THE GEOLOGY OF MOKU'AUIA ISLAND, OAHU, HAWAII, USA

Moku’auia Island, known to locals as “Goat” Island, is an islet that lies off of the northeast point of Oahu Island, Hawaii, lying between the villages of Kahuku and Laie. It is one of many sea stack-type islets that lie along this section of Oahu’s coast and that, together, are associated with ancient Hawaiian legends. Moku’auia is different from the other islets in that it is both the largest and has the lowest profile and elevation of the islets. Moku’auia is visited regularly by local fishermen and surfers, as well as tourists that walk to it during periods of low tide.

Until this study, no detailed geologic map of the islet has been made. On the Geologic Map of the State of Hawaii (Sherrod et al., 2007) it is shown to consist of alluvial sand, coral reef, and a small section of Pleistocene-related sandstone. Using structure-from-motion drone surveys and ground observations, the geology of the islet is now more clear. While, as indicated by Sherrod et. al. (2007), there is a strip of sandstone running along the northwest section of the islet, outcrops of the same rock-type also appear and run along southwestern and eastern sections of the islet. In addition, and most intriguingly, the sections of likely Pleistocene-age corals that form the lowest, observable, stratigraphic layer, which also circles the island, contains primary basalt fragments. These fragments indicate extensive hydrovolcanic mixing. In other cases, clear outcrops of unfragmented basalt lava flows are present on the open-ocean (eastern) side of the island. This suggests that the island went through at least one stage of development in which small, active lava flows overflowed extensive areas of ancient reef. The affects were short-lived, though, as the reef appears to have quickly recovered, incorporating the lava into its structure. However, it is clear that the islet’s formation was not simply due to coral growth and alluvial deposition, but was also directly connected to these previously undocumented lava flows.