ASSESSING THE ROLE OF STEAM EXPLOSIVITY DURING LAVA FRAGMENTATION AND VOLCANICLAST GENERATION IN SHALLOW AND DEEP MARINE ENVIRONMENTS

Lava-water interaction can produce volcaniclasts by steam explosivity, limited at greater depths by higher pressure. Physical evidence for steam explosivity and its role in shallow to deep marine lava fragmentation is still poorly defined. In this study, basaltic volcaniclastic deposits formed in deep and shallow marine environments are compared. Methodology includes field mapping, submersible observations, granulometry, componentry and morphometry analysis.

Historic lava flows on Axial Seamount, Juan de Fuca Ridge, formed volcaniclastic deposits at a depth of ~1,500 mbsl. The average diameter of volcaniclasts is -3.1 ± 2.1 φ. Grains are dominantly angular with ~10% fluidal shapes. Scanning electron microscopy did not identify morphologic or textural characteristics indicative of steam explosivity, which is also supported by a high lava:volcaniclast ratio.

Two Miocene lava flows of the Columbia River Basalt Group (CRBG), the Grande Ronde (GRB) and younger Wanapum basalt (WB) members, formed volcaniclastic material in a shallow marine environment ~0-300 mbsl along the paleo-continental shelf of Oregon. Near Depoe Bay the GRB displays abundant pillow basalt and breccia (~50% pillows by volume), 43.4% matrix to clasts, and has an average volcaniclast diameter of -7.8 ± 0.8 φ, while the WB lacks pillows, has 56.1% matrix to clasts, and has an average grain size of -6.8 ± 0.8 φ . The abundance of pillows in the GRB and coarse grain size support a non-explosive origin. The WB’s fine grain size, indicating energetic fragmentation, and lack of pillows suggest that steam explosivity likely contributed to its fragmentation. Volatile-induced explosivity is unlikely to have produced volcaniclastic CRBG in the study area, as their vents are believed to have been ~600 kilometers away, allowing ample time for magmatic gases to exsolve.

The abundance of coherent lava and coarse volcaniclastic deposits in deep marine settings, compared with the lack of pillows and fine grain size from shallow settings support the idea that steam explosivity during lava-water interaction is limited to shallow marine environments. Interpretations are limited by CRBG alteration, while Axial samples are fine-biased. This will be addressed by analyzing coarse material from submersible recordings, and fine CRBG grain analysis via thin section.