, it is highly likely that the radiocarbon age of Flow B is not good, because it should be between 1,000 and 1,500 years.

Once we determine that the radiocarbon results are consistent with stratigraphy, we have to calibrate the age. Calibration is necessary because, using tree-ring data from around the world, we know that the concentration of atmospheric carbon-14 varied from time to time, and we must account for this variability.

Most ages are reported in years before present, with zero being A.D. 1950, before atmospheric atomic bomb testing altered the amount of carbon-14 in the air. If the age control is good enough based on stratigraphy, radiocarbon ages can be presented in terms of calendar years to facilitate comparison with non-geologic historical events.

So, in geology (and other fields), charcoal can be useful for more than just grilling on the barbeque.