J. D. L. White

Paleohydrology and sedimentology of a post–1.8 ka breakout flood from intracaldera Lake Taupo, North Island, New Zealand

Sudden releases of impounded water from lakes in volcanic regions constitute a major and frequently repeated hazard. An outburst flood from Taupo caldera, New Zealand, released ∼20 km 3 of water, within decades following an ignimbrite-emplacing eruption, ca. 1.8 ka. Paleohydrologic reconstruction of the Taupo flood provides estimates of peak discharge at the outlet in the range 17 000–35 000 m 3 /s. Dimensionless analysis demonstrates that (1) failure of the barrier was essentially instantaneous, (2) the event may be treated hydraulically as a dam break, and (3) the peak discharge was a function of outlet geometry rather than lake volume or breach formation rate. Paleohydraulic reconstructions based on empirical relations derived from historic dam breaches yield only order of magnitude estimates of peak discharge. Calculations based on the physical dimensions of the outlet channel and hydraulic principles are likely to be more accurate and are in close agreement with computer-implemented dynamic-flow–routing models. The latter give peak discharges and maximum stage levels similar to constraints imposed by field evidence and estimates of flow depth and velocity. The long duration of the Taupo flood and the relatively narrow, confined flood route resulted in minimal attenuation of the flood wave compared with modern dam breach events, and flood deposits can be traced as far as 232 km downstream. Caldera lake breakout floods may be among the most far-reaching hazards associated with volcanism.

Incipient granular mass flows at the base of sediment-laden floods, and the roles of flow competence and flow capacity in the deposition of stratified bouldery sands

Extraordinary floods commonly produce thick deposits as a result of the entrainment and transport, often at high concentrations, of large volumes of clastic detritus. This paper describes the sedimentology of a stratified bouldery sand deposit generated by a 20-km3-breakout flood from intracaldera Lake Taupo following the 1.8-ka Taupo eruption. The proximal deposit is characterised by a strongly bimodal grain-size distribution, with downstream-fining boulders enclosed in a poorly sorted and weakly bedded coarse sand to gravel matrix. This combination of characteristics is inferred to reflect simultaneous deposition of small and large particles by two distinct mechanisms: (i) accretion of the sandy matrix from an incipient basal granular mass flow as the flood lost capacity, and (ii) progressive emplacement of the boulders as the flood lost the competence to roll them along the channel floor. The first mechanism is a product of a nonlinear particle concentration gradient in the sediment-laden flow, which caused it to segregate into a basal incipient granular mass flow that formed the transient depositional system indicated by the lithofacies. The second mechanism is a function of the superjacent, turbulent, turbid flow that acted as the primary transporting system. This pairing of transport and depositional systems is likely to characterise many sediment-laden aqueous flows with clay mineral concentrations too low to develop non-Newtonian rheologies, such as jokulhlaups and lahars that entrain unweathered pyroclastic material. Qualitative and quantitative differences in the way bedload is transported in the presence of this basal granular mass flow, as compared to simple water, make impossible accurate estimates of flow competence based on Shields entrainment functions for beds of both uniform and mixed grain size distribution.

Environmental response to a large, explosive rhyolite eruption: sedimentology of post-1.8 ka pumice-rich Taupo volcaniclastics in the Hawke's Bay region, New Zealand

Abstract The 1.8 ka eruption of the Taupo ignimbrite from the Taupo Volcanic Centre in the central North Island of New Zealand produced a ∼30 km3 ignimbrite covering an area ca. 160 km in diameter, centred on Lake Taupo. Remobilisation of its pyroclastic detritus triggered major alluviation in the years to decades after its emplacement. In the Hawkes Bay region in the eastern North Island, three large rivers rise in the ignimbrite-impacted mountainous headwaters, in areas that were buried by up to 40 m of loose, unconsolidated pumiceous material. Analysis of preserved volcaniclastic sediments along valleys shows that resedimentation occurred in two main stages characterised by contrasting sediment:water ratios. The first comprised erosion and redeposition of ignimbrite debris by lahars, e.g. debris flows and hyperconcentrated flows. The second was marked by decreasing sediment:water ratios and the re-establishment of braided fluvial systems in the former river valleys, as recorded by such deposits as trough, planar, low-angle and antidune cross-bedded, planar-bedded, and ripple cross-laminated sands and gravels. However, there is no systematic chronological trend towards lower pumice contents in these fluvial facies. Four characteristic sets of co-occurring lithofacies were produced during the sedimentary response: (1) Laharic remobilisation association; (2) Upper-flow regime-dominated stream-flow association; (3) Lower-flow regime-dominated stream-flow association; and (4) Lacustrine association. Individual lithofacies were strongly controlled by the unusual hydrodynamic properties of pumice, particularly its low and variable density. Automated settling tube analysis of water-saturated pumiceous sediment confirms that sieve analysis is misleading with respect to the hydrodynamic behaviour of such sediments. Recasting granulometric data in terms of effective hydraulic grain size shows better sorting of mixed component sediments, but statistical sorting and mean grain-size parameters do not discriminate between the deposits of tractional and overcapacity parent flows. Therefore, particle behaviour is addressed by using both lithofacies and petrofacies in the description and classification of the remobilisation deposits.

Resedimentation of the 1.8 ka Taupo ignimbrite in the Mohaka and Ngaruroro river catchments, Hawke's Bay, New Zealand

Abstract Deposition of the 1.8 ka Taupo ignimbrite in the Hawkes Bay buried parts of the Ngaruroro and Mohaka River catchments beneath up to 40 m of loose pyroclastic debris. Re‐establishment of the two river systems led to the remobilisation of the loose debris and followed similar patterns in both catchments. An initial period of laharic remobilisation and formation of lahar‐deposit‐dammed lakes was followed by two phases of fluvial remobilisation. During the first of these, streams were shallow and ephemeral to perennial, with sediment‐laden flash‐floods eroding headwards through valley‐ponded primary pyroclastic and laharic deposits. With gradual re‐establishment of vegetation, the sediment:water ratio decreased, and braided rivers with deeper, more stable channels became dominant. Although the general pattern of resedimentation was similar for both river systems, field evidence shows that the volumes of remobilised material, and the likely durations of resedimentation in the different catchments, differed. These variations reflect: (1) the different volumes of initial ignimbrite in the trunk valleys; (2) the different percentage of catchment areas covered by ignimbrite debris; and (3) the different river gradients and valley shapes. However, extremely high sediment yields in the Hawkes Bay region are estimated to have persisted for at least 4–17 yr after the eruption.

Rheological properties of a remobilised‐tephra lahar associated with the 1995 eruptions of Ruapehu volcano, New Zealand

Abstract Volcanic activity at Ruapehu during 1995/96 represents the most voluminous volcanic activity in New Zealand this century. Early in the eruption sequence, lahars were generated by the ejection of water, lake sediments, and pyroclastic material from the summit Crater Lake. Lahars (volcanic debris flows) generated by the remobilisation of pyroclastic fall deposits on the upper slopes of the volcano followed the two largest tephra‐producing eruptions in October 1995, representing a previously unrecognised hazard. Prediction and mitigation of such hazards requires quantitative knowledge of initiation processes, lahar mechanics (i.e., rheology), and appropriate hydrodynamic routing for channelised flows. The rheological properties of a remobilised tephra lahar in the Mangatoetoenui valley were estimated using techniques developed for determining the properties of debris flows based on the geometry of their deposits. Calculated maximum flow velocities of 12–27 m/s, volumetric flow rates of 1300–2000 m3/...

Dynamic interactions between lahars and stream flow: A case study from Ruapehu volcano, New Zealand: Discussion and reply Discussion

[Cronin et al. (1999)][1] presented details of an unusual data set obtained from observations of active lahars made during the 1995 eruptions of Ruapehu volcano, New Zealand. This data set includes both visual observations and time-series variations in stage height and gauged discharge made on

A coniferous tree stump of late Early Jurassic age from the Ferrar Basalt, Coombs Hills, southern Victoria Land, Antarctica

Abstract An upright, partly rooted tree stump preserved within a late Early Jurassic basalt flow of the Ferrar Group, Coombs Hills, is described from southern Victoria Land, Antarctica. The wood structure allows the tree to be identified as a conifer (possibly Podocarpaceae) that from the stump diameter of 35–40 cm was c. 24 m tall. The possibly transported tree may have survived the lava flow because the stump was waterlogged or already mineralised. Conifers of similar age (early Middle Jurassic) are known from fossil forests at Curio Bay and Kawhia Harbour in New Zealand, and this discovery extends the known distribution of forests living in moderately warm, humid climates at very high southern latitudes during the late Early Jurassic.

The evolution of hydrous magmas in the Tongariro Volcanic Centre: the 10 ka Pahoka-Mangamate eruptions

The majority of arc-type andesites in the Tongariro Volcanic Centre are highly porphyritic, hornblende-free, two-pyroxene andesites. An exception is tephras from the c. 10,000 ka Pahoka-Mangamate event. Magmas of these Plinian eruptions bypassed the extensive crustal mush columns under the central volcanoes and sequentially derived a series of almost aphyric rocks spanning a compositional range from dacite to basaltic andesite. Mineral composition, trace element and isotopic data suggest that this eruptive series tapped a mid-crustal magma reservoir, resulting in the initial eruption of an hydrous dacitic magma and several following eruptions characterised by less-evolved and less-hydrous compositions at progressively higher temperatures and substantially lower 87Sr/86Sr ratios. Systematic changes in magma chemistry are also reflected in a sequential change in phenocryst content starting with an early hornblende–plagioclase-dominated assemblage to a late olivine–plagioclase-dominated assemblage.