February 10, 2000
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
How high was Haleakala?
Haleakala, the volcano that forms East Maui, had higher summit elevations in its glorious past. Today the volcano's highest point is 10,023 feet, but one million years ago it may have been as high or higher than the 13,679-foot summit of Mauna Loa. Erosion and island subsidence are the forces that work to bring each Hawaiian island back to sea level. The question "How high?" is really two questions. Many visitors to Haleakala's modern summit are asking, "How much higher would the summit rise above us if there weren't the large erosional valley of Haleakala Crater?" If we restore the now-eroded summit by extending the surface profiles of the volcano's main ridges, we obtain an elevation of 11,200 feet. The restored summit would be about 1,200 feet higher than the present summit.
The other question, "How high relative to sea level?," recognizes that our large island volcanoes slowly subside. Loading the oceanic crust with innumerable lava flows causes the crust to bow under the added weight. For the Big Island, subsidence rates are as high as about 3 mm (0.12 inches) per year. Rates diminish northwestward along the Hawaiian Island chain, approaching zero at O`ahu.
East Maui currently subsides 0.3 mm (0.012 inches) per year, as determined from tide-gauge records maintained since 1951. During the past one million years, however, the average rate has been about 2.1 mm (0.08 inches) per year. This long-term rate is derived from our knowledge of the age of coral reefs on the eastern submarine flank of Haleakala. The reefs are about 750,000 years old and have subsided about 1,650 m (5,400 ft). The reefs have also been tilted downward toward the volcanoes of the Big Island, a result of the great load those volcanoes have added upon the sea floor.
So how high was Haleakala 220,000 years ago, before the summit crater formed? The answer is 12,000 feet. We obtain this answer by taking our estimated summit elevation of 11,200 feet and restoring to it the elevation lost by subsidence at a rate of 1 mm (0.04 inches) per year. This rate is midway between modern and long-term rates for Maui.
Haleakala would have achieved its maximum height about one million years ago, at the end of its shield-building stage. The top of the shield now lies buried by younger lava flows in the floor of Haleakala Crater, probably at an elevation of about 7,000 feet. If we restore the elevation lost by long-term subsidence (2.1 mm or 0.08 inches per year), we obtain a height of 13,900 feet. This answer is too precise, of course, given our many assumptions. A more suitable answer is to describe the ancient summit as lying between 13,500 and 14,500 feet.
Haleakala's eruptions have continued in the past one million years, but the slowly diminishing summit elevations (downward to 12,000 feet by 220,000 years ago) indicate that subsidence has exceeded volcanic buildup. The volcano remains potentially active; but future eruption rates will never be sufficient to build the volcano faster than it subsides. For Haleakala Volcano, it's downhill from here.
Eruptive activity of Kilauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through a network of tubes toward the coast. One flow is entering the ocean at Lae`apuki, and a second flow, located to the east of the first flow, is intermittently entering the ocean near Waha`ula. The public is reminded that the ocean-entry areas are extremely hazardous, with explosions accompanying unpredictable collapses of the new land. The active lava flows are hot and have places with very thin crust. The steam clouds are highly acidic and laced with glass particles.
Numerous breakouts from the eastern flow occur above Pulama pali, and two lobes are burning trees in a kipuka on the pali. The distal section of this flow is slowly filling low areas mauka of Waha`ula and occasionally dribbling into the ocean.
A resident of Holualoa in Kona felt an earthquake at 7:15 a.m. on Tuesday, February 8. The magnitude-3.5 temblor was located 17 km (10.2 mi) southwest of Kealakekua at a depth of 39.4 km (23.6 mi).
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Updated: 14 Feb 2000