Examination of the Axial 2011 and 2015 lava flows from high-resolution AUV bathymetry and ROV dives

Monday, 30 January 2017
Marina/Gretel (Hobart Function and Conference Centre)
Jennifer Brophy Paduan1, David A Clague2, William W. Chadwick Jr.3, David W Caress2, Brian M Dreyer4, Morgane Le Saout2 and Hans J Thomas5, (1)Monterey Bay Aquarium Res Inst, Moss Landing, CA, United States, (2)Monterey Bay Aquarium Research Institute, Watsonville, CA, United States, (3)Oregon State University/NOAA/PMEL, CIMRS, Newport, OR, United States, (4)University of California Santa Cruz, Santa Cruz, CA, United States, (5)Monterey Bay Aquarium Research Institute, Moss Landing, CA, United States
Abstract:
Axial Seamount was surveyed at 1-m resolution with AUVs before and after the eruptions of 2011 and 2015, and the flows were observed and sampled with ROVs. Before and after maps (using ship-collected bathymetry where the AUV had not mapped before) are utilized to identify new fissures, for fine-scale identification of flow morphologies and boundaries, and to determine flow thicknesses to calculate volumes of flows.

The two eruptions began on fissures that overlapped 0.5 km apart on the eastern side of the summit, and flows on the caldera floor came within 12 m of each other. The 2011 eruption migrated down the south rift zone, as had the 1998 eruption. The 2015 eruption propagated northward. Across the NE flank, the 2015 fissures took broad steps west and realigned with the north rift zone, down which the dike propagated. Flows were fed in 2015 from discontinuous en echelon fissures for 19 km, and in 2011 from closely spaced en echelon fissures for 10 km and then built a steep 5-km long ridge of pillow mounds 18.5 km further south. Despite this difference, the 2015 eruption, at 148 x 106 m3 of lava, was half again as voluminous as the 2011 eruption, at 99 x 106 m3 of which 27% was on the summit and upper rift, and 5 times the volume of the 1998 flows. Some new fractures south of the upper south rift flows in 2011 and on the east caldera rim in 2015 did not extrude lava.

Flows in and around the caldera were fluid flows less than 20 m thick and spread along complex, commonly pre-existing channels. Distal flows accumulated to become inflated lobate flows and coalesced pillow mounds up to 130 m thick. As the eruption waned, summits of large pillow mounds deformed and pillow mounds formed over fissures that earlier had emanated sheet flows. Samples from 2015 within the caldera are more primitive (higher MgO content) and spatially and chemically similar to the last eruption in the central caldera ~400 years ago. In contrast, the 2011 lavas are compositionally like all other flows erupted since. Flows from both eruptions are still cooling and discharging warm fluids 4.5 and 1.5 years later, respectively, supporting colonies of bacteria and tubeworms. Clusters of tiny hydrothermal chimneys are growing at the top of a thick inflated 2015 flow on the mid-north rift zone, in a setting where hydrothermal activity had not previously been detected.

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