Geology and ground-water resources of the island of Molokai, Hawaii

The island of Molokai is the fifth largest of the Hawaiian Islands, with an area of 250 square miles. It lies 25 miles southeast of Oahu, and 8.5 miles northwest of Maui. It consists of two principal parts, each a major volcanic mountain. East Molokai rises to 4,970 feet altitude. It is built largely of basaltic lavas, with a thin cap of andesites and a little trachyte. The volcanic rocks of East Molokai are named the East Molokai volcanic series, the basaltic part being separated as the lower member of the series, and the andesites and trachytes as the upper member. Large cinder cones and bulbous domes are associated with the lavas of the upper member. Thin beds of ash are present locally in both members. The lavas of the lower member are cut by innumerable dikes lying in two major rift zones trending eastward and northwestward. A large caldera, more than 4 miles long, and a smaller pit 0.8 mile across existed near the summit of the volcano. The rocks formed in and under the caldera are separated on plate 1 as the caldera complex. Stream erosion has cut large amphitheater-headed valleys into the northern coast of East Molokai, exposing the dikes and the caldera complex.
West Molokai is lower than East Molokai, rising to 1,380 feet altitude. It was built by basaltic lavas erupted along rift zones trending southwestward and northwestward. Many of the flows were unusually fluid. The volcanic rocks of West Molokai Volcano are named the West Molokai volcanic series. Along its eastern side, the mountain is broken by a series of faults along which its eastern edge has been dropped downward. West Molokai Volcano became extinct earlier than East Molokai Volcano, and its flank is partly buried beneath lavas of East Molokai.
Both volcanic mountains were built upward from the sea floor probably during Tertiary time. Following the close of volcanic activity stream erosion cut large canyons on East Molokai, but accomplished much less on drier West Molokai. Marine erosion attacked both parts of the island, producing high sea-cliffs on the windward coast. In late Tertiary or early Pleistocene time the island was submerged to a level at least 560 feet above the present shore line, then reemerged. Later shifts of sea level, probably partly resulting from Pleistocene glaciation and deglaciation, ranged from 300 feet below to 100 feet or more above present sea level. Marine deposits on the southern slope extend to an altitude of at least 200 feet. Eruption of the Kalaupapa basalt built a small lava cone at the foot of the northern cliff, forming Kalaupapa peninsula; and a small submarine eruption off the eastern end of Molokai built the Mokuhooniki tuff cone, the fragments of which now form Hooniki and Kanaha Islands. Deposition of marine and fluviatile sediments has built a series of narrow flats close to sea-level along the southern coast.
Nearly the entire island is underlain, close to sea level, by ground water of the basal zone of saturation. Beneath West Molokai, the Hoolehua Plain between West and East Molokai, and the southern coastal area of East Molokai, the basal water is brackish. Beneath much of East Molokai, fresh basal water is obtainable. Small amounts of fresh water are perched at high levels in East Molokai by thin poorly permeable ash beds. Fresh water is confined at high levels in permeable compartments between poorly permeable dikes in the rift zones of East Molokai, and can be developed by tunnels. Projects to bring the abundant surface and ground water of the large wind ward valleys to the Hoolehua Plain are described. Future developments are suggested. All wells and water-development tunnels are described in tables.