HR: 17:45h
AN: H42L-08    [PDF]
TI: Late Quaternary Volcanogenic Megafloods in the Taupo Volcanic Zone, New Zealand
AU: * Manville, V
EM: v.manville@gns.cri.nz
AF: Institute of Geological & Nuclear Sciences, Wairakei Research Centre, Private Bag 2000, Taupo, 2730 New Zealand
AB: 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 centres. Although many people are familiar with the geologically recent devastating volcanic eruptions from the two currently active rhyolitic volcanoes, Taupo and Okataina, there is much less awareness of the equally damaging sedimentary aftermaths of these events as landscapes re-adjust to burial by thick layers of pyroclastic debris. 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 22.5 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. More 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 cubic metres per second, 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. Further north, Lake Rotorua is also fringed by extensive terrace and shoreline deposits that, coupled with downstream evidence, suggest multiple break-out flood events, while Lake Tarawera has been the source of volcanogenic floods c. 700 years ago and as recently as 1904 A.D.. Outbreak floods from caldera and crater lakes are a recurrent feature of volcanism in New Zealand and constitute a significant new and far-reaching hazard.
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 1821 Floods
DE: 8499 General or miscellaneous
SC: Hydrology [H]
MN: 2003 Fall Meeting