HIGH-RESOLUTION MAPPING AND SAMPLING OF HISTORIC FLOWS ON THE JUAN DE FUCA AND GORDA RIDGES

Since the mid-1980s, 8 eruptions occurred along the Juan de Fuca and Gorda Ridges. MBARI surveyed at 1-m lateral resolution with our Mapping AUV and conducted ROV dives at 6 of them. Prior ROV dive observations were put into spatial context and subsequent dives targeted specific sites to ground-truth the maps, collect lava samples of these and nearby flows to assess chemical variability, and collect sediment cores to obtain absolute ages of underlying flows using ¹⁴C dating of foraminifera that first settled on them. Relative age relationships derived from sediment cover estimates, fault densities, and flow contact overlaps can be calibrated using the absolute ages. We have outlined flow extents, defined fissure systems, gained insights into flow emplacement processes, and constructed field maps and volcanic histories of the spreading ridges, much like volcanologists do on land; all unprecedented for submarine volcanoes.

The North Gorda 1996, North Cleft 1986, and CoAxial 1982-91 and 1993, and the deep south rift portion of the Axial 2011 eruptions produced discontinuous ridges of coalesced pillow mounds atop dikes that intersected the sea floor along parts of their length (Yeo et al., 2013). Adjacent and underlying flows include pillow ridges and ponded sheet flows, whose eruptive fissures have stepped back and forth across the axial valley through time. The Axial 1998 (Chadwick et al, 2013) and 2011 flows (Caress et al, 2012) near the summit are inflated sheet flows that erupted through fissures and flowed down channels created during prior eruptions. Comparing AUV bathymetry before and after the 2011 eruption allowed precise calculation of volume and area of the flow. Axial’s summit caldera has been paved with similar eruptions for the past 725 years (Clague et al, 2013), but explosive eruptions occurred at the summit (Portner et al, 2014) from about 1400 to 1000 years ago, at the same time that vast ponded flows on the deep rifts erupted, which may have triggered caldera collapses.

Prior to these technological advances, our understanding of mid-ocean ridges was based on magnetic anomalies, widely distributed lava samples, and generalized flow type. The synergistic high-resolution mapping and targeted ROV sampling of the ridges has permitted better understanding of how, how much, how often, and when spreading ridges erupt.