IRREVERSIBLE CHANGE OF THE ROTOMAHANA-WAIMANGU HYDROTHERMAL SYSTEM (NEW-ZEALAND) AS A CONSEQUENCE OF A VOLCANIC-ERUPTION

The hydrology of the long-lived Rotomahana-Waimangu hydrothermal syste m of New Zealand was changed irreversibly by the brief 1886 Tarawera R ift basalt eruption. The nature of the pre-1886 surface thermal activi ty indicates that boiling conditions prevailed in the upflow zone bene ath the vicinity of the then-existing shallow Lake Rotomahana. On June 10, 1886, magma erupted through this part of the system, triggering v iolent volcanic and hydrothermal explosions that led to the formation of new fluid conduits and a large crater that filled to form the prese nt Lake Rotomahana. Several years after the eruption, hot springs brok e out along the line of 1886 craters southwest of Lake Rotomahana. The evolution of these features has been punctuated by spectacular geyser s from 1900 to 1904 and a substantial hydrothermal eruption in 1917. T he main effect of the 1886 volcanic eruption on the hydrothermal syste m was the perturbation of pressure gradients, causing abrupt near-surf ace cooling followed by gradual reheating.