The Te Kopia Thermal Area has surface manifestations that extend over an area of about 3 x 1.2 lan along the Paeroa Fault Zone. Steaming ground, fumaroles, mud and acid sulphate pools are present on both the upthrown and downthrown blocks of the Paeroa Fault, but near-neutral pH chloride-bicarbonate springs discharge up to 2 km west of the fault trace. The host rocks comprise mainly Quaternary ignimbrites that dip gently (similar to 7 degrees) eastwards but are vertically displaced by the fault by several hundred metres. Hydrothermal alteration is widespread, and the present thermal activity is now producing kaolinite, alunite, silica residue, hematite and cristobalite. This assemblage occurs both in the otherwise unaltered ignimbrites, as well as overprinting the products of earlier hydrothermal activity. Evidence for changes that have occurred in the nature and extent of thermal activity at Te Kopia include: (1) The widespread overprint of a kaolinite-alunite-cristobalite assemblage upon hydrothermal minerals (including adularia, quartz and mordenite) produced by alkali-chloride, bicarbonate-chloride, and heated groundwaters. (2) The occurrence of silica sinter which deposited from alkali chloride springs that discharged about 3000 years ago. This is now a steam zone. (3) Areas of cold ground that were once hot, as demonstrated by the presence of hydrothermal minerals. (4) The presence of euhedral quartz crystals at the surface on the upthrown fault block. Fluid inclusions in these crystals from two places homogenise at 188 +/-15 degrees C and 196 +/-11 degrees C. The trapped fluids have apparent salinities between 0.2 and 0.4 wt % NaCl equivalent. The quartz crystals thus grew at depths of at least 120 m below the water table. Vertical movements along the Paeroa Fault totalling at least 300 m have uplifted these crystals to their present positions. The surface geology and alteration provide evidence that thermal activity at Te Kopia has been long lived, possibly as long at 120 000 years. However, the hydrology of the system and its thermal regime have changed greatly during its lifetime, mainly in response to movements along the Paeroa Fault.