# Volcano Watch — West Maui's rejuvenated-stage eruptions were about 600,000 and 385,000 years ago

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The four youngest vents on West Maui erupted between 610,000 and 385,000 years ago. These newly determined radiometric ages remind us that sporadic small eruptions are possible on Hawaiian volcanoes even as they verge on extinction.

Earth scientists commonly use the phrase "rejuvenated-stage" for those eruptions that occur long after a Hawaiian volcano has finished its main stages of growth.

In West Maui's case, the main volcano had culminated its growth by about 1.1 million years ago. Nearly 500,000 years passed with no evidence of renewed activity, but the volcano wasn't quite finished. The subsequent eruptions were all from small cinder cones that grew briefly and then died. Lava flows were extruded from each, but the area covered by lava was generally only a few hectares (a few acres).

The oldest of these four small cones is Kilea, which lies a short distance inland from Olowalu on the southwest side of West Maui. Kilea, known today for its petroglyphs, erupted about 610,000 years ago. At nearly the same time, Kekaa Point was active 15 km (9 mi) to the northwest. Lava from Kekaa cone has an age of 580,000 years. The cone today is surmounted by buildings at Kaanapali Resort. The difference between these two ages isn't great, and the analytical error for each determination is about 10,000 years. Therefore Kilea and Kekaa cones could be roughly the same age.

About 200,000 years passed before West Maui was again volcanically active. This time the action took place at Puu Laina, a cinder cone north of Lahaina town. Puu Laina is the largest of the four rejuvenated-stage vents, and it produced more lava than the others. Flows from Puu Laina cover 3.2 square kilometers (800 acres). The age of Puu Laina is 388,000 years.

The fourth cone, Puu Hele, was active about 385,000 years ago, roughly the same time as Puu Laina. Puu Hele has been quarried so extensively that the mound of the cinder cone is gone, and few Maui residents are aware of its location. The quarry lies 2.5 km (1.6 mi) north of Maalaea along the road to Wailuku. No lava flows issued outward from Puu Hele cone. Our sample came from a lava flow or intrusion that was within the cone and exposed by the quarrying.

The ages of these youngest West Maui vents are surprising, because they cluster about two distinct episodes. Two were active about 600,000 years ago, but chemically their lava flows are slightly different. So, too, for the cones active about 385,000 years ago; their lava is dissimilar. Therefore it seems likely that the eruptions from each period were fed from separate small pods of magma that rose into the earth's crust at about the same time.

From a volcano-hazards standpoint, these youngest eruptions are only curiosities. On the one hand they indicate that West Maui and other "extinct" Hawaiian volcanoes could erupt again. But the time between eruptions is so great, and the extent of area affected is so limited, that the hazard is nil. This is the irony for rejuvenated-stage volcanism at nearly extinct volcanoes on Oahu, Molokai, and West Maui-eruptions could occur again, but it's pointless to worry about it.

### Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated at the Puu Oo vent during the past week. Lavamoves away from the vent toward the ocean in a network of tubes and descends Pulama pali in three separate tubes. Two surface flows from breakouts of the tube system also were observed streaming down the pali. Lava continued to enter the ocean in the area east of Kupapau. Many surface flows are active in the coastal flats and on the ocean entry bench.

The public is reminded that the bench of the ocean entry is very hazardous, with possible collapses of the new land. The steam cloud is extremely hot, highly acidic, and laced with glass particles. Swimming at the black sand beach of the bench can be a blistering or even deadly venture.

Four earthquakes were reported felt during the week ending on September 13, 2001. A resident of Honokaa and a resident of Ahualoa felt an earthquake at 3:07 p.m. on Friday, September 7. The magnitude-3.9 earthquake was located 11 km (6.6 mi) southwest of Waikii at a depth of 23.3 km (14.0 mi).

A magnitude-4.7 earthquake at 2:09 p.m. on Monday, September 10 was felt in Hilo, Mountain View, and Volcano. This event marked the start of a swarm of earthquakes from Loihi, the submarine volcano off the southeast coast of Hawaii. On Thursday, September 13, two more earthquakes from Loihi were felt. The first was at 3:11 a.m. with a magnitude of 4.9, and the second was at 8:39 a.m. with a 4.4 magnitude. The earthquakes were located south of the summit area at a depth of 12 km (7.2 mi). The last large swarm of earthquakes from Lo`ihi Volcano occurred during July-August of 1996 when HVO recorded over 4,000 earthquakes from the area.