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May 7, 1998

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


The dating game

The announcement last week that astronomers at Keck Observatory had looked back 12.3 billion years in time was astounding. When you think about it, though, it is pretty amazing that we can determine the age of any natural event that took place before written records, whether it be 12.3 billion years or a few hundred. How is this done?

Most dating methods for material on earth use the principle of radioactive decay. Certain chemical elements contain one or more unstable isotopes that decay at constant rates. If you know how much of one unstable isotope was present in a material at the start and how much is present now, you can determine the material's age. The principle is simple, but the devil is in the detail. Geochronology, the science of dating the past, is difficult and highly technical. The results, however, are vital for determining how fast things happened even if they weren't seen or described in writing.

At HVO, we are interested in determining the ages of eruptions that occurred hundreds to thousands of years before written accounts were kept. This interest has a practical purpose: the better we know when eruptions happened in the past, the better we can estimate the likelihood of them occurring in the future.

How do we measure the age of an eruption so long ago? We look for charcoal. With practice and luck, it is not too hard to find a place where lava flows buried forest or grassland. Most of the vegetation burns away in such circumstances, but a small amount of charcoal remains. The charcoal generally occurs beneath the thin edge of the flow, where high temperatures didn't last long enough to completely burn up the vegetation. The geologist searches diligently under thin toes of pahoehoe or along the edges of `a`a flows, hoping to spot tiny rootlets or twigs of charcoal. A day is successful if even one piece of charcoal is found!

The charcoal is then sent to a laboratory that specializes in determining radiocarbon, or C-14, ages. Carbon-14 is produced mainly by interactions of cosmic rays with atmospheric nitrogen. Living plants and animals take in carbon from the air; most of the carbon is the stable isotope, C-12, but a tiny amount is an unstable isotope, C-14. The amount of C-14 relative to C-12 is nearly constant in the atmosphere. When the plant or animal dies (such as when charcoal forms), the intake of carbon stops. The amount of stable C-12 stays the same in the charcoal, but the amount of unstable C-14 decreases with time. The amount of C-14 that remains relative to the amount of C-12 is a measure of when the plant died, which is the age of the lava flow that burned it.

The final age is not quite exact, because of analytical uncertainties and because of small fluctuations with time in the atmospheric ratio of C-14 to C-12. The ages are reported in "radiocarbon" years before present, where the "present" is 1950, when atmospheric testing of hydrogen bombs loaded the atmosphere with artificially produced C-14. Sometimes the age is corrected from "radiocarbon" to truly "calendar" years by using a scale developed between tree-ring calendar ages and radiocarbon ages. In Hawai`i, trees rarely have annual rings, so we depend on the scale worked out for temperate climates in order to derive a precise calendar age.

When HVO geologists report that, for example, a flow covered part of Kulani about 450 years ago, they "ain't just blowin' smoke." There must indeed have been smoke, but it would have come from lava cooking plants into charcoal and starting the carbon-14 clock.

Eruption and Earthquake Update

The east rift zone eruption of Kilauea Volcano from the Pu`u `O`o vent continued unabated during the past week. The lava flows through a network of tubes to the seacoast and enters the ocean at two locations-Waha`ula and Kamokuna. The public is again reminded that these two areas are extremely dangerous, and the National Park Service has restricted access to them because of frequent explosions accompanying collapses of the growing lava delta.

There were five earthquakes reported felt. Residents of Mountain View and Hawaiian Ocean View Estates felt an earthquake at 1:33 a.m. on Sunday, May 3. The magnitude 3.9 earthquake was located 11.5 km (7 mi) south of the summit of Kilauea Volcano at a depth of 30 km (18 mi). Two earthquakes near Kauluoa Point, 11 km (6.6 mi) south of Honaunau, were felt in Kona and Volcano on Tuesday night, May 5, at 10:07 and 10:23 p.m. The pair of earthquakes originated from a depth of 13.6 km (8.2 mi) and had magnitudes of 3.3 and 3.9, respectively. A magnitude 3.5 earthquake located 2.5 km (1.5 mi) west of Pahala was felt at Ka`u Hospital on Wednesday afternoon at 3:00 p.m. The earthquake originated from a depth of 10.5 km (6.3 mi). A magnitude 4.3 earthquake shook residents from Pepe`ekeo to Ho`okena on Thursday, May 7 at 1:15 in the afternoon. The earthquake was located 5 km (3 mi) northwest of Pahala at a depth of 8.1 km (4.9 mi).


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Updated: 7 May 1998