# Volcano Watch — New ages show Haleakalā is a postshield volcano

Release Date:

Haleakalā volcano, on Maui, is still in its postshield stage of volcanic evolution, as determined by 50 new isotopic ages. The volcano was long thought to have passed beyond that stage, with a lengthy eruptive lull lasting several hundred thousand years. The new ages show, however, that the lull does not exist and that lavaflows have erupted persistently during the past one million years.

The idea that Hawaiian volcanoes progress through four stages has been widely accepted by geologists since the 1930s. Our understanding of why these stages occur increased when the plate tectonic theory was developed in the 1960s.

A volcano like Kīlauea or Mauna Loa grows fastest and erupts most frequently during the preshield and shield-building stages, when it is closest to the center of the Hawaiian hot spot. Hot spots, with their origin deep in the Earth's interior, are relatively stable compared to the moving plates that cover the Earth's surface. As the Pacific plate moves northwestward at 10 cm per year, it carries the shield-stage volcano away from its heat source. As a result, the volcano erupts less frequently. Also, the lava erupted will differ chemically from that produced during the shield stage because of the diminished heat supply. These changes define the character of the third stage, called postshield volcanism. Mauna Kea and Hualālai, located about 70 km from the hot spot, are examples of volcanoes that have entered their postshield stage.

Haleakalā, nearly 200 km from the hot spot, has been active for two million years. It remains active, having erupted several times in the past 1,000 years. Until now, its recent lava flows were assigned to the fourth stage, the rejuvenation stage, for the following reason. Several large canyons cut into the volcano's flanks. Geologists assumed that a substantial time break and absence of eruptions were required for such erosion to gain the upper hand and carve deeply into Haleakalā. Eruptions that followed such a lengthy episode of erosion, geologists reasoned, were the indicators of a rejuvenated volcanic system. Thus, an estimation of time has always been a critical part in assessing volcanic rejuvenation. But Haleakalā's lava flows had never been sampled systematically to test the existence of a volcanic lull.

Our new ages, determined mostly by the potassium-argon method of dating, show that, although volcanism has fluctuated during the past one million years, it has never stopped completely. The large canyons on Haleakalā's north, east, and south flanks were eroded in only a few tens of thousands of years, beginning after 150,000 years ago, based on the new ages. No lengthy period of erosion and eruptive quiet was required. Lavasupply has diminished during the postshield stage, but it has never been curtailed completely. Chemically, lava flows of the past ten thousand years are similar to those erupted throughout the past 600,000 years. These flows are erupted from the same alignment of vents that has been building Haleakalā's rift zones for eons.

Haleakalā will erupt again, given the frequency of its past eruptions and long eruptive history. What's new in our understanding is that the recent, and coming, eruptions are the waning efforts of a postshield-stage volcano instead of a rejuvenated volcano. We have to look farther up the island chain, to West Maui, Molokai, and Oahu, to find volcanoes that have erupted products of the rejuvenated stage.

### Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated at the Puu Oo vent during the past week. The fluid, shield-building lava moves away from the vent toward the ocean in a network of tubes and descends Pulama pali in several separate tubes. After-dark visitors to the end of the Chain of Craters road can view the glow of surface flows above the pali and watch the molten lava stream down the pali and fan out at the base. Many surface flows are observed in the coastal flats, and lava continues to enter the ocean at Kamoamoa and the area east of Kupapau.

The public is reminded that the benches of the ocean entries are very hazardous, with possible collapses of the unstable new land. The steam clouds are extremely hot, highly acidic, and laced with glass particles. Swimming at the black sand beaches of the benches can be a blistering or even deadly venture.

One earthquake was reported felt during the week ending on December 27. Residents of the Kau district felt an earthquake at 10:33 p.m. on December 23. The magnitude-2.9 earthquake was located 13 km (7.8 mi) northwest of Naalehu at a depth of 10 km (6 mi).