Implications for the thermal regime of acoustic noise measurements in Crater Lake,Mount Ruapehu,New Zealand

Hydrophone measurements of acoustic noise levels in the Crater Lake of Mount Ruapehu, New Zealand were made on 18 January 1991 from an inflatable rubber boat on the lake. The greatest sound pressures were recorded in the 1–10 Hz band, with sound levels generally decreasing about 20 dB per decade from 10 Hz to 80 kHz. The low frequency noise did not have an obvious relationship to the tremor observed at a seismic station within 1 km of the lake. The comparatively low levels of middle and high frequency sound meant that at the time of measurement, direct steam input did not make a significant contribution to the heating of Crater Lake. This is consistent with the earlier conclusion that during the last decade a major part of the heat input of Crater Lake has come from lake water that was heated below the lake and recycled back into the lake.     

Acoustic noise and temperature monitoring of the Crater Lake of Mount Ruapehu Volcano

Acoustic signals in Ruapehu Crater Lake, which are now being telemetered via a satellite transmission system, show promise as a possible precursor of increased volcanic activity from Ruapehu. The start of a recent period of rapid heating of Crater Lake was preceded by low-frequency (2 Hz) acoustic signals. These accompanied similar frequency seismic signals, but seemed to be produced independently. Audio-frequency (350–3000 Hz) acoustic noise also showed a very clear peak shortly before the lake temperature started to rise.     

Evolution of a vent-hosted hydrothermal system beneath Ruapehu Crater Lake,New Zealand

A two-year chemical monitoring program of Ruapehu Crater Lake shows that it has evolved considerably since the volcano's more active eruptive periods in the early 1970s. The present pH (20°C) of 0.6 is about one half unit more acid than the baseline values in the 1970s, whereas S/Cl ratios have increased markedly owing in part to declining HCl inputs into the lake, but also to absolute increases in SO₄ levels which now stand at the highest values ever recorded. Increases in K/Mg and Na/Mg ratios over the 20-year period are attributed to hydrothermal reaction processes in the vent which are presently causing dissolution of previously formed alteration phases such as natroalunite. These observations, combined with results of a recent heat budget analysis of the lake, have led to the development of hydrothermal convection model for the upper portion of the vent. Possible vent/lake chemical reaction processes between end member reactants have been modelled with the computer code CHILLER. The results are consistent with the view that variations in lake chemistry, which are initiated by the introduction of fresh magmatic material into the vent, reflect the extent of dissolution reaction progress on the magmatic material and/or its alteration products. The results also provide insights into the role of such vent processes in the formation of high sulfidation-type ore deposits.     

Temperature observations of Crater Lake,Mt Ruapehu,New Zealand,using Temperature Telemetry Bouys

The temperature of the Crater Lake of the active volcano Ruapelm has been recorded by Temperature Telemetry Buoys, to determine if lake temperature is correlated with volcanic activity. These buoys had to be specially designed to cope with the unfavourable environment of Crater Lake. A buoy contains a thermistor to measure the lake temperature, and a radio transmitter to transmit a short signal every few minutes, the interval between signals being a function of temperature. The temperature records obtained from these buoys show that the temperature near the lake surface can vary considerably within a few hours. Some of these variations appear to be caused by disturbances in convective heat transfer occurring in the lake. The occurrence of these short term temperature variations means that there is no simple relation between Crater Lake temperatures and the volcanic activity of Ruapehu. Some rapid increases in temperature followed volcanic earthquakes, but one of the biggest increases in temperature occurred just before a group of earthquakes upder the lake.     

A multiple-parameter approach to andesitic tephra correlation, Ruapehu volcano, New Zealand

A multi-parameter approach was used to correlate andesitic tephras in a complex tephra sequence ranging in age from ca. 23 to ca. 75 ka on the eastern ring plain of Ruapehu volcano, North Island. Field properties, combined with ferromagnesian mineral assemblages and mineral compositions, were required to map and correlate this sequence. Three tephra units could be identified based on their unique physical appearance, but other tephras could not be correlated on this basis alone. Hornblende and olivine proved to be valuable marker minerals enabling further distinction of two of the marker units recognised by field properties, as well as defining two further marker tephras. Unweathered titanomagnetite crystals, present in all of the tephras, were subjected to major-element analysis by electron microprobe. Canonical discriminant function analysis (DFA) of these analyses enabled the grouping and discrimination of tephra units, further aiding the identification of defined marker units, as well as defining new marker units. The titanomagnetite chemistry showed a strong relationship to the ferromagnesian mineralogy, showing that the ferromagnesian phenocrysts formed from the same melt or under the same melt conditions prior to eruption of each tephra. Canonical DFA was also applied to hornblende and olivine mineral analyses to identify further marker beds and to confirm identifications of previously defined units. This statistical analysis was found to be invaluable in reducing the large amount of compositional data from this study into a useable form for andesitic tephra correlation and mapping.     

The use of chemical indicators in the surveillance of volcanic activity affecting the Crater Lake on Mt Ruapehu,New Zealand

Between January 1966 and December 1973 approximately 100 water samples wert collected from the Crater Lake of Mt Ruapehu. From analyses of the samples, changes in chloride and magnesium concentrations and pH emerged as the most useful indicators for the occurrence of the two major processes associated with eruptive activity: Chloride concentrations vary in response to changes in fumarolic activity, arising from the degassing of magma; rises in magnesium concentrations are due to interaction of lake water with freshly injected, hot andesitic material. Similarly, variations in temperature, pH and the ratio Mg/Cl enable the effects of dilution and evaporation to be considered. The thermal power required to maintain elevated lake temperatures ～200 MW during quiet periods, reaching 1000 MW during active periods, is largely transferred by fumarolic steam.     

Shallow seismicity beneath Ruapehu Crater Lake: results of a 1994 seismometer deployment

 Virtually all the seismicity within Ruapehu Volcano recorded during a 2-month deployment in early 1994, with 14 broadband seismographs around the Tongariro National Park volcanoes in the North Island of New Zealand, was associated with the active vent and occurred within approximately 1 km of Ruapehu Crater Lake. High-frequency volcano-tectonic earthquakes and low-frequency events (similar to bursts of 2 Hz volcanic tremor) were both found to have sources in this region. The high-frequency events, which often consisted of a smaller precursor event followed approximately 2 s later by the main event, had sharp onsets and were locatable using standard techniques. The depth of these events ranged from the surface down to approximately 1500 m below Crater Lake. The low-frequency events did not have sharp onsets and were located by phase-correlation methods. Nearly all occurred under a small region on the east side of Crater Lake, at depths from 200 to 1000 m below the surface. This low-frequency earthquake source region, in which no high-frequency events occurred, may be the steam zone within the actual vent of Ruapehu Volcano. Received: 30 June 1996 / Accepted: 16 February 1998     

The hydrothermal vent system of Mount Ruapehu,New Zealand — a high frequency MT survey of the summit plateau

High frequency magnetotelluric (MT) measurements made on the summit plateau of Mount Ruapehu, some 1 km to the north of the presently active vent beneath Crater Lake, have been used to derive the electrical resistivity structure associated with the volcanic hydrothermal vent system. The entire summit plateau area is underlain at shallow depth by low resistivity which is inferred to be the result of hydrothermal alteration caused by rising volcanic gases mixing with local groundwater. Two areas of localised higher resistivity, one between 200 and 500 m depth beneath the central part of the plateau, and one at a depth of 1000 m below the northern part of the plateau, are interpreted as being the result of hydrothermal alteration at higher temperature forming chlorite dominated alteration products. These regions are believed to represent the locations of further heat pipes within the volcanic system. Both correlate with the locations of eruption centres on Ruapehu active within the last 10 ka.     

The Taurewa Eruptive Episode: evidence for climactic eruptions at Ruapehu volcano, New Zealand

 The ca. 10,500 years B.P. eruptions at Ruapehu volcano deposited 0.2–0.3 km³ of tephra on the flanks of Ruapehu and the surrounding ring plain and generated the only known pyroclastic flows from this volcano in the late Quaternary. Evidence of the eruptions is recorded in the stratigraphy of the volcanic ring plain and cone, where pyroclastic flow deposits and several lithologically similar tephra deposits are identified. These deposits are grouped into the newly defined Taurewa Formation and two members, Okupata Member (tephra-fall deposits) and Pourahu Member (pyroclastic flow deposits). These eruptions identify a brief (<ca. 2000-year) but explosive period of volcanism at Ruapehu, which we define as the Taurewa Eruptive Episode. This Episode represents the largest event within Ruapehu's ca. 22,500-year eruptive history and also marks its culmination in activity ca. 10,000 years B.P. Following this episode, Ruapehu volcano entered a ca. 8000-year period of relative quiescence. We propose that the episode began with the eruption of small-volume pyroclastic flows triggered by a magma-mingling event. Flows from this event travelled down valleys east and west of Ruapehu onto the upper volcanic ring plain, where their distal remnants are preserved. The genesis of these deposits is inferred from the remanent magnetisation of pumice and lithic clasts. We envisage contemporaneous eruption and emplacement of distal pumice-rich tephras and proximal welded tuff deposits. The potential for generation of pyroclastic flows during plinian eruptions at Ruapehu has not been previously considered in hazard assessments at this volcano. Recognition of these events in the volcanological record is thus an important new factor in future risk assessments and mitigation of volcanic risk at Tongariro Volcanic Centre. Received: 5 July 1998 / Accepted: 12 March 1999     

Eruption-induced modifications to volcanic seismicity at Ruapehu,New Zealand,and its implications for eruption forecasting

Broadband seismic data collected on Ruapehu volcano, New Zealand, in 1994 and 1998 show that the 1995-1996 eruptions of Ruapehu resulted in a significant change in the frequency content of tremor and volcanic earthquakes at the volcano. The pre-eruption volcanic seismicity was characterized by several independent dominant frequencies, with a 2 Hz spectral peak dominating the strongest tremor and volcanic earthquakes and higher frequencies forming the background signal. The post-eruption volcanic seismicity was dominated by a 0.8-1.4 Hz spectral peak not seen before the eruptions. The 2 Hz and higher frequency signals remained, but were subordinate to the 0.8-1.4 Hz energy. That the dominant frequencies of volcanic tremor and volcanic earthquakes were identical during the individual time periods prior to and following the 1995-1996 eruptions suggests that during each of these time periods the volcanic tremor and earthquakes were generated by the same source process. The overall change in the frequency content, which occurred during the 1995-1996 eruptions and remains as of the time of the writing of this paper, most likely resulted from changes in the volcanic plumbing system and has significant implications for forecasting and real-time assessment of future eruptive activity at Ruapehu.     

Relations of cinder cones to the magmatic evolution of Mount Mazama,Crater Lake National Park,Oregon

Cinder cones at Crater Lake are composed of high-alumina basaltic to andesitic scoria and lavas. The Williams Crater Complex, a basaltic cinder cone with andesitic to dacitic lava flows, stands on the western edge of the caldera, against an andesite flow from Mount Mazama. Bombs erupted from Williams Crater contain cores of banded andesite and dacite, similar to those erupted during the climatic eruption of Mount Mazama.Major- and trace-element variations exhibit an increase in incompatible elements and a decrease in compatible elements, consistent with crystal fractionation of olivine, plagioclase, clinopyroxene, orthopyroxene, and magnetite. LREE patterns in the rocks are irregular; each successive basalt is enriched in LREE relative to the preceding andesite.Compositional variations in the magmas of the cinder cones suggest that three magmatic processes were involved, partial melting, fractional crystallization, and magma mixing. Partial melting of more than one source produced primary basaltic magma(s). Subsequent mixing and fractional crystallization produced the more differentiated basaltic to andesitic magmas.     

Controls on accumulation of a volcaniclastic fan,Ruapehu composite volcano,New Zealand

The Whangaehu fan is the youngest sedimentary component on the eastern ring plain surrounding Ruapehu volcano. Fan history comprises constructional (830–200 years bp) and dissectional (<200 years bp) phases. The constructional phase includes four aggradational periods associated with both syneruptive and inter-eruptive behavior. All four aggradational periods began when deposition by large lahars changed flow conditions on the fan from channelized to unchannelized. Subsequent behavior was a function of the rate of sediment influx to the fan. The rate of sediment influx, in turn, was controlled by frequency and magnitude of volcanic eruptions, short-term climate change, and the amount of sediment stored on the volcano flanks. Fanwide aggradation occurred when rates of sediment influx and deposition on the fan were high enough to maintaìn unchannelized flow conditions on the fan surface. Maintenance of an undissected surface required sedimentation from frequent and large lahars that prevented major dissection between events. These conditions were best met during major eruptive episodes when high frequency and magnitude eruptions blanketed the volcano flanks with tephra and rates of lahar initiation were high. During major eruptive episodes, volcanism is the primary control on sedimentation. Climatic variations do not influence sediment accumulation. Local aggradation occurred when lahars were too small to maintain unchannelized flow across the entire fan. In this case, only the major channel system received much sediment following the deposition from the initial lahar. This localized aggradation occurred if (1) the sediment reservoir on the flank was large enough for floods to bulk into debris flows and (2) sedimentation events were frequent enough to maintain sediment supply to only some parts of the fan. These conditions were met during both minor eruptive and inter-eruptive episodes. In both cases, a large sediment reservoir remained on the volcano flanks from previous major eruptive intervals. Periods of increased storm activity produced floods that bulked to relatively small debris flows. When the sediment reservoir was depleted, the fan entered the present dissectional phase. Syneruptive and noneruptive lahars are mostly channelized and sediment bypasses the fan. Fan deposits are rapidly reworked. This is the present case at Ruapehu, even though the volcano is in a minor eruptive episode and the climate favors generation of intense storm floods.     

Transport and emplacement mechanisms of channelised long-runout debris avalanches,Ruapehu volcano,New Zealand

The steep flanks of composite volcanoes are prone to collapse, producing debris avalanches that completely reshape the landscape. This study describes new insights into the runout of large debris avalanches enhanced by topography, using the example of six debris avalanche deposits from Mount Ruapehu, New Zealand. Individual large flank collapses (>1 km³) produced all of these units, with four not previously recognised. Five major valleys within the highly dissected landscape surrounding Mount Ruapehu channelled the debris avalanches into deep gorges (≥15 m) and resulted in extremely long debris avalanche runouts of up to 80 km from source. Classical sedimentary features of debris avalanche deposits preserved in these units include the following: very poor sorting with a clay-sand matrix hosting large subrounded boulders up to 5 m in diameter, jigsaw-fractured clasts, deformed clasts and numerous rip-up clasts of late-Pliocene marine sediments. The unusually long runouts led to unique features in distal deposits, including a pervasive and consolidated interclast matrix, and common rip-up clasts of Tertiary mudstone, as well as fluvial gravels and boulders. The great travel distances can be explained by the debris avalanches entering deep confined channels (≥15 m), where friction was minimised by a reduced basal contact area along with loading of water-saturated substrates which formed a basal lubrication zone for the overlying flowing mass. Extremely long-runout debris avalanches are most likely to occur in settings where initially partly saturated collapsing masses move down deep valleys and become thoroughly liquified at their base. This happens when pore water is available within the base of the flowing mass or in the sediments immediately below it. Based on their H/L ratio, confined volcanic debris avalanches are two to three times longer than unconfined, spreading flows of similar volume. The hybrid qualities of the deposits, which have some similarities to those of debris flows, are important to recognise when evaluating mass flow hazards at stratovolcanoes.     

A fluid dynamics approach to modelling the 18th March 2007 lahar at Mt. Ruapehu,New Zealand

Lahars are water-sediment mass flows from a volcanic source. They can be triggered by a variety of mechanisms and span a continuum of flow rheology and hydraulic properties, even within the same event. Lahars are extremely powerful landscaping agents and represent a considerable hazard potential. However, this highly dynamic character and a lack of direct measurements has made modelling lahars difficult. This study therefore applies a fluid dynamics model; Delft3D, to analyse the 18th March 2007 dam break lahar at Mount Ruapehu, New Zealand. The modelled lahar routed through the Whangaehu gorge in ~30 min, crossed the Whangaehu fan in ~60 min, and then over a further 3 h travelled an additional ~22 km distance along the Whangaehu River to the Tangiwai bridge. The modelled mean frontal velocity was 6.5 m s⁻¹ along the gorge although peak velocity reached up to 19.6 m s⁻¹. The modelled lahar flow front progressively slowed across the fan but along the River it accelerated from 2.1–3.3 m s⁻¹. Calculated peak velocity along the River was <4.5 m s⁻¹. These results generally compare well with gauged records, with historical records, and with other modelling approaches. However, discrepancies in frontal velocity and time to peak stage arise due to (1) specifying roughness, which arises from slope variations between adjacent computational nodes, and which is stage-dependant, and (2) due to rapid topographic changes that produce frequent hydraulic jumps, which are inadequately accommodated in the numerical scheme. The overall pattern of discharge attenuation, and of relationships between topographic and hydraulic variables, is similar to that calculated for lahars on other volcanoes. This modelling method could be applied at other similar sites where a likely source hydrograph and high-resolution topographic data are available. These results have important implications for hazard management at Ruapehu and for examining geomorphic and sedimentary impacts of this lahar.     

Lacustrine shore platforms at Lake Waikaremoana,North Island,New Zealand

This paper examines the morphology and processes governing the development of shore platforms at Lake Waikaremoana, North Island, New Zealand. Shore platforms at Lake Waikaremoana are recent features, and were formed when a new sequence of shoreline development was initiated, due to lowering of the lake by 5 m in 1946 for hydroelectric power development. Three predominant platform morphologies were identified around the lake. These include gently sloping platforms (c.1·5 to 3·9°), ramp platforms (c.6·8 to 9·2°), and concave ramp platforms (c.7·9 to 12°). Platform widths ranged from 11 to 31 m, with the gently sloping platforms characterized by the widest morphologies. Erosion rates were estimated using perched sandstone boulders and were found to range from 3·4 to 12·5 mm a^?1, with a mean erosion rate of 5·9 mm a^?1. Higher rates of erosion were identified at lower platform elevations, due to a greater frequency of wetting and drying cycles coincident with storm waves, while lower erosion rates were identified at higher elevations. Field evidence suggests that shore platforms at Lake Waikaremoana were likely initiated and continue to develop as a result of subaerial wetting and drying cycles. Waves, coincident with fluctuating lake levels, play an important role by removing the weathered material from the platforms, and appear to control the width of the platforms. A conceptual model of platform development is presented, and analogies are drawn between this model, and the formation of shore platforms in oceanic environments. Copyright © 2002 John Wiley & Sons, Ltd.     

Volcanic hazard assessment for Ruapehu composite volcano,taupo volcanic zone,New Zealand

Ruapehu is a very active andesitic composite volcano which has erupted five times in the past 10 years. Historical events have included phreatomagmatic eruptions through a hot crater lake and two dome-building episodes. Ski-field facilities, road and rail bridges, alpine huts and portions of a major hydroelectrical power scheme have been damaged or destroyed by these eruptions. Destruction of a rail bridge by a lahar in 1953 caused the loss of 151 lives. Other potential hazards, with Holocene analogues, include Strombolian and sub-Plinian explosive eruptions, lava extrusion from summit or flank vents and collapse of portions of the volcano. The greatest hazards would result from renewed phreatomagmatic activity in Crater Lake or collapse of its weak southeastern wall. Three types of hazard zones can be defined for the phreatomagmatic events: inner zones of extreme risk from ballistic blocks and surges, outer zones of disruption to services from fall deposits and zones of risk from lahars, which consist of tongues down major river valleys. Ruapehu is prone to destructive lahars because of the presence of 10⁷ m³ of hot acid water in Crater Lake and because of the surrounding summit glaciers and ice fields. The greatest risks at Ruapehu are to thousands of skiers on the ski field which crosses a northern lahar path. Three early warning schemes have been established to deal with the lahar problems. Collapse of the southeastern confining wall would release much of the lake into an eastern lahar path causing widespread damage. This is a long-term risk which could only be mitigated by drainage of the lake.     

Nature and origin of cone-forming volcanic breccias in the Te Herenga Formation, Ruapehu, New Zealand

 Volcanic breccias form large parts of composite volcanoes and are commonly viewed as containing pyroclastic fragments emplaced by pyroclastic processes or redistributed as laharic deposits. Field study of cone-forming breccias of the andesitic middle Pleistocene Te Herenga Formation on Ruapehu volcano, New Zealand, was complemented by paleomagnetic laboratory investigation permitting estimation of emplacement temperatures of constituent breccia clasts. The observations and data collected suggest that most breccias are autoclastic deposits. Five breccia types and subordinate, coherent lava-flow cores constitute nine, unconformity-bounded constructional units. Two types of breccia are gradational with lava-flow cores. Red breccias gradational with irregularly shaped lava-flow cores were emplaced at temperatures in excess of 580  °C and are interpreted as aa flow breccias. Clasts in gray breccia gradational with tabular lava-flow cores, and in some places forming down-slope-dipping avalanche bedding beneath flows, were emplaced at varying temperatures between 200 and 550  °C and are interpreted as forming part of block lava flows. Three textural types of breccia are found in less intimate association with lava-flow cores. Matrix-poor, well-sorted breccia can be traced upslope to lava-flow cores encased in autoclastic breccia. Unsorted boulder breccia comprises constructional units lacking significant exposed lava-flow cores. Clasts in both of these breccia types have paleomagnetic properties generally similar to those of the gray breccias gradational with lava-flow cores; they indicate reorientation after acquisition of some, or all, magnetization and ultimate emplacement over a range of temperatures between 100 and 550  °C. These breccias are interpreted as autoclastic breccias associated with block lava flows. Matrix-poor, well-sorted breccia formed by disintegration of lava flows on steep slopes and unsorted boulder breccia is interpreted to represent channel-floor and levee breccias for block lava flows that continued down slope. Less common, matrix-rich, stratified tuff breccias consisting of angular blocks, minor scoria, and a conspicuously well-sorted ash matrix were generally emplaced at ambient temperature, although some deposits contain clasts possibly emplaced at temperatures as high as 525  °C. These breccias are interpreted as debris-flow and sheetwash deposits with a dominant pyroclastic matrix and containing clasts likely of mixed autoclastic and pyroclastic origin. Pyroclastic deposits have limited preservation potential on the steep, proximal slopes of composite volcanoes. Likewise, these steep slopes are more likely sites of erosion and transport by channeled or unconfined runoff rather than depositional sites for reworked volcaniclastic debris. Autoclastic breccias need not be intimately associated with coherent lava flows in single outcrops, and fine matrix can be of autoclastic rather than pyroclastic origin. In these cases, and likely many other cases, the alternation of coherent lava flows and fragmental deposits defining composite volcanoes is better described as interlayered lava-flow cores and cogenetic autoclastic breccias, rather than as interlayered lava flows and pyroclastic beds. Reworked deposits are probably insignificant components of most proximal cone-forming sequences. Received: 1 October 1998 / Accepted: 28 December 1998     

A facies model for a quaternary andesitic composite volcano: Ruapehu,New Zealand

Ruapehu composite volcano is a dynamic volcanic-sedimentary system, characterised by high accumulation rates and by rapid lateral and vertical change in facies. Four major cone-building episodes have occurred over 250 Ka, from a variety of summit, flank and satellite vents. Eruptive styles include subplinian, strombolian, phreatomagmatic, vulcanian and dome-related explosive eruptions, and extrusion of lava flows and domes. The volcano can be divided into two parts: a composite cone of volume 110 km³, surrounded by an equally voluminous ring plain. Complementary portions of Ruapehu's history are preserved in cone-forming and ring plain environments. Cone-forming sequences are dominated by sheet- and autobrecciated-lava flows, which seldom reach the ring plain. The ring plain is built predominantly from the products of explosive volcanism, both the distal primary pyroclastic deposits and the reworked material eroded from the cone. Much of the material entering the ring plain is transported by lahars either generated directly by eruptions or triggered by the high intensity rain storms which characterise the region. Ring plain detritus is reworked rapidly by concentrated and hyperconcentrated streams in pulses of rapid aggradation immediately following eruptions and more gradually in the longer intervals between eruptions.