BH250-231b

Puʻu Waʻawaʻa is one of the most unusual volcanic centers in Hawaiʻi and a remarkable example of explosive volcanism within a hotspot setting normally dominated by basaltic lava flows. Located on the north flank of Hualālai Volcano, the cone rises prominently above surrounding lava fields and is well known for its deeply eroded flanks, reflected in its Hawaiian name meaning “many-furrowed hill.” Unlike the typical gently effusive eruptions associated with most Hawaiian volcanoes, Puʻu Waʻawaʻa records a period when magma evolved to become more silica-rich, more viscous, and gas-charged, allowing explosive eruptions to occur. However, the chemistry of your samples shows that these rocks are not as silica-rich as many classic continental pumice and obsidian deposits, but instead represent intermediate to evolved alkaline Hawaiian magmas.

These eruptions produced a wide variety of pyroclastic materials, including pumice, ash, scoria, volcanic bombs, and blocks of obsidian. The presence of pumice indicates volatile-rich magma that expanded violently as pressure was released, creating frothy vesicular fragments that cooled in the air. Obsidian formed when evolved lava or hot ejecta cooled rapidly, preventing extensive crystal growth and preserving a natural volcanic glass. In your obsidian sample, the presence of aligned plagioclase laths recording a preferred orientation is especially significant, as these microlites preserve the direction of magma flow during emplacement. This flow texture shows that the melt was still mobile as crystals formed and were rotated into alignment by viscous movement prior to final quenching. Scoria and basaltic fragments represent hotter, more mafic eruptive phases, likely associated with Strombolian-style bursts or lava fountaining. Together, these rock types show that the volcano experienced multiple eruptive stages and likely involved magma mixing or transitions between basaltic and more evolved trachytic to benmoreitic magma compositions.

BH250-227d (pumice; SiO₂ 60.93%), BH250-231b (obsidian; SiO₂ 62.93%), and BH250-231c (basalt/scoria; SiO₂ 45%) capture this complex volcanic history very well. The pumice and obsidian are chemically more intermediate than many rhyolitic pumices or obsidians elsewhere in the world, yet they still developed highly vesicular and glassy textures because of rapid cooling and abundant dissolved gases. The flow-aligned plagioclase laths in the obsidian further demonstrate that crystallization and deformation occurred simultaneously within a moving viscous melt. This shows that explosive eruptions and volcanic glass formation do not require extremely high silica contents, but can also occur in evolved alkaline magmas such as those found in Hawaiʻi.

Geologically, Puʻu Waʻawaʻa is one of the most important volcanic sites to visit in Hawaiʻi because it shows that Hawaiian hotspot volcanoes are capable of producing far more diverse magmas than simple basalt alone. It provides evidence for fractional crystallization, magma differentiation, gas buildup, explosive fragmentation, and changing eruptive behavior through time. In addition, obsidian from this area was historically used by Native Hawaiians for tools and cutting implements, adding cultural significance to its geological importance. The summit cone and surrounding deposits therefore represent both a rare volcanic laboratory and an important part of Hawaiʻi’s natural history.

The locality is also rewarding in the field. From the access road, it is roughly a 3 km hike to the cone. If you have extra time and energy, climbing the summit provides excellent panoramic views across the north flank of Hualālai, where many other volcanic cones, lava flows, and surrounding landscapes can be seen. It is an outstanding place to appreciate both the geology and scenery of Hawaiʻi.