Intrusive Rocks of the Leeward Slope of the Koolau Range, Oahu

The Koolau dome, forming the east half of the island of Oahu, is notably elongate, built about a linear rift zone in which numerous feeder dikes occur in a dike complex over 30 miles long. Scattered dikes and sills occur in the leeward parts of the dome, the concentration being progressively reduced with the distance away from the dike complex. There are secondary rift zones and subcomplexes with increased concentrations of dikes and sills, which trend at right angles to the main rift zone. The dikes and sills show three stages and patterns of columnar jointing which are related to the cooling history. Shallow intrusives are vesicular and banded but not columnar jointed. Dikes with a thickness of about 2 feet are preponderant to such an extent as to suggest that this is an optimum determined by the amount of lateral crowding-together of contracted lava formations which can be achieved by the pressures of invading lava.