Jérôme A. Lecointre

Environmental hazards of fluoride in volcanic ash: a case study from Ruapehu volcano, New Zealand

Abstract The vent-hosted hydrothermal system of Ruapehu volcano is normally covered by a c. 10 million m3 acidic crater lake where volcanic gases accumulate. Through analysis of eruption observations, granulometry, mineralogy and chemistry of volcanic ash from the 1995–1996 Ruapehu eruptions we report on the varying influences on environmental hazards associated with the deposits. All measured parameters are more dependent on the eruptive style than on distance from the vent. Early phreatic and phreatomagmatic eruption phases from crater lakes similar to that on Ruapehu are likely to contain the greatest concentrations of environmentally significant elements, especially sulphur and fluoride. These elements are contained within altered xenolithic material extracted from the hydrothermal system by steam explosions, as well as in residue hydrothermal fluids adsorbed on to particle surfaces. In particular, total F in the ash may be enriched by a factor of 6 relative to original magmatic contents, although immediately soluble F does not show such dramatic increases. Highly soluble NaF and CaSiF6 phases, demonstrated to be the carriers of ‘available’ F in purely magmatic eruptive systems, are probably not dominant in the products of phreatomagmatic eruptions through hydrothermal systems. Instead, slowly soluble compounds such as CaF2, AlF3 and Ca5(PO4)3F dominate. Fluoride in these phases is released over longer periods, where only one third is leached in a single 24-h water extraction. This implies that estimation of soluble F in such ashes based on a single leach leads to underestimation of the F impact, especially of a potential longer-term environmental hazard. In addition, a large proportion of the total F in the ash is apparently soluble in the digestive system of grazing animals. In the Ruapehu case this led to several thousand sheep deaths from fluorosis.

Changes in Whangaehu river lahar characteristics during the 1995 eruption sequence, Ruapehu volcano, New Zealand

Abstract During the 1995 Ruapehu eruptive sequence multiple lahars occurred in the Whangaehu river, which drains Ruapehus Crater Lake. During the earlier phreatic and phreatomagmatic eruptions, lahars were generated by expulsion of waters from the Lake, but once the lake had emptied, lahars were formed by remobilisation of seasonal snowpack laden with saturated freshly erupted tephra. Four types of lahars occurred during the eruptive sequence: (1) Initial snow-slurry lahars, composed of granular snow and ice incorporated by eruptively expelled Crater Lake waters which left behind frozen deposits with 2.5–20% clastic sediment. (2) Large dilute lahars, generated as the volumes of ejected lake water increased and removed much of the readily available snow. At least one third of the pre-eruption Crater Lake volume was expelled during one day producing the largest lahars of the series. These lahars were hyperconcentrated flows for up to 84 km from source, leaving extensive deposits along the channel margins. (3) Concentrated lahars; smaller volume lahars generated as the frequency of eruptions and volumes of expelled water declined. These lahars were able to maintain high sediment concentrations, measured at 46–52% by volume suspended sediment at 42 km from source. Their high sediment concentrations were maintained by erosion and incorporation of sand from the deposits of earlier flows which were lining the channel margins. (4) Remobilised tephra lahars, generated following the two largest tephra eruptions of the sequence. Seasonal snowpack was covered by water-saturated tephra. Warmer spring temperatures and heavy rainfall events caused collapse and remobilisation of snow and tephra, producing several lahars in catchments draining eastern Ruapehu.

1995 Ruapehu lahars in relation to the late Holocene lahars of Whangaehu River, New Zealand

Abstract Observations of active flows, their deposits, and the effects of lahars generated during the 1995 eruptions of Ruapehu are compared to those of the last 2000 years. The 1995 lahars were generated by similar mechanisms, and had similar volumes and flow rheologies, to those of the last 135 years. However, the large number of lahars in the 1995 sequence and the eventual emptying of the entire Crater Lake on Mt Ruapehu by lahars is distinctive in the context of the historic record. The 1953 and 1975 lahars, although of smaller total volume than the largest 1995 lahar, had higher peak discharges and consequently greater effects on life and property within 57 km from the source. This implies that failure of a lake dam is the most efficient mechanism for generating a fast lahar with high peak discharge, and that eruptions through Crater Lake are more efficient at generating a lahar with high peak discharge when the lake is full. The scant 1995 lahar deposits preserved indicate that the geological record...

Quaternary lahar stratigraphy of the western Ruapehu ring plain, New Zealand

Abstract One of the most widespread hazards associated with explosive activity at Mt Ruapehu is the production of potentially destructive lahars. Volcanic activity at this andesite stratovolcano has produced numerous lahars throughout Quaternary time, with the resulting deposits being preserved principally on the lower slopes of the composite cone and its ring plain. This paper describes the stratigraphy of volcaniclastic sequences on the western Ruapehu ring plain, in order to improve the record of known events. The lithologic record is principally one of lahar deposits interbedded with datable andesitic tephras, loesses, paleosols, and rhyolitic marker beds, allowing a chronological reconstruction of volcanic events in the region during the last 120 000 yr. The main laharic surfaces include the extensive Porewan‐aged (>65 000 yr B.P.) uplifted Erua plateau (Waimarino Formation, new) and voluminous Ratan‐Ohakean (10 000–65 000 yr B.P.) sequences of stacked diamictons east of the Waimarino Fault (Horopito...

Transformation, internal stratification, and depositional processes within a channelised, multi‐peaked lahar flow

Abstract On 25 September 1995, phreatomagmatic explosions through Crater Lake at Ruapehu volcano, New Zealand, generated a closely spaced sequence of lahars. From direct observations of the flows and timely description of their deposits, we infer these debris flows transformed to hyperconcentrated streamflows not by dilution with incorporated water they overran, as previously proposed. Rather, the described debris flows diluted by selective deposition of their coarse clasts as they thinned and decelerated while spreading >700 m laterally over the Whangaehu fan. Deposits recording this transformation are veneering (<100 mm thick) layers of muddy sandy gravel interspersed with many boulders and cobbles. Downstream of their transformation to hyperconcentrated streamflows, ephemeral near‐channel deposits indicate the flows were vertically stratified. A new depositional model for these hyperconcentrated streamflows includes a basal, coarse, sediment‐concentrated “channel flow” that emplaced transitory near‐channel sediment wedges. The near‐channel sediment was bouldery, massive, and poorly sorted, like debris‐flow deposits elsewhere in the Whangaehu catchment. The upper and marginal parts of the lahars (the surface layer) were diluted, finer hyperconcentrated flows that left voluminous overbank deposits. The overbank sediment is poorly sorted gravelly sand, with some degree of horizontal bedding, like other hyperconcentrated flow deposits elsewhere in the catchment. The rapid erosion of channel‐flow deposits within days to months of the events indicates that geologic records will only preserve lateral‐flow deposits of such lahars. Hence, long after an event, interpretation of hyperconcentrated streamflow mechanisms from geologic deposits can be misleading without the near‐channel record.

Onetapu Formation: The last 2000 yr of laharic activity at Ruapehu volcano, New Zealand

Abstract The Onetapu Formation represents a sequence of at least 17 volcanic diamictons that were emplaced over the last c. 2000 14C yr by lahars in the Whangaehu River catchment, covering part of the eastern flank of Ruapehu volcano and its adjacent ring plain. Interbedded andesitic tephras from the Tufa Trig Formation help in reconstructing the chronology of volcanogenic events that led to the formation of the Crater Lake basin and its subsequent evolution. Onetapu Formation contains deposits laid down by lahars spanning several orders of magnitude. Channel deposits left by the largest lahars are bouldery to distances reaching c. 50 km downstream from the source region. Much of the coarsest fraction of the medium to smaller lahar deposits (i.e., volume <5 × 107 m3) was emplaced close to the source on the Whangaehu Fan, where the river debouches onto the ring plain. Mainly thin, fine grained, pebbly overbank debris‐flow deposits are preserved downstream of the fan, with increasing proportions of sandy hyperconcentrated‐flow deposits. The largest Onetapu lahars are inferred to have been generated by partial collapse of the wall of Crater Lake as a result of powerful explosive eruptions. In contrast, smaller lahars were the consequence of phreatic/phreatomagmatic activity within Crater Lake, expelling lake waters onto the flanks of the volcano.

Late Quaternary evolution of the Rotoaira Basin, northern Tongariro ring plain, New Zealand

Abstract The last 100 000 yr record of volcaniclastic sedimentation in the Rotoaira Basin reflects the critical role played by tectonic and eruptive activity that led to: (1) the catastrophic emplacement of Te Whaiau Formation (60–55 ka); (2) major episodes of effusive activity (c. 30 ka and c. 15 ka); and (3) syn‐ and post‐eruptive lahar aggradation on the shores of Lake Rotoaira. Stratigraphic unconformities, fluvial and aeolian reworking, and subsequent volcanogenic sedimentation indicate that climate change driven periods of erosion contributed significantly to the evolution of the landscape during the late Quaternary. Waters that accumulated in a newly expanded graben formed Lake Rotoaira. An attempt to core through the sediments on the lakes floor revealed a >6.5 m thick mantling of Taupo ignimbrite, restricting chronostratigraphic sampling to the last 1.85 ka. Lake Rotoaira pollen profiles record: (1) the destruction of the indigenous forest by the Taupo ignimbrite eruption; (2) the following period of vegetation recovery (c. 300 yr); and (3) periods of human occupation of the lakeshores (bracken spores and Pinus pollen) in the younger sediments. Coarse sand generated by the 1995–96 tephra‐producing Ruapehu eruptions also contributed to the recent lake sedimentation.