THE MOHOKEA CALDERA.
Upon the southwest flank of Mauna Loa the evenness of the slope is interrupted by the presence of an irregular pit, as if the rock had been removed by an immense scoop. My attention was first called to it by conversation with Mr. Joseph S. Emerson of the Hawaiian Trigonometrical Survey. He inquired into the reason of the depression, and the association with it of certain hills resembling the "Buttes" of the Cordilleras region of the United States. A paper by him descriptive of the region is en titled Characteristics of Kau, published in the American Journal of Science, December, 19o2. I ventured to call the depression the Mohokea Caldera in Volume 14 of the Bulletin of the Geo logical Society of America, and gave a further account of it in the same publication, Volume 17, after a second examination of the ground in 1905.
In place of the map with contours I have constructed a small relief map of the caldera thinking that its features may be more readily appreciated. Plate 24A. First is the general situation back from the harbor of Punaluu towards Mauna Loa. Second, are the elevations called buttes. Third, the valleys running north westerly between the lines of buttes and the sides of the depres sion. Fourth, the isolated peaks of Kaumaikeohu to the north east and Puu Iki on the north rim.
The Mohokea depression is situated in Kau, in the southwest ern part of the island of Hawaii, to the north of the harbor of Honuapo, which at present is the end of the sea voyage for those who skirt the leeward side of the great island on the way from Honolulu to Kilauea. There is a line of stages from Honuapo to the volcano, rising gradually for a distance of thirty miles to the altitude of 4,040 fet. Hilea, about four miles from the seaport, is the best point from which to traverse the depression. It is the residence of the head overseer of the sugar plantation, who very kindly accompanied me to the principal points of in terest in the caldera. From the house, situated upon lava, the road ascends a steep hill covered by volcanic ashes to about 1,200 feet altitude, and thence another thousand feet to Makanao, where the soil seems to have originated from rock decomposition. This
hill is on the southeast side of Kaiholena, the highest elevation in the district.
Mauna Loa is an elongated dome 13,65o feet in height, sloping gradually to the sea or to an intersection with an adjacent vol cano. On the northwest side, next to Hualalai, the base is 4,50o feet ; on the northeast side, next to the extinct Mauna Kea, at the sheep ranch Humuula, the col is 6,600 feet ; on the southeast side, next to Kilauea, the base exceeds 4,00o feet. The slopes to the sea at Hilo and South cape are gradual for distances of thirty miles. The mass of Kilauea is often regarded as being on the flank of Mauna Loa, because there is no marked col between the two. Kialuea is as well defined a caldera, with its own periods of eruption, as Makuaweoweo. The locations of the eruptions from Kilauea range from Nanawale, in Puna, on the east, to Puna luu on the west, which is on the seashore only three miles from Hilea. A very conspicuous fault extends twenty miles from Kohaualea westerly to near the flow of 1823. The land makai (shoreward) of this fault has dropped down I,Ioo feet. A some what similar but more irregular escarpment may be traced from near Kapapala to Waiohinu, eighteen miles in length, but is on the south slope of the mass of Mauna Loa. The caldera of Mohokea has this escarpment for its southern boundary. It is an elliptical depression, six miles long northwest and southeast, and five miles wide northeast and southwest, but truncated by the escarpment named. It has been hollowed out from the basaltic sheets of Mauna Loa. The total area is about thirty square miles.
Mohokea differs from the other calderas in three respects: 1. It is not inclosed on all sides, so as to be properly a pit. It is open on the makai side.