Editing Mount Ruapehu (section)

==Geology==
{{#tag:mapframe|[{{Wikipedia:Map data/Northern North Island Volcanics}},{{Wikipedia:Map data/Mount Tongariro}}]
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| text =Map centred on Mount Ruapehu to show approximate selected surface volcanic deposits around it with [[andesite|andesitic]] deposits shaded {{legend-inline|#fb7977|red}}. Vents and cones or craters active in the last 15,000 years are shaded {{legend-inline|#fdbc3b|orange-yellow}} with craters in {{legend-inline|#ffff00|yellow outline}}. Lakes in vents are outlined in {{legend-inline|#0000ff|blue}}. To the north the volcanic deposits are continuous with those from [[Mount Tongariro]] and beyond this is [[Lake Rotoaira]] and the andesitic deposits of [[Pihanga]]. Mixed (e.g. [[lahar]]s) and sedimentary deposits are not coloured. Clicking on the map enlarges it, and enables panning and mouseover of volcanic deposits name/wikilink and ages before present for wider volcanic context. The key to the shading of other volcanics that are shown (active in last million years odd) with panning is {{legend-inline|#cc6600|[[basalt]]}} (shades of brown/orange), {{legend-inline|#aa4400|[[Monogenetic volcanic field|monogenetic]] basalts}}, {{legend-inline|#efaf00|undifferentiated basalts}}, {{legend-inline|#ff6600|arc basalts}}, {{legend-inline|#ff4400|arc ring basalts}}, {{legend-inline|#bb00bb|[[dacite]]}}, {{legend-inline|#ff2200|basaltic andesite}}, {{legend-inline|#ff00ff|[[rhyolite]]}}, ([[ignimbrite]] is lighter shades of violet), and {{legend-inline|#aaaaaa|plutonic or intusive}} (gray). White shading has been used for postulated calderas (usually subsurface now). 
| width =250 
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| latitude =-39.283333
| longitude =175.566667
| icon =no
| zoom =9
}}
Ruapehu is a composite [[andesite|andesitic]] [[stratovolcano]] located at the southern end of the [[Taupō Volcanic Zone]] and forming part of the Tongariro Volcanic Centre.<ref name="Hackett1985">{{cite thesis |type=Ph.D. |last=Hackett |first=W. R. |date=1985 |title=Geology and Petrology of Ruapehu Volcano and Related Vents |publisher=Victoria University of Wellington}}</ref> Volcanism at Ruapehu is caused by the subduction of the [[Pacific Plate]] under the [[Australian Plate]] at the [[Hikurangi Trough]] to the east of the [[North Island]]. Ruapehu has erupted from multiple craters over its lifetime, however, only one crater is presently active, a deep crater at the southern end of the summit plateau which is filled with hot, acidic water, dubbed Crater Lake (Te Wai ā-moe). The lake water currently covers separate north and central vents.<ref>{{Cite web|url=https://www.geonet.org.nz/vabs/4cVdxClElNOctJS9LaQXty|title =Mt Ruapehu status update: Minor volcanic unrest continues while Crater Lake temperature is low. Volcanic Alert Level remains at Level 1|publisher=[[GeoNet (New Zealand)|GeoNet]]|access-date=21 July 2024|date=13 October 2020}}</ref>

Ruapehu sits on a [[Basement (geology)|basement]] of [[Mesozoic]] [[greywacke]] overlain by a thin layer of sediments of the [[Wanganui Basin]], composed of sands, silts, shell beds, and [[limestone]].<ref name="Price2012">{{cite journal |last1=Price |first1=R. C. |last2=Gamble |first2=J. A. |last3=Smith |first3=I. E. M. |last4=Maas |first4=R. |last5=Waight |first5=T. |last6=Stewart |first6=R. B. |last7=Woodhead |first7=J. |title=The Anatomy of an Andesite Volcano: a Time–Stratigraphic Study of Andesite Petrogenesis and Crustal Evolution at Ruapehu Volcano, New Zealand |journal=Journal of Petrology |date=October 2012 |volume=53 |issue=10 |pages=2139–2189 |doi=10.1093/petrology/egs050 |bibcode=2012JPet...53.2139P |doi-access=free}}</ref> It has not been clearly established when Ruapehu first began erupting, only that eruptions began at least 250,000 years ago and possibly as early as 340,000 years ago.<ref name="Gamble2003">{{cite journal |last1=Gamble |first1=John A. |last2=Price |first2=Richard C. |last3=Smith |first3=Ian E.M. |last4=McIntosh |first4=William C. |last5=Dunbar |first5=Nelia W. |title=40Ar/39Ar geochronology of magmatic activity, magma flux and hazards at Ruapehu volcano, Taupo Volcanic Zone, New Zealand |journal=Journal of Volcanology and Geothermal Research |date=February 2003 |volume=120 |issue=3–4 |pages=271–287 |doi=10.1016/S0377-0273(02)00407-9 |bibcode=2003JVGR..120..271G}}</ref> Ruapehu has been built in four distinct stages of relatively intense eruptive activity followed by periods of relative quiet. Each of these four stages of activity has left behind distinct rock formations, named the Te Herenga Formation (erupted 250,000–180,000 years ago), the Wahianoa Formation (erupted 160,000–115,000 years ago), the Mangawhero Formation (erupted 55,000–15,000 years ago), and the Whakapapa Formation (erupted 15,000–2,000 years ago).<ref name="Gamble2003" /> Each of these rock formations is composed of lava flows and tuff [[breccia]]s, and studies of these formations has revealed how volcanic activity at Ruapehu has developed over time.<ref name="Hackett1985" /><ref name="Gamble2003" /> During the Te Herenga stage of activity, magma rose quickly through the crust during eruptions. However, by 160,000 years ago a complex network of magma [[Dike (geology)#Magmatic dikes|dikes]] and [[Sill (geology)|sills]] had formed in the crust under the volcano, and lava erupted since that time shows signs of extensive mixing between different magma chambers prior to eruptions.<ref name="Price2012" />

[[File:MtRuapehu 23oct2002.jpg|thumb|300px|A composite satellite image looking west across Ruapehu, with the older eroded volcano [[Hauhungatahi]] visible behind it, and the cone of [[Mount Ngauruhoe|Ngauruhoe]] visible to the right.]]
In modern times, volcanic activity has been centred on Crater Lake. There are two active vents under the lake, dubbed North Vent and Central Vent.<ref>{{cite web |title=GeoNet Volcanic Alert Bulletin RUA – 2020/08 |url=https://www.geonet.org.nz/vabs/4cVdxClElNOctJS9LaQXty |website=[[GeoNet (New Zealand)|GeoNet]] |access-date=14 November 2020 |language=en |archive-date=14 November 2020 |archive-url=https://web.archive.org/web/20201114170807/https://www.geonet.org.nz/vabs/4cVdxClElNOctJS9LaQXty |url-status=live }}</ref> Activity is characterized by cyclic heating and cooling of the lake over periods of 6–12 months. Each heating cycle is marked by increased seismic activity under the crater and is accompanied by increased emission of [[volcanic gas]]es, indicating that the vents under Crater Lake are open to gas escape.<ref name="Werner2006">{{cite journal |last1=Werner |first1=C. |last2=Christenson |first2=B.W. |last3=Hagerty |first3=M. |last4=Britten |first4=K. |title=Variability of volcanic gas emissions during a crater lake heating cycle at Ruapehu Volcano, New Zealand |journal=Journal of Volcanology and Geothermal Research |date=June 2006 |volume=154 |issue=3–4 |pages=291–302 |doi=10.1016/j.jvolgeores.2006.03.017 |bibcode=2006JVGR..154..291W}}</ref><ref name="Kilgour2010" /> Evidence suggests that an open-vent system such as this has been in place throughout Ruapehu's 250,000 year history. This prevents build-up of pressure and results in relatively small, frequent eruptions (every 20–30&nbsp;years on average) at Ruapehu compared to other andesitic volcanoes around the world.<ref name="Gamble2003" /><ref name="Werner2006" />

Crater Lake is emptied by major eruptions, such as the ones in 1945 and 1995–1996, but refills after eruptions subside, fed by melting snow and vented steam.<ref name="Kilgour2010" /> In historic times, major eruptions have deposited a [[tephra]] dam across the lake's outlet, preventing lake overflow into the [[Whangaehu River|Whangaehu]] valley. The dam collapses after several years causing a large lahar down the valley. The tephra dam created by the 1945 eruptions collapsed on 24&nbsp;December&nbsp;1953, sending a lahar down the Whangaehu River and causing the [[Tangiwai disaster]]. 151 people died when the lahar swept away the Tangiwai railway bridge just before an express train crossed it. Another dam was deposited by the 1995–1996 eruptions, which collapsed on 18&nbsp;March&nbsp;2007. A warning system, the [[Eastern Ruapehu Lahar Alarm and Warning System]] (ERLAWS) system began operation on the mountain in 2002<ref name=":0" /> to detect such a collapse and alert the relevant authorities. The ERLAWS system detected the 2007 lahar, and roads were closed and railway traffic stopped until the lahar had subsided.<ref name="Keys 2008" />

===Early eruptive history===

The earliest known volcanic activity in Tongariro National Park was approximately 933,000 ± 46,000 years ago at [[Hauhungatahi]], northwest of Ruapehu.<ref name="RuapehuTongariro2021Review">{{Cite journal |last1=Leonard |first1=Graham S. |last2=Cole |first2=Rosie P. |last3=Christenson |first3=Bruce W. |last4=Conway |first4=Chris E. |last5=Cronin |first5=Shane J. |last6=Gamble |first6=John A. |last7=Hurst |first7=Tony |last8=Kennedy |first8=Ben M. |last9=Miller |first9=Craig A. |last10=Procter |first10=Jonathan N. |last11=Pure |first11=Leo R. |last12=Townsend |first12=Dougal B. |last13=White |first13=James D. L. |last14=Wilson |first14=Colin J. N. |date=2021-05-02 |title=Ruapehu and Tongariro stratovolcanoes: a review of current understanding |url=https://www.tandfonline.com/doi/full/10.1080/00288306.2021.1909080 |journal=New Zealand Journal of Geology and Geophysics |language=en |volume=64 |pages=389–420 |doi=10.1080/00288306.2021.1909080 |issue=2–3 |bibcode=2021NZJGG..64..389L |s2cid=235502116 |access-date=5 June 2022 |archive-date=5 June 2022 |archive-url=https://web.archive.org/web/20220605141941/https://www.tandfonline.com/doi/full/10.1080/00288306.2021.1909080 |url-status=live |hdl=10468/11258 |hdl-access=free }}</ref> Subsequently, andesitic [[Clastic rock#Igneous clastic rocks|clasts]] found 100&nbsp;km southwest of Ruapehu, near [[Whanganui]], demonstrate that volcanism was likely present in the Ruapehu area 340,000 years ago.<ref name="Gamble2003" /> However, the oldest rocks on Ruapehu itself are approximately 250,000 years old.<ref name="Gamble2003" /> Eruptions during this period are believed to have built a steep [[volcanic cone]] around a central crater, which would have been located somewhere near the present-day upper Pinnacle Ridge.<ref name="Hackett1985" /> Cone-building eruptions ceased about 180,000 years ago, and the cone began to be eroded away by glacial action. Rock formations that date to this period are collectively named the Te Herenga Formation, and today these formations be seen at Pinnacle Ridge, Te Herenga Ridge, and Whakapapanui Valley, all on the northwestern slopes of Ruapehu.<ref name="Hackett1985" /><ref name="Gamble2003" />

Approximately 160,000 years ago, cone-building eruptions began again, this time from a crater that is thought to have lain northwest of present-day Mitre Peak (Ringatoto)—southeast of the original Te Herenga vent.<ref name="Hackett1985" /> Eruptions continued until approximately 115,000 years ago, and the lava erupted during this period is known as the Wahianoa Formation. This formation has also been heavily eroded by glacial activity, and it now forms the southeastern flanks of modern Ruapehu.<ref name="Hackett1985" /><ref name="Price2012" /> The formation consists of lava flows and tuff [[breccia]]s.<ref name="Waight1999">{{cite journal |last1=Waight |first1=T. E. |last2=Price |first2=R. C. |last3=Stewart |first3=R. B. |last4=Smith |first4=I. E. M. |last5=Gamble |first5=J. |title=Stratigraphy and geochemistry of the Turoa area, with implications for andesite petrogenesis at Mt Ruapehu, Taupo Volcanic Zone, New Zealand |journal=New Zealand Journal of Geology and Geophysics |date=December 1999 |volume=42 |issue=4 |pages=513–532 |doi=10.1080/00288306.1999.9514858|doi-access=free|bibcode=1999NZJGG..42..513W }}</ref>

Beginning approximately 55,000 years ago, a third phase of cone-building eruptions began, creating the Mangawhero Formation. This formation was erupted onto the eroded Wahianoa Formation in two phases: the first occurring 55,000–45,000 years ago and the second 30,000–15,000 years ago.<ref name="Gamble2003" /> Multiple summit craters were active during this period, all lying between Tahurangi and the northern summit plateau. [[Parasitic cone|Parasitic eruptions]] also occurred at Pukeonake, a scoria cone to the north-west of Ruapehu and at several isolated craters near [[Ohakune]]. The Mangawhero Formation can be found over most of modern Ruapehu, and it forms most of the mountain's high peaks as well as the [[Turoa|Turoa skifield]].<ref name="Hackett1985" /><ref name="Waight1999" />

===Holocene activity===
<div style="float:right;">
<!--Source of most of data is Marija Voloschina 2020 and Bonadonna 2005. The number of tephra layers of the 1995-96 eruption is 2, Nagakawa 1999. Donoghue 1997. The Tn notation for Tephra layer is Voloschina 2020a and b, the Tfn notation is Donoghue 1997
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<small>Medium size [[tephra]] eruptions of Ruapehu since the 232 CE<br />Taupo eruption<ref name="Voloschina2020a"/><ref name="Bonadonna2005">{{cite journal|last1=Bonadonna |first1= C.|last2= Houghton |first2= B.F. |title=Total grain-size distribution and volume of tephra-fall deposits| journal=Bulletin of Volcanology|volume= 67|pages= 441–456 |year=2005 |issue= 5|doi=10.1007/s00445-004-0386-2|bibcode= 2005BVol...67..441B|s2cid= 53386767}}</ref><ref name="Nagakawa 1999"/><ref name="Donoghue 1997">{{cite journal|last1=C|first1= S. L.|last2= Neall |first2= V. E. |last3=Palmer |first3= A. S. |last4=Stewart |first4= R. B. |year=1997 |title= The volcanic history of Ruapehu during the past 2 millenia based on the record of Tufa Trig tephras| journal= Bulletin of Volcanology |volume= 59 |issue= 2|pages=136–146 |doi=10.1007/s004450050181|bibcode= 1997BVol...59..136D|s2cid= 128492446}}</ref></small>
</div>

Lava flows that have been erupted from Ruapehu since the [[Last Glacial Maximum|last glacial maximum]] are called the Whakapapa Formation. These flows all erupted between 15,000 and 2,000 years ago from a number of different craters on the summit of Ruapehu as well as from craters on the northern and southern flanks of the mountain.<ref name="Hackett1985" /><ref name="Price2012" /> Rangataua on the southern flanks had a large lava flow between 15,000 and 10,000 years ago.<ref>{{cite journal|last1 =Doll|first1 =P.|last2 =Eaves|first2 =S.R.|last3 =Kennedy|first3 =B.M.|last4 =Blard|first4 =P.H.|last5 =Nichols|first5 =A.R.L.|last6 =Leonard|first6 =G.S.|last7 =Townsend|first7 =D.B.|last8 =Cole|first8 =J.W.|last9 = Conway|first9 =C.E.|last10 =Baldwin|first10 =S.|last11 =Fénisse|first11 =G.|year =2024|title =Cosmogenic 3 He chronology of postglacial lava flows at Mt Ruapehu, Aotearoa/New Zealand|journal =Geochronology|volume = 6|issue =3|pages = 365–395|doi =10.5194/gchron-6-365-2024|doi-access=free|bibcode =2024GeChr...6..365D}}</ref>

Approximately 10,000 years ago, a series of major eruptions occurred, not just on Ruapehu, but also at the [[Tama Lakes]] between Ruapehu and [[Tongariro]] volcanoes. This period of intense eruptions is called the Pahoka-Mangamate event and is thought to have lasted between 200 and 400 years.<ref name="Nairn 1998">{{cite journal |last1=Nairn |first1=Ian A |last2=Kobayashi |first2=Tetsuo |last3=Nakagawa |first3=Mitsuhiro |title=The ~10 ka multiple vent pyroclastic eruption sequence at Tongariro Volcanic Centre, Taupo Volcanic Zone, New Zealand |journal=Journal of Volcanology and Geothermal Research |date=November 1998 |volume=86 |issue=1–4 |pages=19–44 |doi=10.1016/S0377-0273(98)00085-7}}</ref> On Ruapehu, lava was erupted from Saddle Cone—a flank crater on the northern slopes—and from another crater on the southern slopes. This southern crater erupted three times, and lava flows from this crater travelled nearly 14&nbsp;km to the south.<ref name="Hackett1985" /><ref name="Price2012" />

There is evidence that a [[sector collapse]] on the northwestern slopes about 9,400 years ago formed the amphitheatre that now comprises the [[Whakapapa skifield]] and left an extensive avalanche deposit on the northwestern ring plain that can still be seen today.<ref name="Price2012" /> Since then eruptions have been an order of magnitude lower in intensity and volume.<ref name="Pardo2012">{{cite journal |last1=Pardo |first1=Natalia |last2=Cronin |first2=Shane J. |last3=Palmer |first3=Alan S. |last4=Németh |first4=Karoly |title=Reconstructing the largest explosive eruptions of Mt. Ruapehu, New Zealand: lithostratigraphic tools to understand subplinian–plinian eruptions at andesitic volcanoes |journal=Bulletin of Volcanology |date=2011-10-27 |volume=74 |issue=3 |pages=617–640 |doi=10.1007/s00445-011-0555-z |s2cid=129769124}}</ref> Accordingly, most of the {{cvt|150|km3|cumi}} cone and {{cvt|150|km3|cumi}} ring-plain is older than 10,000 years.<ref name="RuapehuTongariro2021Review" /> Eruptions between 10,000 and 2,500 years ago generated lava flows that all flowed into the Whakapapa amphitheatre and created the slopes of the modern skifield.<ref name="Price2012" />

For the past 2,000 years, activity at Ruapehu has been largely focused through a crater lake at the summit.<ref name="Gamble2003" /> Eruptive activity has typically consisted of relatively small but explosive [[phreatomagmatic eruption]]s occurring every few decades and lasting several months each.<ref name="Gamble2003" /><ref name="Price2012" /> The eruptive record is only well understood from [[tephra]] deposits before 1950 for 1718 years from the Taupō [[Hatepe eruption]] in 232 CE. Over this period there are 30 assigned tephra units by composition studies to Ruapehu and in the 370 years before 1950 the mean time between these months of eruptive period was 40 years.<ref name="Voloschina2020a">{{cite thesis|first1=Marija |last1=Voloschina |title=Eruption dynamics and frequency-magnitude relationships of explosive eruptions at Mt. Ruapehu, New Zealand over the past 1800 years : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Science at Massey University, Palmerston North, New Zealand|url=https://mro.massey.ac.nz/handle/10179/16439 |year =2000 |access-date=2023-04-16}}</ref><ref name="Voloschina2020b">{{cite journal|first1=Marija |last1=Voloschina |first2=Gert |last2=Lube |first3=Jonathan |last3=Procter |first4= Anja |last4=Moebis |first5= Christian |last5=Timm |title=Lithosedimentological and tephrostratigraphical characterisation of small-volume, low-intensity eruptions: The 1800 years Tufa Trig Formation, Mt. Ruapehu (New Zealand) |journal=Journal of Volcanology and Geothermal Research |volume=402 |year=2020 |issn=0377-0273 |doi=10.1016/j.jvolgeores.2020.106987 |bibcode=2020JVGR..40206987V |s2cid=224945660 |url=https://www.sciencedirect.com/science/article/pii/S0377027320301475 }}</ref>

In recorded history, these eruptions have occurred about 50&nbsp;years apart, in 1895 ([[lahar]]),<ref name="Blot1979">{{cite journal|first1=Claude |last1=Blot |year=1979 |title= Deep earthquakes that were precursors of the eruption of Ruapehu, New Zealand on 22 June 1969 |journal= New Zealand Journal of Geology and Geophysics| volume=22 |issue=3 |pages= 413–414 |doi=10.1080/00288306.1979.10424110 |bibcode=1979NZJGG..22..413B }}</ref> 1945 and 1995–1996.<ref name="GNS" /> Minor [[phreatic eruption|phreatic]] or hydrothermal eruptions occur every few years on average, with notable minor eruptions occurring in 1969, 1975, and 2007.<ref name="Kilgour2010" /><ref name="Nairn 1979" /> More than 600 eruptive events of various sizes have been documented since 1830.<ref name="Scott 2013" />

===1945 eruptions and 1953 lahar===

Ruapehu entered an eruptive phase in March 1945 after several weeks of volcanic tremors. The first indication of an eruption was reported on 8 March, with ashfall seen on the eastern slopes.<ref name="Johnston1997">{{cite book |last1=Johnston |first1=D. M. |title=A Chronology of the 1945 eruption of Ruapehu volcano, New Zealand |date=1997 |publisher=Institute of Geological and Nuclear Sciences science report (97/2)}}</ref> A [[lava dome]] was observed in Crater Lake on 19&nbsp;March but was destroyed in a series of explosive eruptions over the following week. A second, larger lava dome appeared in May, which continued to grow over the following months and had emptied Crater Lake of water by July.<ref name="Johnston1997" />

Eruptions increased from August through November. A particularly powerful eruption in the early hours of 21 August was heard in Hawkes Bay and the [[Tararua District]], loud enough to awaken people from sleep and cause alarm.<ref name="Johnston1997" /> Eruptions began declining in December and had ended by January.

The eruptions dispersed ash across most of the North Island, and [[eruption column]]s could be seen from as far afield as [[Palmerston North]], [[Whanganui]], and [[Hawkes Bay]]. Ash caused disruption to several North Island communities, entering houses, causing eye and throat irritation, and damaging paintwork on cars. Crop damage was reported in Ohakune, and the water supply at [[Taumarunui]] was disrupted.<ref name="Johnston1997" /><ref name="TeAra">{{cite web |title=Ruapehu 1861 – 1945 |url=https://teara.govt.nz/en/historic-volcanic-activity/page-4 |website=Te Ara Encyclopedia of New Zealand |access-date=29 July 2020 |archive-date=14 August 2020 |archive-url=https://web.archive.org/web/20200814034114/https://teara.govt.nz/en/historic-volcanic-activity/page-4 |url-status=live }}</ref>

After eruptions subsided in late December, Crater Lake slowly began refilling, with a "boiling lake" already filling the bottom of the crater by mid-January.<ref name="Johnston1997" /> A tephra dam had formed at the lake's normal outlet during the eruptions, which eventually collapsed on 24&nbsp;December&nbsp;1953 causing a lahar that led to the [[Tangiwai disaster]] with the loss of 151 lives when the Tangiwai railway bridge across the Whangaehu River collapsed while the lahar was in full flood, just before an express train crossed it.

===1969 and 1975 eruptions===

Ruapehu saw a period of heightened activity between 1966 and 1982, with multiple small eruptions occurring in Crater Lake and two larger eruptions in 1969 and 1975, which ejected rocks across the summit region and produced significant lahars.<ref name="Scott 2013" />

The eruption in 1969 occurred in the early hours of 22 June. It was a moderate phreatic eruption, which blasted rocks up to 1&nbsp;km northwest of the crater and sent lahars down several valleys. The Whakapapa skifield was left covered in mud. This was the largest eruption since 1945.<ref name="Healy 1978">{{cite journal |last1=Healy |first1=J. |last2=Lloyd |first2=E. F. |last3=Rishworth |first3=D. E. H. |last4=Wood |first4=C. P. |last5=Glover |first5=R. B. |last6=Dibble |first6=R. R. |title=The eruption of Ruapehu New Zealand on 22 June 1969 |journal=DSIR Bulletin |date=1978 |volume=224 |page=79}}</ref>

A larger phreatic eruption occurred at 3:59&nbsp;a.m. on 24 April 1975, blasting rocks up to 1.6&nbsp;km northwest of the crater, against the wind, and depositing ash more than 100&nbsp;km to the southeast. Nine minutes of seismic activity preceded the eruption, but crater dilation had been measured two weeks earlier. Nearly half of the water in Crater Lake was erupted into the air, which subsequently rained down onto the summit, generating lahars down several river valleys. Lahars which travelled down the [[Whakapapa River|Whakapapa]] and [[Manganui o te Ao River|Manganui o te Ao]] rivers entered the [[Whanganui River]] and poisoned it, which affected much of the aquatic life downstream.<ref name="Nairn 1979" /><ref name="Paterson 1976">{{cite journal |last1=Paterson |first1=B. R. |last2=Page |first2=C. E. |last3=Cudby |first3=E. J. |title=The effects of lahars from the 1975 April Mt. Ruapehu eruption and the threat of future eruptions on Tongariro power development |journal=New Zealand Geological Survey, Engineering Geology Section |date=1976}}</ref> Additionally, the lahars damaged ski installations on the Whakapapa ski field, several bridges and hydroelectric tunnel intakes, but no loss of life occurred.<ref name="Nairn 1979">{{cite journal |last1=Nairn |first1=I. A. |last2=Wood |first2=C. P. |last3=Hewson |first3=C. A. Y. |title=Phreatic eruptions of Ruapehu: April 1975 |journal=New Zealand Journal of Geology and Geophysics |date=March 1979 |volume=22 |issue=2 |pages=155–170 |doi=10.1080/00288306.1979.10424215|doi-access=free |bibcode=1979NZJGG..22..155N }}</ref><ref name="Montgomery 1993">{{cite book |last1=Montgomery |first1=R. |last2=Keys |first2=H. |title=Volcanic hazard management in Tongariro National Park |date=1993 |publisher=Department of Conservation |isbn=0-478-01523-2}}</ref>

Three days later, on the morning of 27 April, Ruapehu erupted again. A series of five eruptions occurred between 7:10&nbsp;a.m. and 10:18&nbsp;a.m., sending surges of mud, rocks, and ash northwards across the summit plateau and producing eruption columns up to 500&nbsp;m high.<ref name="Nairn 1979" />

The 1975 eruptions deepened Crater Lake from 55 to 60&nbsp;m to more than 90&nbsp;m.<ref name="Nairn 1979" />

===1995–1996 eruptions===

Earthquake swarms to the west of Ruapehu between November 1994 and September 1995 marked the beginning of renewed heightened activity at the volcano. Bursts of earthquake activity immediately preceded rapid rises in the temperature of Crater Lake, with the surface temperature reaching 51.4&nbsp;°C in January 1995—one of the highest temperatures recorded in 30 years and about 10&nbsp;°C higher than its usual peak temperature.<ref name="Hurst 1999">{{cite journal |last1=Hurst |first1=A.W |last2=McGinty |first2=P.J |title=Earthquake swarms to the west of Mt Ruapehu preceding its 1995 eruption |journal=Journal of Volcanology and Geothermal Research |date=May 1999 |volume=90 |issue=1–2 |pages=19–28 |doi=10.1016/S0377-0273(99)00019-0 |bibcode=1999JVGR...90...19H}}</ref><ref name="GNS_2005">{{cite web |title=Changes made since Ruapehu's eruptions / Media Releases and News / News and Events / Home - GNS Science |url=https://www.gns.cri.nz/Home/News-and-Events/Media-Releases-and-News/Changes-made-since-Ruapehu-s-eruptions |website=www.gns.cri.nz |access-date=14 November 2020 |archive-date=15 November 2020 |archive-url=https://web.archive.org/web/20201115113715/https://www.gns.cri.nz/Home/News-and-Events/Media-Releases-and-News/Changes-made-since-Ruapehu-s-eruptions |url-status=live }}</ref> A minor eruption occurred on 26 April, which sent waves against the walls of the crater and damaged some monitoring equipment there. A second eruption on 29 June destroyed the equipment and produced a lahar. Chemical analysis showed that magma was interacting with water under the lake.<ref name="Scott 2013">{{cite book |last1=Scott |first1=B. J. |title=A revised catalogue of Ruapehu volcano eruptive activity: 1830–2012 |date=2013 |publisher=GNS Science report (2013/45)}}</ref><ref name="Hurst 1999" />

The first significant eruption took place at 8:05&nbsp;a.m. on 18 September 1995, raining tephra onto the summit region and sending lahars down the mountain. On 23 September, an even larger eruption blasted rocks up to 1.5&nbsp;km from the crater, sent lahars down three valleys, and generated an eruption column 12&nbsp;km high.<ref name="GNS_2005" /> Phreatomagmatic eruptions occurred through the rest of the month and throughout October, with some eruptions continuing for hours at a time. Ash fell up to 250&nbsp;km downwind. Explosive eruptions on 11 October emptied Crater Lake of water.<ref name="Scott 2013" /><ref name="Nagakawa 1999">{{cite journal |last1=Nakagawa |first1=M. |last2=Wada |first2=K. |last3=Thordarson |first3=T. |last4=Wood |first4=C. P. |last5=Gamble |first5=J. A. |title=Petrologic investigations of the 1995 and 1996 eruptions of Ruapehu volcano, New Zealand: formation of discrete and small magma pockets and their intermittent discharge |journal=Bulletin of Volcanology |date=11 July 1999 |volume=61 |issue=1–2 |pages=15–31 |doi=10.1007/s004450050259 |bibcode=1999BVol...61...15N |s2cid=128710842}}</ref>

[[File:Crater Lake, Ruapehu, New Zealand 13.JPG|thumb|300px|Crater Lake and Tahurangi, the highest peak (top right) in 2016. The 1996 tephra dam is the bluish dark area at lake edge directly below Tahurangi.]]
Following this, activity died off until 15 June 1996 when renewed seismic activity was recorded. This was followed by eruptions on 17 and 18 June which once again emptied the partially refilled Crater Lake of water. [[Types of volcanic eruptions#Strombolian|Strombolian]] eruptions occurred on 27 June and throughout July and August, producing eruption columns more than 10&nbsp;km high and shooting rocks 1.4&nbsp;km from the crater.<ref name="Scott 2013" /><ref name="Nagakawa 1999" />

These eruptions produced more than 7 million tonnes of ash, which contaminated water supplies, destroyed crops, and led to the deaths of livestock.<ref>{{cite web |title=Remembering Ruapehu – ten years on |url=https://www.nzherald.co.nz/technology/remembering-ruapehu-ten-years-on/EZ7CRNK6SX344A5TNABCMPTW7A/ |work=[[The New Zealand Herald]] |access-date=14 November 2020 |language=en-NZ |archive-date=20 November 2020 |archive-url=https://web.archive.org/web/20201120015714/https://www.nzherald.co.nz/technology/remembering-ruapehu-ten-years-on/EZ7CRNK6SX344A5TNABCMPTW7A/ |url-status=live }}</ref> Ash in the Tongariro River also damaged the intake turbines at the Rangipo power station,<ref name="waikatoregion">{{cite web |title=Mount Ruapehu erupts |url=https://www.waikatoregion.govt.nz/Services/Regional-services/Regional-hazards-and-emergency-management/Volcanic-activity/Mount-Ruapehu-erupts/ |website=www.waikatoregion.govt.nz |access-date=14 November 2020 |archive-date=28 November 2020 |archive-url=https://web.archive.org/web/20201128090204/https://www.waikatoregion.govt.nz/Services/Regional-services/Regional-hazards-and-emergency-management/Volcanic-activity/Mount-Ruapehu-erupts/ |url-status=live }}</ref> and ash clouds caused airport closures as far away as [[Auckland Airport|Auckland]] and [[Wellington Airport|Wellington]].<ref name="teara6">{{cite web |last1=Taonga |first1=New Zealand Ministry for Culture and Heritage Te Manatu |title=Ruapehu since 1945 |url=https://teara.govt.nz/en/historic-volcanic-activity/page-6 |website=[[Te Ara: The Encyclopedia of New Zealand]] |access-date=16 November 2020 |language=en |archive-date=12 November 2020 |archive-url=https://web.archive.org/web/20201112003122/https://teara.govt.nz/en/historic-volcanic-activity/page-6 |url-status=live }}</ref> The eruptions also caused closures to the three ski fields on the mountain, costing the region an estimated $100 million in lost revenue.<ref name="teara6" /> During the 1995–1996 summer period between the eruptions, Ruapehu Alpine Lifts ran its chairlifts up the mountain and organised guided tours to within 500 metres of the crater. Hundreds of tourists visited, even though the volcano was still emitting steam and toxic sulphur gas and the Department of Conservation was warning that further eruptions were possible.<ref>{{cite news |last1=Bell |first1=Stephen |date=1 January 1996 |title= Steaming crater big drawcard |work=Dominion |id={{ProQuest|314851359}}}}</ref>

Both the 1995 and 1996 eruptions were filmed and streamed to the internet via a custom-built 'volcano-cam', possibly the first such camera in the world. The website hosting the feed during the 1996 eruption received up to 4000 hits an hour.<ref>{{Cite web |last=Stokes |first=Don |date=18 June 1996 |title=An Internet Camera: VolcanoCam |url=https://www.don.nz.net/ruapehu/index.html |url-status=live |archive-url=https://web.archive.org/web/20241107054754/https://www.don.nz.net/ruapehu/index.html |archive-date=7 November 2024 |access-date=2024-11-07 |website=www.don.nz.net}}</ref><ref>{{Cite web |title=1995: Don Stokes |url=https://downtothewire.co.nz/video-archive/view/don-stokes-1995/index.html |access-date=2024-11-07 |website=downtothewire.co.nz}}</ref><ref>{{cite news |date=1 July 1996 |title=Natural wonders ready for download |work=[[Waikato Times]] |id={{ProQuest|313848593}}}}</ref>

=== Lahar danger ===
After the 1996 eruption it was recognised that a catastrophic lahar could again occur when Crater Lake burst the volcanic ash dam blocking the lake outlet as it did in 1953. In 1997, the government proposed digging a trench through the blockage in the wall of Crater Lake, but this was plan was opposed by conservation groups and Māori. A [[Ngāti Rangi]] spokesman noted that the mountain was sacred, and said: "as far as we are concerned if these things [lahars] do happen well we step aside. Let them go past".<ref>{{cite news |title=Ruapehu Maori oppose lahar plan |date= 23 October 1998 |work= Evening Post |id={{ProQuest|314621185}}}}</ref>
Other plans considered were building a stop bank on the Desert Road, or creating an early-warning system, which would be much more expensive than digging a trench.<ref>{{cite news |last1=Watkins |first1=Tracy |date= 8 June 2001 |title= DOC drops plans to cut trench on Ruapehu |work= Daily News |id={{ProQuest|315136006}}}}</ref><ref>{{cite news |last1= Wallis | first1 =Anna |date= 13 June 2001 |title= Rift opens over lahar warning system | work=The Evening Standard |id={{ProQuest|314298532}}}}</ref> In 2001, the [[Eastern Ruapehu Lahar Alarm and Warning System]] (ERLAWS) system was installed on the mountain to detect a crater wall collapse and alert the relevant authorities. It began operating in 2002.<ref name=":0">{{cite news |date=10 July 2002 |title=Teething problems delay Ruapehu lahar warning system |work=Dominion Post |id={{ProQuest|337940728}} |quote=The alarm system had been installed in February and successfully commissioned.[...] However, the alarm system would not be 100 per cent till Genesis Power's communications system, which is being used to relay signals from the alarm sensors to the Tokaanu Power Station, was fully operational. The system is being upgraded and is expected to be completed within a month.}}</ref> The lake gradually filled with snowmelt and had reached the level of the hard rock rim by January 2005. The lahar finally occurred on 18 March 2007 (see below).

===2006 and 2007 activity===

Ruapehu erupted at 10:24&nbsp;p.m. on 4 October 2006. The small eruption was marked by a magnitude 2.9 volcanic earthquake and sent waves 4–{{cvt|5|m|ft}} tall crashing into the wall of the crater. No ash was erupted into the atmosphere, and the eruption is presumed to have occurred entirely underwater.<ref name="Mordret2010">{{cite journal |last1=Mordret |first1=A. |last2=Jolly |first2=A.D. |last3=Duputel |first3=Z. |last4=Fournier |first4=N. |title=Monitoring of phreatic eruptions using Interferometry on Retrieved Cross-Correlation Function from Ambient Seismic Noise: Results from Mt. Ruapehu, New Zealand |journal=Journal of Volcanology and Geothermal Research |date=March 2010 |volume=191 |issue=1–2 |pages=46–59 |doi=10.1016/j.jvolgeores.2010.01.010 |bibcode=2010JVGR..191...46M}}</ref>

[[File:Ruapehu ast 2007084 lrg cropped.jpg|thumb|300px|Fresh lahar channels scar Ruapehu's eastern slopes, 2007.]]
At 11:22&nbsp;a.m. 18 March 2007, the tephra dam which had been holding back Crater Lake burst, sending a [[lahar]] down the mountain. An estimated 1.9–3.8&nbsp;million cubic metres of mud, rock, and water travelled down the Whangaehu river.<ref name="Keys 2008" /> ERLAWS activated, sending an alarm to [[pager]]s at 11:25&nbsp;a.m. and automatically activating warning lights and barrier arms to close roads and stop trains. There was no serious damage and no injuries. A toilet block at the Tangiwai memorial was destroyed, but the memorial had already been closed due to the lahar threat.<ref name="Keys 2008" /> One family was trapped for around 24&nbsp;hours after the lahar swept away the access route to their home.<ref name="NZ_Herald_10429432">{{cite news |url=http://www.nzherald.co.nz/section/story.cfm?c_id=1&objectid=10429432 |title=Photos: Lahar could have been much worse |date=18 March 2007 |agency=[[NZPA]] |work=[[The New Zealand Herald]]   |access-date=26 October 2011}}</ref>

At 8:16&nbsp;p.m. on 25 September 2007, [[Harmonic tremor|volcanic tremor]] was detected underneath Ruapehu, which was followed at 8:26&nbsp;p.m. by an explosive [[Types of volcanic eruptions#Surtseyan|surtseyan]] eruption.<ref name="Kilgour2010">{{cite journal |last1=Kilgour |first1=G. |last2=V. Manville |last3=Pasqua |first3=F. Della |last4=A. Graettinger |last5=Hodgson |first5=K.A. |last6=Jolly |first6=G.E. |title=The 25 September 2007 eruption of Mount Ruapehu, New Zealand: Directed ballistics, surtseyan jets, and ice-slurry lahars |journal=Journal of Volcanology and Geothermal Research |date=March 2010 |volume=191 |issue=1–2 |pages=1–14 |doi=10.1016/j.jvolgeores.2009.10.015 |bibcode=2010JVGR..191....1K}}</ref> The explosive phase of the eruption lasted for less than a minute and blasted ash, mud, and rocks northward, reaching to about 2&nbsp;km from Crater Lake.<ref name="Kilgour2010" /><ref name="Jolly2010">{{cite journal |last1=Jolly |first1=A.D. |last2=Sherburn |first2=S. |last3=Jousset |first3=P. |last4=Kilgour |first4=G. |title=Eruption source processes derived from seismic and acoustic observations of the 25 September 2007 Ruapehu eruption—North Island, New Zealand |journal=Journal of Volcanology and Geothermal Research |date=March 2010 |volume=191 |issue=1–2 |pages=33–45 |doi=10.1016/j.jvolgeores.2010.01.009 |bibcode=2010JVGR..191...33J}}</ref> Two climbers were caught in the eruption at Dome Shelter, an alpine hut approximately 600&nbsp;m from the crater, when the hut was struck by the surge.<ref name="Kilgour2010" /> The climbers nearly drowned before the hut floor gave way and the water drained into the basement seismometer vault. One of them, a 22-year-old primary school teacher, had a leg pinned and crushed by a boulder as the water subsided. A rescue operation was mounted after his companion, who was unable to free him, went down the mountain for help.<ref name="Kilgour2010" /><ref name="Stuff2007">{{cite web |last1=Mussen |first1=D |title=Mt Ruapehu eruption survivor William Pike inspires a generation of Kiwi 'Pike-lets' |url=https://www.stuff.co.nz/national/education/76103430/mt-ruapehu-eruption-survivor-william-pike-inspires-a-generation-of-kiwi-pike-lets |website=[[Stuff (website)|Stuff]] |date=22 January 2016 |access-date=1 August 2020 |archive-date=31 October 2022 |archive-url=https://web.archive.org/web/20221031153315/https://www.stuff.co.nz/national/education/76103430/mt-ruapehu-eruption-survivor-william-pike-inspires-a-generation-of-kiwi-pike-lets |url-status=live }}</ref>

The eruption initiated lahars down the Whangaehu valley and the [[Whakapapa skifield]].<ref name="GN07">{{cite web |title=GeoNet Volcanic Alert Bulletin RUA-2007/03 |url=https://www.geonet.org.nz/vabs/2KAPkCsfpm2ka6Sc4E0u62 |website=[[GeoNet (New Zealand)|GeoNet]] |access-date=1 August 2020 |language=en |archive-date=24 October 2021 |archive-url=https://web.archive.org/web/20211024030510/https://www.geonet.org.nz/vabs/2KAPkCsfpm2ka6Sc4E0u62 |url-status=live }}</ref> ERLAWS detected the lahars in the Whangaehu valley.<ref name="Keys 2008">{{cite journal |last1=Keys |first1=H. |last2=Green |first2=P. |title=Ruapehu Lahar New Zealand 18 March 2007: Lessons for Hazard Assessment and Risk Mitigation 1995–2007 |journal=Journal of Disaster Research |date=August 2008 |volume=3 |issue=4 |pages=284–296 |doi=10.20965/jdr.2008.p0284|doi-access=free }}</ref><ref>{{cite web |title=GeoNet Volcanic Alert Bulletin RUA-2007/02 |url=https://www.geonet.org.nz/vabs/6A6Ty6YEvYMEMugma6ikYW |website=[[GeoNet (New Zealand)|GeoNet]] |access-date=3 August 2020 |language=en |archive-date=24 October 2021 |archive-url=https://web.archive.org/web/20211024040658/https://www.geonet.org.nz/vabs/6A6Ty6YEvYMEMugma6ikYW |url-status=live }}</ref> A snow groomer on the Whakapapa skifield narrowly avoided being caught in the lahar there.<ref name="Kilgour2010" />

===Current activity and future hazards===

Only one eruptive event has been recorded at Ruapehu since the 2007 eruption—a minor event on 13 July 2009 when a small volcanic earthquake beneath Crater Lake caused the lake water level to rise 15&nbsp;cm and triggered a snow slurry lahar in the upper Whangaehu valley.<ref name="Scott 2013" /><ref name="Wunderman 2011">{{cite journal |last1=Wunderman |first1=R. |title=Report on Ruapehu (New Zealand) |journal=Bulletin of the Global Volcanism Network |date=2011 |volume=36 |issue=7 |doi=10.5479/si.GVP.BGVN201107-241100}}</ref> Since then, Crater Lake has continued its regular cycle of heating and increased gas emissions, although with periods of sustained high temperatures that occurred in 2011, 2016 and 2019.<ref name="Wunderman 2011" /><ref name="NZHerald 2019">{{cite news |title=Scientists monitoring Ruapehu as crater lake heats up |url=https://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=12221130 |access-date=27 August 2020 |work=[[The New Zealand Herald]] |date=10 April 2019 |language=en-NZ |archive-date=18 July 2020 |archive-url=https://web.archive.org/web/20200718030405/https://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=12221130 |url-status=live }}</ref><ref name="GeoNet 2019-04-10">{{cite web |url=https://www.geonet.org.nz/vabs/4kjCTeqLSLY8mwsONDFDlq |title=Te Wai ā-moe, Mt Ruapehu: Increases in lake temperature and seismic activity |author=Agnes Mazot |date=10 April 2019 |access-date=10 April 2019 |archive-date=10 April 2019 |archive-url=https://web.archive.org/web/20190410123129/https://www.geonet.org.nz/vabs/4kjCTeqLSLY8mwsONDFDlq |url-status=live }}</ref>

Eruptions at Ruapehu are expected to continue much as they have for the past 2,000 years, with frequent minor eruptions and more significant events every 20–30 years, although the possibility of larger events like the Pahoka-Mangamate event cannot be ruled out.<ref name="Gamble2003" /> The previous activity trend until 10,000 years ago was about {{cvt|7.5|km3|cumi}} erupted each 10,000 years.<ref name="RuapehuTongariro2021Review" /> Minor eruptions, such as the one in 2007, especially if they are hydrothermal, can occur at any time without warning. However, in historic times, major eruptions such as the ones in 1995–96 have only occurred within periods of enhanced activity.<ref name="Scott 2013" /><ref name="RuapehuTongariro2021Review" />

The main recent volcanic hazard at Ruapehu is from lahars. Two major lahar paths run through the Whakapapa skifield, and in recent times, lahars have travelled through the ski field in 1969, 1975, 1995, and 2007.<ref name="Kilgour2010" /><ref name="Keys 2007">{{cite journal |last1=Keys |first1=Harry J.R. |title=Lahars of Ruapehu Volcano, New Zealand: risk mitigation |journal=Annals of Glaciology |date=2007 |volume=45 |issue=1 |pages=155–162 |doi=10.3189/172756407782282390|bibcode=2007AnGla..45..155K |doi-access=free}}</ref> An eruption warning system operates in the ski field to warn skiers in the event of another eruption.<ref name="DOCrisk">{{cite web |title=Volcanic risk in Tongariro National Park |url=https://www.doc.govt.nz/volcanicrisk |website=[[Department of Conservation (New Zealand)|Department of Conservation]] |language=en-nz |access-date=17 November 2020 |archive-date=25 October 2020 |archive-url=https://web.archive.org/web/20201025075728/https://www.doc.govt.nz/volcanicrisk |url-status=live }}</ref> Lahars also represent a significant hazard to surrounding river valleys, particularly the Whangaehu River, which is crossed by national highways, the [[North Island Main Trunk]] railway line, and electricity transmission lines.<ref name="Keys 2007" /> Large, destructive lahars have been observed in the Whangaehu River in 1862, 1895, 1953, 1975, and 2007.<ref name="Keys 2007" /><ref name="Purves 1990">{{cite thesis |type=Ph.D. |last=Purves |first=A. M. |date=1990 |title=Landscape Ecology of the Rangipo Desert |publisher=Massey University}}</ref>

[[GNS Science]] continuously monitors Ruapehu using a network of seismographs, GPS stations, microphones and webcams. Chemical analysis of the water in Crater Lake is regularly undertaken along with airborne gas measurements.<ref>{{cite web |title=Monitoring volcanic unrest / Ruapehu / New Zealand Volcanoes / Volcanoes / Science Topics / Learning / Home – GNS Science |url=https://www.gns.cri.nz/Home/Learning/Science-Topics/Volcanoes/New-Zealand-Volcanoes/Ruapehu/Monitoring-volcanic-unrest |website=www.gns.cri.nz |access-date=4 August 2020 |archive-date=13 July 2020 |archive-url=https://web.archive.org/web/20200713032247/https://www.gns.cri.nz/Home/Learning/Science-Topics/Volcanoes/New-Zealand-Volcanoes/Ruapehu/Monitoring-volcanic-unrest |url-status=live }}</ref> Live data can be viewed on the [https://www.geonet.org.nz/volcano/monitoring/ruapehu GeoNet website].