Augustine S. Furumoto

Seismic studies on the Ontong Java Plateau, 1970☆

Abstract Marine seismic studies were conducted over the Ontong Java Plateau in the south-western Pacific Ocean during 1970 using two ships. Seismic refraction surveys revealed that crustal thickness of the plateau ranges from 35 to 42 km. A high-velocity basal crustal layer was detected in the northern part of the plateau but not in the southern part. Seismic reflection profiling showed the sediments to be about 1.5 km thick. In the sedimentary column were numerous intrusions with very little magnetic expression. These intrusions are probably not of igneous origin. There is a large discrepancy between observed gravity values and theoretical values calculated from the crustal structure.

Crustal Structure of the Hawaiian Archipelago, Northern Melanesia, and the Central Pacific Basin by Seismic Refraction Methods*

Abstract The crustal structure of the Hawaiian Archipelago, northern Melanesia, and parts of the Central Pacific Basin have been studied by seismic refraction methods. The systematic variation found in crustal thickness in the Hawaiian Islands is explainable by a hypothesis of differential subsidence. The crustal structure of northern Melanesia points to tensional forces in an east-west direction and compressional forces in a north-south direction. In the Central Pacific Basin, a 7.4 km/sec layer in the lower crust seems to be present over a wide area.

Nature of the magma conduit under the east rift zone of Kilauea Volcano, Hawaii

From a combination of results of gravity, magnetic and seismic refraction surveys, the dike complex under the east rift zone of Kilauea Volcano in Hawaii was found to extend for 110 km from the summit area of the volcano to a point 60 km at sea beyond the eastern tip of the island. Near the summit the complex is 20 km wide, and at about 40 km distance from the summit, the complex narrows to 12 km wide. The main body of the dike complex is 2.3 km deep, but some parts are as shallow as 1 km.From extrapolation of temperature data of a deep well and from analysis of magnetic data, it was inferred that temperature of the dike complex is above the Curic point of 540°C. The internal part of the complex can approach the melting point of 1060°C.The dike complex was formed by numerous excursions of magma from the holding reservoir under the volcano summit. The theory of forceful intrusion of magma into rift zones accounts for the magma excursions and migration of the passageways.Gravity and seismic velocity data indicate that density of the material left in the dike complex is 3.1 g/cm3. In the light of recent density determinations of Hawaiian rocks under high pressure and temperature, it is concluded that during Hawaiian volcanic activity, less dense components of the parent magma crupt through surface vents while the more dense components remain trapped below. Samples of the dense material from the dike complex are required before we can have a complete picture of the parent magma of Hawaiian volcanoes.The dike complex is the source of thermal energy for a commercial quality geothermal reservoir that was found by drilling.

Structure of Hawaiian volcanoes from seismic refraction data

Numerous seismic refraction traverses were carried out over the Hawaiian Islands to outline the structure of the volcanoes and determine the thickness of the layers of the crust. Results indicate that there is a progressive thickening of the crust in the area from younger to older volcanic islands. High velocity mantle-like material was detected under rift zones and in volcanic plugs. Volcanic regions were found to be intersected by flexure zones in the crust and in the mantle.The seismic refraction method, in conjunction with gravity and magnetic methods, was found to be successful in outlining the horizontal and vertical structures of rift zones and plugs of volcanoes.