HYBRID SCORIA CONES: AN EXAMPLE FROM HAWAII

Scoria cones are the most common and widely spread volcanic landform on Earth and are also common to other planetary bodies including the Moon. Erosion and mining can dissect these easily remobilized deposits and provide opportunities to study the volcanic plumbing systems feeding these pyroclastic constructs, commonly expressed as dikes that cross cut the beds of scoria comprising the cone. In monogenetic volcanism, the dikes are expected to be sourced from crustal magma chambers that propagate through pre-existing fractures, joints and faults to the surface to feed the eruption that builds the scoria cone and associated lava flows. Logically, exposed dikes that are hosted by and intrude scoria cones are almost always interpreted as the magma source for the pyroclastic construct. This inferred relationship usually holds true for the great majority of isolated, monogenetic cones. Here, however, we describe a complex stratigraphic relationship between Pu’u Huluhulu scoria cone of Mauna Kea volcano and its internal dike system that is sourced from Mauna Loa volcano.

Mauna Kea’s Pu’u Huluhulu scoria cone is a kipuka located on Humu’ula saddle between Mauna Kea and Mauna Loa volcanoes. The cone has been dissected by erosion and mining for road base, exposing dikes that crosscut the layered scoria. Our geologic mapping establishes stratigraphic relationships between units and the relative timing of events, whilst, our lab work shows the volcanic units to be petrographically and geochemically distinct. The scoria comprises Laupahoehoe volcanics of Mauna Kea’s post-shield stage and the dikes and associated lava flows comprise Mauna Loa shield building tholeiites. In addition, sulfur analyses on the quench margins of the dikes (>0.12 wt%) indicate the Mauna Loa dikes are not degassed. Based on our observations, we deduce that the dikes exposed in the Pu’u Huluhulu scoria cone are derived from a previously unmapped radial vent eruption of Mauna Loa Volcano that hijacked the preexisting Mauna Kea scoria cone and used it as an eruptive center. The broader implications for this result are that the simple model for dike emplacement in scoria cones may not hold true for complex volcanic fields that involve the interaction of disparate magma sources. Care must be taken when interpreting these seemingly simple volcanic constructs.