Hydrological evolution and chemical structure of a hyper-acidic spring-lake system on Whakaari/White Island, NZ

White Island has a long and varied history of acid spring discharge and shallow ephemeral lake formation on its main crater floor. In the 12 months prior to the onset of the 1976-2000 eruptive episode, mass discharge from the spring system increased ca. 10-fold, pointing to a strong coupling of the hydrothermal environment to the evolving magmatic system. Between 1976 and 1978, the formation of numerous eruption vents to 200 m depth in the Western Sub-crater abruptly changed the hydraulic gradients in the volcano, resulting in the reversal of groundwater flow in the massif towards the newly-formed crater(s). This affected not only the style of volcanic activity (leading to phreatic-phreatomagmatic-magmatic eruption cycles), but also led to the demise of the spring system, with discharge from the main crater declining by a factor > 100 by 1979. Eruptive activity ended shortly after a moderate Strombolian eruption in mid-2000, after which ephemeral lakes started to form in the eruption crater complex.

Between 2003 and 2015 there were three complete lake filling and evaporative cycles, reflecting varying heat flow through the conduit system beneath the lake. Over these cycles, lake water concentrations of Cl and SO₄ varied between ca. 35-150 and 5-45 g/L respectively, with pH values temporally ranging from + 1.5 to - 1. Springs appeared on the Main Crater floor in 2004, and their discharges varied with lake level, pointing to the lake level being a primary control over the piezometric surface in the crater area. Springs closest to the crater complex show direct evidence of crater lake water infiltration into the crater floor aquifer, whereas distal spring discharges show compositional variations reflecting vertical displacement of the interface between shallow, dilute condensate and underlying acidic brine fluids.

Source components for the spring fluids include magmatic vapour, dissolved andesitic host rocks, seawater and meteoric water. Lake waters, on the other hand, consist predominantly of magmatic vapour, meteoric water and solutes derived from host andesites and their altered derivatives. δ²H and δ¹⁸O signatures of the enclosing acid brine fluids, indicate they are predominantly seawater which have been affected by both vapour loss, but also mixing with arc-type vapour.

An interesting finding of this study is that crater floor deformation correlates directly to both lake level and volatile emissions, in an apparent poroelastic response to the establishment of a hydrostatic water column in the eruption crater complex, and a net decrease in permeability owing to hydrothermal mineralization in the conduit (predominantly elemental sulfur and sulfate minerals). The hydrostatic pressurization of the vent environment also leads to increased gas pressures and flows through fumarolic channels, and consequent expansion of fumarolic areas on the main crater floor.

A period of unrest, which commenced in August 2012 and lasted until October 2013, included the extrusion of a small dome into the eruption crater complex. This activity, and related high heat flow, led once again to evaporation of the lake, and ongoing phreatic eruption activity which has provided interesting insights into the role which elemental sulfur, associated hydrothermal alteration minerals and of course water play in regulating pressures in the magmatic-hydrothermal environment.