# Volcano Watch — Youngest lava flows on East Maui probably older than A.D. 1790

September 9, 1999

In our efforts to refine the geologic map of Haleakalā, we recently obtained radiocarbon ages from the youngest lava flows, those at La Perouse Bay. The ages indicate these flows were emplaced sometime between A.D. 1480 and 1600.

In our efforts to refine the geologic map of Haleakalā, we recently obtained radiocarbon ages from the youngest lava flows, those at La Perouse Bay. The ages indicate these flows were emplaced sometime between A.D. 1480 and 1600. This finding shakes the long-held assumption that the flows are vintage A.D. 1790. The charcoal that produced the ages was sought to test the 1790 hypothesis, and therein lies an unfinished story of scientific investigation.

The lava flows in question lie 2.5 miles south of Makena, south of the resorts that line Maalaea Bay from Kihei to Wailea. Erupted from a prominent spatter cone, Kalua o Lapa, the flows spread outward and built the promontory of Cape Kinau and, to the east, La Perouse Bay. The La Perouse flows, as they became known informally, were thought to have been emplaced in the time between the voyages of La Perouse (1786) and Vancouver (1793) because of the subtly different charts produced by geographers from those journeys. But neither of these charts is accurate enough for definitive comparison.

Our initial interest in these lava flows was to use them to calibrate our knowledge of the Earth's magnetic pole. The magnetic north pole changes through time, so that a compass needle at a stationary site will point in a different direction today compared with 10 or 1,000 years ago. Lava flows contain minerals that record the magnetic orientation existing at the time they form. By determining the magnetic orientation of precisely dated flows, we can learn of the exact path followed during magnetic polar variation.

The surprise came when we compared the magnetic record of the La Perouse flows with Big Island flows emplaced in 1802; although thought to be similar in age, the pole positions were substantially different. This raised serious doubt that the La Perouse flows were erupted in 1790. Indeed, the La Perouse magnetic poles more closely match those from Big Island flows whose ages range from 350 to 460 radiocarbon years before present.

This information led us to look for charcoal beneath a La Perouse flow and beneath spatter at the Kalua o Lapa vent, which we found at both sites. The laboratory ages are 390 and 460 radiocarbon years before present (before A.D. 1950); analytical uncertainty is plus or minus 50 years. Unlike many isotopic dating clocks, the radiocarbon clock must be calibrated to account for changes in the atmospheric abundance of the carbon-14 isotope, which varies as a consequence of cosmic-ray bombardment. When calibrated, these ages correspond to dates ranging respectively from A.D. 1428 to 1640 and from 1402 to 1609. Thus the two charcoal ages are roughly coincident, within analytical error. They indicate an age substantially older than the previously assumed age of A.D. 1790.

Our effort to resolve the discrepancy between different scientific findings, however, is unfinished. People questioned in 1841 about the age of the flow stated that their grandparents saw it. Their reports indicate a lava-flow age of about A.D. 1750. Oral history is important and not to be overlooked among the numbers from laboratories. Perhaps some of our Maui-born readers have additional knowledge to share about the La Perouse flows that could resolve all pieces of the puzzle.

### Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated during the past week. Lava is erupting from Puu O`o and flowing through a network of tubes from the vent to the sea. Lava is entering the ocean near Kamokuna and enlarging the bench. The public is reminded that the ocean-entry area is extremely hazardous, with explosions accompanying unpredictable collapses of the unstable, new land. The steam clouds are highly acidic and laced with glass particles.

A magnitude-3.7 earthquake was reported felt by a resident of Hawaiian Ocean View Estates at 2:09 a.m. on September 7. The earthquake was located 30 km (18 mi) north of South Point at a depth of 1.4 km (0.9 mi).