Historic Littoral Cones in Hawaii

Abstract

Littoral cones are formed by steam explosions resulting when lava flows enter the sea. Of about 50 littoral cones on the shores of Mauna Loa and Kilauea on the island of Hawaii, three were formed in historic time: 1840, 1868, and 1919. Five new chemical analyses of the glassy ash of the cones and of the feeding lava show that there is no chemical interchange between molten lava and sea water during the brief period they are in contact. The littoral cone ash contains a lower Fe2O3 / (Fe203 + FeO) ratio than does its feeding lava because drastic chilling reduces the amount of oxidation. A large volume of lava entering the sea (probably more than 50 million cubic yards) is required to produce a littoral cone. All the historic littoral cones were fed by aa flows. The turbulent character of these flows and the included cooler, solid material allows ingress of sea water to the interior of the flow where it vaporizes and explodes. The cooler, more brittle lava of the aa flows tend to fragment and shatter more readily upon contact with water than does lava of pahoehoe flows.