FIRE AND WATER: LATE QUATERNARY MEGAFLOODS IN THE TAUPO VOLCANIC ZONE, NEW ZEALAND

The Taupo Volcanic Zone in the central North Island of New Zealand is an area of intense Quaternary silicic volcanism, characterized by frequent caldera-forming explosive eruptions from multiple short-lived, nested, and/or overlapping volcanic centers. Both of the largest caldera lakes, Taupo and Rotorua, are fringed by extensive terrace and shoreline deposits laid down in the aftermath of explosive rhyolitic eruptions that provide evidence for prolonged highstand periods followed by sudden falls in lake level, that, in some cases, are linked to catastrophic outbreak floods. The Rotorua caldera, formed at 220 ka during eruption of the Mamaku Ignimbrite, contains both post-Mamaku lake sediments and extensive highstand terraces developed after eruption of the Rotoiti Breccia from the adjacent Okataina caldera at 64 ka again blocked the outlet. Further south, Lake Taupo, the largest lake in New Zealand, partially occupies a composite caldera and volcano-tectonic collapse structure formed during the 26.5 ka Oruanui eruption and last modified during the 1800a Taupo eruption. Post-eruption, Lake Taupo refilled to ~500 metres above sea level (mASL), an increase of c. 140 m over its modern level, and impounded an additional 80 cubic kilometers of water. The highstand lake initially overtopped the lowest area on the caldera rim to establish a semi-stable outlet controlled by a sill of older welded ignimbrite. Headward erosion through thick but unwelded Oruanui pyroclastic further east established a new, lower outlet at ~ 405 mASL prior to 21 ka, releasing c. 60 cubic km of water in a flood that transported enormous boulders and which may have permanently changed the course of the largest river in the North Island. Most recently, the 1800a Taupo eruption also blocked the outlet to the lake, resulting in a 34 m rise in lake level. Catastrophic failure of the ignimbrite dam following overtopping generated a flood whose discharge peaked at 17 000 – 30 000 cumecs, releasing 5 years of normal outflow in 3 weeks. The geomorphic expression of this flood is found in boulder deposits, erosion surfaces, and overbank aggradation deposits and buried forests that can be traced for over 200 km downstream. Caldera-lake outbreak floods are a recurrent feature of volcanism in New Zealand and constitute a significant new and far-reaching hazard.