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]
[Illustration: Ahinahina (silversword).]
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."