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HAWAII NATURE NOTES
GEOLOGY THE ORIGIN OF THE SCENIC FEATURES The Island of Maui was built by two volcanoes. That forming West Maui is deeply dissected into several high peaks. The old summit crater now is encompassed by the head of Iao Valley. Mt. Kukui, the highest point, has an elevation of 5,788 feet. East Maui is built of three series of lava products from Haleakala Volcano whose flows extended westward across the present isthmus to come to rest against the base of West Maui Volcano. These represent three great periods of activity, the latter two being separated by a long interval of quiescence that was characterized by intense erosion and mild, if any, eruptions. Geologists designate these three periods by the names Honomanu, Kula, and Hana. The mountain was built over three rifts, northerly, easterly, and southwesterly, each extending about fifteen miles. In earliest (Honomanu) time, about the beginning of the Ice Ages, a symmetrical shield like Mauna Loa was built of pahoehoe and aa basalts 8,500 feet above present sea-level. During the next (Kula) cycle, eruptions were more explosive in nature; flows were composed of more viscous andesite between which layers of ash and soil accumulated. Big cinder cones and extensive ash beds were formed at this time. Like Mauna Kea today, the Honomanu dome was capped by a craterless mound of cinders, 2,500 feet high, that was studded with many lesser cones. The summit was a mile east of the present top on Red Hill and a thousand feet higher than it is today. As Kula eruptions declined and grew less frequent, running water cut deeply into the sides of the mountain and excavated four great valleys, Keanae, Kaupo, Kipahulu, and Waihoi, that had broad heads, thousands of feet deep. Numerous lesser valleys were later to be buried more or less by lava flows. Most of the eastern summit ridge was worn away; Kaupo and Keanae Valleys met near the summit and fused into a great depression like that near the head of Iao Valley today. At one time, a great flow of mud, probably triggered by an earthquake, swept all before it as it moved down Kaupo Valley into the sea. Its remnants today are 350 feet deep at Puu Maneoneo near the coastal road. A similar mass movement of rock on soft mud was started by an earthquake on April 2, 1868 at Wood Valley, west of the Kilauea Section of Hawaii National Park; the flow, in its precipitous descent, buried a village with 31 people and more than 500 head of stock. In recent times, volcanism again quickened at Haleakala. giving the third (Hana) series of volcanics. This veneered the east and west slopes of the volcano, covered the floor of the depression, and pushed great lava flows through Koolau and Kaupo Gaps to the sea. Large flows and cones mask the divide that delimited the two great valleys. During Hana time, the northern rift alone remained inactive. The most recent activity, dated by Hawaiian legend as 1750, is represented by two bare, black flows above La Perouse Bay, the southwest corner of the island. Haleakala Crater, 7 miles long and 2-1/2 miles wide, is locally proclaimed the largest extinct crater on earth, but the claim like the name is inaccurate, Nevertheless, it possesses a most unusual geological origin and beauty that give it a worthy place among the National Parks.
The summit depression of Haleakala stimulates speculation, and competent geologists have come up with widely differing hypotheses regarding its origin. In the account of their visit (see The Historical Background, p. 29), the first foreign visitors naturally used the term "crater," which has been in vogue ever since. Pickering and Drayton of the Wilkes Expedition remark, "The crater of Haleakala, if so it may be called, is a deep gorge."10
Drayton's sketch was the first published map of the crater. James Dana, the great geologist with the Expedition, sailed past the mountain and later wrote a physiographic description based on notes made by Pickering and Drayton. In the official report, he expressed the idea, suggested by the crude map, that the mountain has been ripped apart by mighty convulsions that attended the most recent activity, so that the northeastern (Hana) part was separated along a zigzag crack from the rest of the mountain by the width of Keanae and Kaupo Valleys.11 During the great eruptions that attended the rending, lava covered the floor and poured in great floods through Koolau and Kaupo Gaps.
W. D. Alexander, who surveyed the crater in 1869, believed: ". . . this is a real terminal crater, and not merely 'a deep gorge open at the north and east' or a caldera. I have indeed heard the theory proposed that the mountain is but a wreck of a complete dome with a small terminal crater, the whole top of which has fallen in and been carried away, as is supposed to have been the case with some of the volcanoes of Java, and the caldera of Palma."12
C. E. Dutton, volcanologist of the United States Geological Survey, objected to Dana's explanation and wrote that the depression is "strictly homologous" to Kilauea Crater, that is, a collapsed caldera.13 He assumed that this had been tapped by the upper ends of Keanae and Kaupo drainages. In 1887, Dana had opportunity to make a quick trip through the crater and down Kaupo Valley, so that he tempered his earlier opinion and decided that Keanae and Kaupo valleys might be graben.14 Reginald Daly of Harvard rejected the hypothesis that the depression was like Kilauea Crater, since arcuate faults so prominent at Kilauea are apparently absent at Haleakala.15 In his paper on petrography, Whitman Cross stated, "What is commonly called the crater of Haleakala appears to me to be, in some part at least, a result of erosion."16 At about the same time, Sidney Powers stated his belief that Kipahulu and Waihoi Valleys are graben, but he based his opinion on "authentic reports" and does not claim that he saw the valleys.17
H. T. Stearns analyzed carefully all the supporting evidence and objections to the various viewpoints.18 Of all of his profession, he made the most thorough field surveys, with the conclusion that the "crater" is chiefly erosional, affected by the recession of two great amphitheater-headed valleys, instead of by collapse, sliding away of the side of a cone, or explosion. Small craters may have existed at the time, but not a large one resembling the present depression. He believed that the big size results from the fact that the heads were offset and not in a straight line. The shape of the depression is what would result upon the fusion of two amphitheater heads similar to those of Waikoi, Kipahulu, and Manawainui valleys of today. Each of these is a typical Hawaiian valley, narrow at the base, but with a broad amphitheater at the head. Kipahulu is separated from Waihoi and from Kaupo Valleys by narrow divides. It can be assumed that a similar divide once separated the amphitheaters at the heads of the early Kaupo and Keanae drainages. Stearns further believes that once the rift zone was reached, stream erosion was greatly accelerated because of the loosely knit structure, the presence of many weak cinder cones, and the dike complex. This complex, as in other places in Hawaii, must have yielded perennial spring water to accelerate erosion. His summary is as follows: "No stratigraphic or constructural evidence was found to support the hypothesis that Haleakala Crater is a true caldera, that it was formed by renting, or that Keanae and Kaupo valleys tapping this depression are grabens. Instead, detailed mapping and the examination of water tunnels show that Haleakala dome has been eroded by a number of great valleys. The hypothesis is presented that the so-called "crater" of Haleakala is chiefly, if not entirely, the result of the coalescence of the amphitheater heads of Kaupo and Keanae valleys and that renewed volcanic activity has partly masked their former divide and partly filled these valleys with lava flows."
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nature_notes/havo/vol6-59i.htm
24-Mar-2006
