The cone is capped by a crater 540 m in diameter (1770 ft), although the northern rim is below sea level and the crater is flooded by the sea. Molokini was active about 230,000 years ago -- give or take 90,000 years -- according to an age measured from lava fragments contained in the cone.

The age was obtained recently by Yoshitomo Nishimitsu, a graduate student at Kyoto University, Japan. Working in conjunction with scientists from the Hawaiian Volcano Observatory, he used the potassium-argon method of dating to measure ages from 60 lava flows on Haleakala.

(Editor's Note: This article is reprinted, with minimal editing, from the U.S. Geological Survey Hawaiian Volcano Observatory's weekly Volcano Eruption Bulletin. As in this case, the scientists there prove to be good science writers as well.)

Knowing the age of Molokini will improve our understanding of Haleakala's volcanic history and the likelihood of future eruption along its rift zones.

Molokini lies along Haleakala's southwest rift zone. Much of the rift zone is mantled with lava, cinders, and ash erupted during the past 50,000 years. For that reason, geologists have always assumed that Molokini was a fairly young volcanic formation.

But the 230,000-year age suggests that Molokini is much older, probably older than Haleakala Crater itself.

Molokini would be similar to cinder cones elsewhere along the southwest rift zone except that it erupted through water. When magma erupts explosively in shallow water, the liquid water heats, expands rapidly and changes to steam, adding to the eruptive force.

The extra force shatters the extruded lava, which exposes more hot material -- and hence more steam and more force as the eruption grows.

Near-shore eruptions are some of the most dangerous that Hawaiian volcanoes can produce.

Shallow marine eruptions have two consequences for the appearance of the resulting cone. The first is grain size, because the ripping power of these marine eruptions leads to finer-grained deposits than in cinder cones onshore. The second is the abundance of volcanic glass, because the lava fragments are quickly cooled by water before crystals can form.

Glass is a geologically unstable material. It alters rapidly to brownish-yellow clays, giving Molokini its earthy yellow color. In contrast, cinders erupted on land are reddish and black.

For those dying to know, the Molokini deposits are basanite, a type of basalt with fairly low amounts of silicon and high concentrations of sodium and potassium. (Geochemists would say it contains 45 percent SiO2, 4.4 percent Na2O and 1.4 percent K2O.)

Visible crystals are sparse, even under a magnifying glass. Lava like this is typical of Haleakala's flows erupted during the past 500,000 years.

The ocean near Molokini is a popular skin-diving location reached easily by boat. The island itself is off limits, however, because it serves as a bird sanctuary. We at the Hawaiian Volcano Observatory take this opportunity to thank Drs. Fern Duvall (State of Hawaii's Department of Land and Natural Resources) and Marilet Zablan (U.S. Fish and Wildlife Service) for permission to collect geologic samples.

The trip to Molokini was made possible through the expert piloting of Chief Petty Officer Robert Schmidt and Petty Officer Richard Magaña of the U.S. Coast Guard.

[Contact: Carolyn Bell]

14-Feb-2001