Points in the geological history of the islands Maui and Oahu

Abstract

"The subjects prominently illustrated by the islands Maui and Oahu are: the conditions of extinct volcanoes in different stages of degradation; the origin of long lines of precipice cutting deeply through the mountains; the extent and condition of one of the largest of craters at the period of extinction; and the relation of cinder and tufa cones to the parent volcano. The other islands of the group present facts bearing on these subjects, but the writer's knowledge of them is too imperfect for review in this place." Dana describes Haleakala Crater, using W.W. Alexander's new government map of Maui. He describes young scoria cones, some with exotic blocks of dense basalt. He favors the formation of the two crater gaps by fracturing and subsidence but leaves open the possibility of other explanations. He cites the results of Preston (1888), who determined, by measurement of gravity, that Haleakala's bulk density was greater than that of its surface lavas, thus supporting consolidation through collapse of an original crater. He quotes a second-hand report of a lateral eruption about 150 years before. Dana infers that coral sand deposits near Wailuku were formed under conditions close to those of the present. Dana concludes his observations on Maui with a cursory description of West Maui Volcano, identifying three centers of volcanism in the eroded edifice, and remarking on the eccentric position of the summit craters of each volcano, relative to the entire volcanic mass. Dana spent several days on Oahu, assembling new facts and reconsidering his earlier conclusions (Dana, 1849). He reviews ideas on the formation of the Koolau Pali, now interpreting the caldera center as being closer to the pali. He confirms that the pali originated by catastrophic subsidence and now extends this idea to include all of the vertical sea cliffs found in the island, including those on Molokai, Lanai, Kauai, Niihau, as well as the scarp bounding Kealakekua Bay on the island of Hawaii. He speculates as follows on a subsidence mechanism (p. 95): "A great volcano is a disgorger of lava in vast floods and so it makes its mountain; and it may make also empty cavities at the same time and as a consequence. As long as the ascensive force keeps the liquid lava-column of the active volcano fully up to the summit crater, the mountain may have only local cavities. But whenever a great discharge takes place, a coequal cavity may result; and if the discharge is from fissures at the base of the cone, 10,000 to 18,000 feet below the sea level (not a greater depth than exists in the neighboring seas) an enormous cavity may be left, which only the renewed action of the ascensive force would fill. If the mountain then became extinct with no return of the liquid, it would be a hollow mountain; and the greatest of subsidences which the Hawaiian facts seem to indicate, are small compared with the possible consequences of such a condition." Dana includes a new map of Oahu with several cross-sections and well data from Diamond Head, Punchbowl, and the Hulihee Palace grounds. He reviews evidence for subsidence, concluding that the coral deposits encountered in deep wells are consistent with, but do not necessarily prove, Darwin's ideas on subsidence. He proposes additional deep borings to settle the matter. He repeats his observations on the raised reef and coral sand deposits, indicating that the island has undergone recent uplift.