Discovery of a trachyte ignimbrite sequence at Hualālai, Hawaii

Abstract

Ignimbrites are common in many intraplate ocean islands but have been missing from the known geological record in Hawaii. During a recent field campaign, the remnants of a trachytic ignimbrite sequence have been discovered at Hualālai volcano, fortuitously preserved from subsequent basaltic lava flow cover. We provide a preliminary description of these deposits, as well as bulk and glass chemical analyses to determine their potential relationship with other nearby trachytes from Pu’u Wa’awa’a (PWW) and Pu’u Anahulu (PA). The results suggest that these ignimbrites are from neither PWW nor PA, but instead may relate to trachytes that are found as maar wallrock blocks some 20 km distant. Therefore, despite being rare overall in Hawaii, the ignimbrites—and more generally trachytes—were probably widespread around Hualālai. Compared to other intraplate ocean islands, the combination of a fast-moving plate, high magma supply, and eruption rates underneath Hawaiian volcanoes may explain the scarcity of ignimbrites preserved at the surface. Their presence at Hualālai could reflect unusual conditions of edifice stress during the transition from shield to post-shield volcanism.