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 80-100,000 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 3 feet 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 1,000-1,500 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 barbecue.
Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.
Volcano Activity Updates
Kilauea continues to erupt at its summit and East Rift Zone. During the past week, the summit lava lake level varied between about 100-130 feet below the vent rim within Halema‘uma‘u Crater. On the East Rift Zone, the “61g” lava flow continued to advance across the coastal plain and enter the ocean. The lava flow does not pose an immediate threat to nearby communities.
Mauna Loa is not erupting. Seismicity remains elevated relative to the long-term background rate, but has not changed significantly over the past week. Earthquakes are occurring mostly in Mauna Loa’s south caldera and upper Southwest Rift Zone at depths less than 3 miles. GPS measurements show deformation related to inflation of a magma reservoir beneath the summit and upper Southwest Rift Zone, with inflation occurring mainly in the southwestern part of the magma storage complex.
One earthquake was reported felt on the Island of Hawaii this past week. At 12:11 a.m. Thursday, a magnitude-3.7 earthquake occurred 4.7 miles northwest of Mauna Kea Summit at a depth of 15.3 miles.
Visit the HVO website (https://hvo.wr.usgs.gov) for past Volcano Watch articles, Kilauea daily eruption updates, Mauna Loa weekly updates, volcano photos, recent earthquakes info, and more; call for summary updates at 967-8862 (Kilauea) or 967-8866 (Mauna Loa); email questions to askHVO@usgs.gov.
