ANALYTICAL MODELING AND PETROLOGY OF A TUBE-FED LAVA FLOW ON MAUNA LOA VOLCANO, HAWAII

Understanding the full range of lava flow behavior is essential for hazard assessment, and a unique opportunity to do so is found at an elevation of ~2700 m on Mauna Loa’s northeastern flank, where breakouts from a large lava tube system have fed a series of exceptionally thin flows. Featuring pahoehoe lobes less than 10 cm thick in some locations, these flows appear to have transported lava blocks up to 2 m in diameter out of the tube. The unusual morphology and emplacement conditions of this flow field have not yet been studied in detail; here, we report whole rock chemistry, petrography, field data, and results from analytical modeling of this system. In hand sample, the glassy breakout-fed flows are denser than the underlying pre-breakout tube material, primarily due to the increased vesicularity of the latter. Vesicles within the breakout-fed flows are variable in size (1 - 10 mm) and preferentially elongated, while those in the pre-breakout flow are equant and average 1-3 mm in diameter. Both flow types are aphyric, with rare 1 mm olivine phenocrysts visible in the pre-breakout flows. Preliminary XRF analysis shows that the two flow types are chemically indistinguishable and resemble other Mauna Loan tholeiites, indicating that the contrast between the two is likely controlled by physical and/or thermal processes. Different hypotheses for breakout-fed flow emplacement include: (1) pressurization and overflow of the tube due to increased effusion rate at the vent, and (2) blockage due to tube collapses. Block emplacement can be explained by flotation (blocks are less dense than breakout-fed lava) or forceful ejection (blocks are denser than breakout-fed lava). Stokes settling velocities of the largest blocks indicate that if they were suspended in lava with viscosity <50 Pa s and not buoyant during emplacement, breakout exit velocities must have been >5 m/s. MELTS (Gualda et al, 2012) was used to calculate phase equilibria for the measured bulk compositions—at P = 1 bar and fO₂ constrained by the QFM buffer, liquidus temperatures are 1171°– 1173 °C. Depending on volatile content, these temperatures yield lower bounds for melt viscosity and density of 26 – 159 Pa s and 2778 – 2850 kg/m³. Additional fieldwork to collect samples and link morphology derived effusion rates with flow conditions in the tube is currently underway.