# Volcano Watch — Haleakalā Crater formed between 145,000 and 120,000 years ago

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Haleakalā Crater is a large erosional valley at the summit of Haleakalā volcano, East Maui. It formed after the rimrock lava flows were erupted around the top of the volcano about 145,000 years ago, give or take about 10,000 years.

Determined by the potassium-argon method of dating, this age and other supporting evidence were newly obtained by scientists from the Hawaiian Volcano Observatory and their colleagues at Kyoto University, Japan.

An interpretation first offered in the 1940s is that the crater formed as two large valleys coalesced from the south and north flanks of the volcano. The canyon openings, Koolau and Kaupo Gaps, expanded into the summit area along the main rift zone that crosses the volcano.

Kaupo Gap on the south flank probably originated when a large section of the mountain slid southward. The resulting debris-flow deposit—a large landslide—is exposed in sea cliffs about 9 km (6 miles) south of the crater in the Kaupo area. A lava flow that overlies the debris flow at the 100-m (340-ft) elevation is about 120,000 years old. The eastern segment of Haleakalā Crater, which opens southward through Kaupo Gap, must therefore have formed largely in the time between 145,000 and 120,000 years ago.

Less clear is the origin of the northern gap, Koolau, which is on the rainy side of East Maui. Keanae Stream downslope of Koolau Gap has thick deposits of sand and gravel exposed near the coast at Keanae Point. These sedimentary deposits are overlain by a lava flow about 43,000 years old. No debris-flow deposits are exposed along the flanks of Keanae Valley. We presume that the north-flank landslides were smaller than those on the south flank, and that streams carved away the debris as each slide formed. This reworked debris was deposited as the thick sand and gravel at the mouth of Keanae Valley before the 43,000-year-old lava flowed down the canyon from Haleakalā Crater.

Canyons and valleys are difficult to date because they destroy evidence of their growth as they deepen and widen. In the case of Haleakalā Crater, lava flows from five sites around the rim of Haleakalā Crater range in age from about 200,000 years to 145,000 years. These lava flows issued from the axial rift zone, which would have been the summit of the volcano prior to extensive erosion. The deepening valleys etched their way along either side of the rift zone, diverting lava flows into the canyons. From that time forward, the central flanks were barren of lava, and the crater rim was never again buried. Conceivably some lava flows slightly younger than 145,000 years might exist as rimrock, but more extensive dating is required to find those needles in the volcano's lava haystack.

The exact timing of crater formation probably never will be determined, because the crater likely grew in stages. To recognize that such a short duration, only 30,000 years, is sufficient for extensive erosion is an exciting discovery in our understanding of Hawaiian geology. Large landslides are probably the key to understanding rapid rates of erosion, because they redistribute large amounts of rock quickly. Erosion is an ongoing process, but its rate varies greatly across thousands of years.

### Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated at the Puu Oo vent during the past week and provided visitors with an occasional glimpse of surface flow activity on Pulama pali and on the coastal flats. Lava is pooling in the coastal flats and not entering the ocean at this time. A broad active flow front extends 1.5 km (0.9 mi) from the end of the Royal Gardens access road to the west. The closest flow to the ocean is 0.7 km (0.4 mi) from the sea coast.

One earthquake was reported felt during the week ending on March 8. A resident of Hawaiian Ocean View Estates subdivision felt an earthquake at 7:42 a.m. on Sunday, March 4. The magnitude-3.0 earthquake was located 27 km (16 mi) north of Ka Lae at a depth of 2.44 km (1.5 mi).