The hydrology of the long-lived Rotomahana-Waimangu hydrothermal system of New Zealand was changed irreversibly by the brief 1886 Tarawera Rift basalt eruption. The nature of the pre-1886 surface thermal activity indicates that boiling conditions prevailed in the upflow zone beneath the vicinity of the then-existing shallow Lake Rotomahana. On June 10, 1886, magma erupted through this part of the system, triggering violent volcanic and hydrothermal explosions that led to the formation of new fluid conduits and a large crater that filled to form the present Lake Rotomahana. Several years after the eruption, hot springs broke out along the line of 1886 craters southwest of Lake Rotomahana. The evolution of these features has been punctuated by spectacular geysers from 1900 to 1904 and a substantial hydrothermal eruption in 1917. The main effect of the 1886 volcanic eruption on the hydrothermal system was the perturbation of pressure gradients, causing abrupt near-surface cooling followed by gradual reheating.