Environmental impact of the 1.8 ka Taupo eruption, New Zealand: Landscape responses to a large-scale explosive rhyolite eruption
Abstract
Large-scale ignimbrite eruptions from rhyolitic caldera volcanoes can trigger geologically instantaneous changes in sedimentary systems over huge areas by either burying existing environments or overloading them with vast quantities of unconsolidated particulate material. The post-eruption readjustment of the landscape to such perturbations is one of the most dramatic processes in physical sedimentology, exemplified here by the 1.8 ka Taupo eruption in the central North Island of New Zealand. This eruption generated voluminous fall deposits, then climaxed with emplacement of a c. 30 km 3 non-welded ignimbrite over a near-circular area of c. 20 000 km 2. Approximately 90% of the area, but < 50% of the ignimbrite volume, is represented by a landscape-mantling unit that covered the pre-eruption topography to a depth varying from c. 10 m in proximal areas to less than 15-30 cm distally. The remainder of the ignimbrite deposit is represented by landscape-modifying material that ponded in valley bottoms and depressions to thicknesses of up to 70 m, with no systematic variation in thickness with distance from source. The headwaters of many of the North Island's largest rivers were impacted by both the primary pyroclastic fall and flow material. Large-scale post-eruption remobilisation of this material, coupled with the re-establishment of fluvial systems, occurred in a distinct sequence as recorded by the evolution of sedimentary facies in different sub-environments. Following an initial period dominated by mass flows, re-establishment of fluvial systems began with the headward erosion of box canyons through the ponded ignimbrite deposits, a process often associated with the break-out of temporary lakes. Aggradational streams developed in these channels rapidly evolved from shallow, ephemeral, sediment-laden outbursts associated with flash flood events to deeper, permanent braided rivers, before declining sediment yields led to retrenchment of single thread rivers and a return to pre-eruption gradients and bedloads years to decades later. Typically the modern profile of many streams and rivers follow closely their pre-eruption profiles, and incision and erosion is overwhelmingly confined to the deposits of the eruption itself. Although the general remobilisation pattern is similar for all impacted river systems, detailed studies of the Waikato, Rangitaiki, Mohaka, Ngaruroro and Whanganui catchments show that the relative timing and scale of each eruption response phase differs between each catchment. These reflect differences in catchment physiography and hydrology, and the volume and type of pyroclastic material deposited in each. Ultimately, the landscape response reflects the relative spatial distributions of, and the volumetric ratios between, the volumes of pyroclastic debris, water, and accommodation space in the basin (cf. Kataoka and Manville, this volume).
- Publication:
-
Sedimentary Geology
- Pub Date:
- October 2009
- DOI:
- 10.1016/j.sedgeo.2009.04.017
- Bibcode:
- 2009SedG..220..318M