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Volcano Watch — Sun exposure can help determine age of lava flows

December 7, 2006

A major challenge in mapping a volcano is to reconstruct reliably what happened in the past. Geologists employ a variety of different techniques to determine the ages of past volcanic events.

Photo showing exposure-age dating.

The most familiar method is radiocarbon dating, also known as Carbon-14. Charcoal is produced when plant material is buried by lava flows. Living plants and animals take in carbon from the air, most of which is the stable isotope C-12 and a tiny amount of the unstable isotope C-14.

The amount of C-14 relative to C-12 is nearly constant in the atmosphere. When a plant dies, 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, indicating the age of the lava flow that burned it.

Another method, called exposure-age dating, also uses the ratio of specific isotopes to measure the age of the ground's surface. A simple way to think of exposure-age dating is to look at the color of lava flows in your neighborhood. We all recognize that young lava flows are black in color. But exposure to the sun eventually causes weathering and a color change. An analogy is that when we go down to the beach and lay out in the sun, the amount of change in skin color correlates to the amount of time we are exposed to the sun.

Similarly, the surface of a lava flow changes in color from black to gray, to brown, and tan, then orange, and finally red. This color progression can be used as a general guide to the age of the lava. Therefore, if you see a dark brown flow next to a tan colored one you can be reasonably sure that the dark brown flow is younger and has weathered less than the tan one.

Scientists have discovered ways to use exposure to cosmic rays in a more quantitative way. We use different ratios of several elements to more accurately calculate the age of a surface where we are not able to find charcoal. There are several different environments where finding charcoal is very difficult. This would include the summit of Mauna Loa or, in a desert region like Miloli'i, due to a lack of vegetation.

It is in environments like these that the quantitative exposure-age technique works best. This technique is most useful if the rocks are always exposed to the cosmic rays and has the right mineralogic composition. If obstacles interfere with the cosmic rays, such as soil cover, thick vegetation or shielding created by adjacent rocks, the consequence is an imprecise or inaccurate result.

How does this technique work? Cosmic rays are constantly bombarding rocks on the Earth's surface. This barrage produces special isotopes, which act as clocks that measure the length of time since they were created. As a result, the number of special isotopes in a rock correlates to the length of time the surface of the rock was exposed to the sun-hence, exposure-age dating. Under favorable conditions this technique can be used to measure the length of time volcanic ejecta was deposited on a surface.

Recently, we used this technique to date explosive ejecta around the summit of Mauna Loa. The ejecta sit on top of lava flows at the summit. Age determinations, based on exposure-age dating techniques, range from 500 to 900 years before present for large blocks on the surface. These ages are consistent with radiocarbon ages obtained from summit lava that flowed down slope into vegetated areas, where the ages of the flows obtained from Carbon-14 range from 1,000 to 1,500 years before present.

The key to the future is to know the past. We are employing a number of techniques to unlock secrets of volcanism in Hawaii. Many people come to Hawaii to get a tan. Who would have thought that "tanning" of flows could be used to determine lava flow ages?


Volcano Activity Update

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater. Lava is fed through the PKK lava tube from its source on the southwest flank of Pu`u `O`o to the ocean. About 1 kilometer south of Pu`u `O`o, the Campout flow branches off from the PKK tube. The PKK and Campout tubes feed two widely separated ocean entries, at East Lae`apuki and East Ka`ili`ili, respectively. Both entries are located inside Hawaii Volcanoes National Park.

In the last week, intermittent breakouts from the Campout tube have occurred on the slope of Pulama pali and on the coastal plain.

Access to the sea cliff near the ocean entries is closed, due to significant hazards. The surrounding area, however, is open. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

There were six earthquakes beneath Hawai`i Island reported felt within the past week. Three are aftershocks of the October 15 magnitude-6.7 earthquake. A magnitude-4.6 aftershock occurred at 2:22 a.m. H.s.t. on Sunday, December 3, and was located 20 km (13 miles) west of Waikoloa Village at a depth of 14.1 km (8.8 miles). A magnitude-2.7 earthquake at 2:41 p.m. was located 34 km (21 miles) west-northwest of Waikoloa Village and a magnitude-2.5 earthquake at 4:14 p.m. was located 19 km (12 miles) southwest of Waikoloa Village. Elsewhere, a magnitude-3.0 earthquake occurred at 11:42 p.m. on Friday, December 1, and was located 12 km (7 miles) east-southeast of Honoka`a at a depth of 12 km (7 miles). The smallest event reported felt was a magnitude-1.6 earthquake on November 30 at 9:15 p.m. 14 km (8 miles) southeast of Waimea at a depth of 12 km (7.5 miles). On Kīlauea Volcano a magnitude-2.1 earthquake on Thursday, November 30, occurred 18 km (9 miles) south-southwest of the summit at a depth of 6.2 km (3.9 miles).

Mauna Loa is not erupting. During the past week, earthquake activity remained low beneath the volcano's summit (one earthquake was located). Extension of distances between locations spanning the summit, indicating inflation, continues at slow rates.