Decay of hydrothermal output following the 1998 seafloor eruption at Axial Volcano: Observations and models

Observations of the decay of heat and particle discharge following the reinvigoration of a hydrothermal field by a dike intrusion and eruption can provide insights about subsurface hydrothermal circulation. In January 1998, a lava eruption at the summit of Axial Volcano, Juan de Fuca Ridge, created numerous new vents along a 5‐km length of the caldera boundary. Five cruises over the next 3.5 years found the inventory of hydrothermal heat and particles (determined by optical backscattering) in the overlying plume decaying as time t −0.9 . These results are consistent with the 1993 CoAxial eruption, where the heat flux decayed as ∼ t −1.2 and ∼ t −0.7 at the Flow and Floc sites, respectively. All three eruption sites decayed significantly faster than t −0.5 , the rate attributable to cooling only by simple conduction through the impermeable layer separating magma from hydrothermal circulation. We present a more realistic model of hydrothermal cooling that adds conductive heat loss by ascending fluid to the surrounding rock walls, increasing the theoretical loss rate to t −0.75 . We suggest that variability in the observed decay rates are not sampling variations but reflect real differences in the local heat supply and cooling processes. The decay rate was slowest, and matched our model result, where no seafloor eruption occurred and a long‐lasting heat source was likely present (Floc). The rate was fastest where a shallow source erupted a thick lava pavement (Flow). The Axial eruption, where a substantial magma chamber emplaced only a thin sheet flow, may be intermediate between these extremes.