Geology and ground-water resources of the island of Niihau, Hawaii
Niihau lies 17 1 / 2 miles southwest of Kauai. Its area is 72 square miles, and its highest point has an altitude of 1,281 feet. The population is about 180, chiefly Hawaiians. The annual rainfall at Kiekie, the ranch headquarters, generally ranges between 18 and 26 inches. The chief industries are the raising of sheep and cattle and production of honey. The island is privately owned. The main mass of the island is composed of a deeply weathered remnant of a basalt dome of Tertiary age, cut by a dike complex trending NE-SW. These Tertiary rocks are herein named the Paniau volcanic series. The central vent lay about 2 miles out to sea to the east of the present island. The dome, after deep gulches were cut into it by stream erosion and it was cliffed all around by the sea, was partly submerged. During Pleistocene time a broad wave-cut platform on the north, west, and south sides was built above sea level and widened by the eruption of lavas and tuffs, from 9 vents now visible and other vents now buried, to form a low coastal plain. These Pleistocene volcanic rocks are named the Kiekie volcanic series. Ash from Lehua Island, a Pleistocene tuff cone, has been drifted into duties on the north end of Niihau. Lithified dunes that extend below sea level, and the small outcrops of emerged fossiliferous limestone above sea level, indicate the plus 100-foot, minus 60-foot, plus 25-foot, and plus 5-foot eustatic stands of the sea correlative with changes in the volume of the polar ice caps and concurrent changes in the configuration of ocean basins. Calcareous dune and beach deposits, short stretches of nullipore reef and beach rock, and playa and alluvial deposits constitute the Recent rocks. No perennial streams exist on the island but about a dozen playa lakes, fresh or brackish during rainy weather, lie on the plain. The domestic water supply is rain caught from roofs. Only three wells on the island yield water with less than 25 grains of salt per gallon (260 parts per million of chloride). Typically, water holes for stock are about 15 feet across, 5 feet deep, and 8 feet wide. They have been dug in the lowlands where the depth to water is usually less than 5 feet. Forty-six dug wells and water holes exist, the water of some of which has become too salty for stock. Two or three deep wells were drilled 500 to 1,000 feet below sea level, but they encountered salty water. Three wells, not yet used, have been excavated in the Tertiary basalts. One of them has an infiltration tunnel at the bottom. Two seeps perched on vitric tuff beds more than 500 feet above sea level carry large quantities of salt leached from spray that falls on their recharge areas. Several sites are recommended for developing additional water for stock. The island, however, will always be short of domestic water because of aridity, unfavorable geologic structures, continuous deposition of salt spray, and abundant authigenic salts in the lake beds. Among the lavas of the Paniau volcanic series, of Tertiary age, olivine basalts probably predominate but ordinary basalts are abundant. Picrite-basalt of the primitive type, containing abundant olivine phenocrysts, also occurs. Andesites are probably present but are rare. Most of the lavas of the Kiekie volcanic series, of Pleistocene age, are olivine basalt, but one is transitional between olivine basalt and picrite-basalt. In many of the Pleistocene lavas the late-crystallized augite is titanian. A single occurrence of melilite-nepheline basalt has been reported. Chemical analyses of five rocks are listed.