Plant Molds in Hawaiian Basalts: Was Oahu a Desert, and Why?

Abstract

Plant molds are common in the basaltic lavas of Hawaii, and this study quantitatively documents their distribution in terms of the number of molds per unit length of section. The number varies with lava structural type, being lower in distal-type aa than in pahoehoe or proximal-type aa. This study defines the expectation of finding plant molds among the lava flows of Oahu given a climatic and volcanic regime like today's in Hawaii. Plant molds prove to be very scarce on Oahu, only 2% of the expected number being found. The exposed parts of the volcanoes on Oahu are smaller, causing lava flows to be superposed at shorter intervals, but this can account for only a small part of the discrepancy. Oahu was evidently a near-desert 3.9 to 1.8 m.y. ago when its Waianae and Koolau shield volcanoes were active. This conclusion is supported by the virtual absence of earthy soils among, and the negligible contemporaneous chemical weathering of the shield-building flows. It is proposed that when it formed, Oahu stood 2000 m or more higher than today. It projected above the Trade Winds and hence was arid, just as the upper parts of Mauna Kea, Mauna Loa, and Heleakala are arid today.