Mount Ruapehu, New Zealand

Mount Ruapehu in eruption during 1995 eruption sequence.  Image courtesy TRTWorld blog, 2016

Mount Ruapehu is a massive andesitic stratovolcano in the southern end of the Taupo Volcanic Zone on the North Island of New Zealand.  It is 2,797 m high and has a permanent ice cap.  It is the largest active volcano in New Zealand.

The volcano has three major peaks and a smallish crater lake between the peaks.  This lake refills between major eruptions and empties periodically, producing deadly lahars.  There have been over 60 lahars from Ruapehu in the last 150 years, most, though not all associated with catastrophic drainage of the lake.

Happy Valley ski lift on Mt. Ruapehu open for business, May 31, 2018.  Image courtesy Snowsbest.com

The highest mountain in the North Island ends up being permanently glaciated and is home to one of the nation’s major ski resorts.  There is so much recreation associated with Ruapehu that it is difficult to sort through the massive number of recreational hits for actual volcano information.  It is clearly a popular location.

The volcano is 23 km NE of Ohakune and 23 km SW of Lake Taupo.  It is situated in Tongariro National Park.  The surrounding region is sparsely populated, with just under 120,000 within 100 km and less than 7,000 within 30 km.

Ski trail map for Turoa Ski Resort on the flanks of Ruapehu.  Image courtesy Liftopia.com, 2019

The two commercial ski resorts are on the northern and southern flanks of the volcano.  They are the largest ski areas in New Zealand.  They are accessible by vehicle and chair lifts.

Weather on the mountain is variable with occasional violent snowstorms that have trapped people on the mountain overnight or closed the road to vehicles without chains, studs or 4-wheel drive.  Overall climate on the mountain is maritime polar or tundra.  Glaciers are relatively short, less than 2 km long.

Some scenes in the Peter Jackson Lord of the Rings trilogy were filmed on Mount Ruapehu.

There are multiple webcams on Ruapehu that mostly support the ski season.  The Mt. Ruapehu Turoa web site has output from 6 of them available.

Sept. 30, 2013 ISS astronaut photo of Ruapehu (center left), Mount Ngauruhoe (right of center) and Tongariro (far right).  Crater lake on top of Ruapehu clearly visible.  Small lake on the flank of Tongariro is also visible.  North is to the right.  Image courtesy NASA Earth Observatory

Region

Ruapehu is located at the southern end of the Taupo Volcanic Zone (TVZ).  This is the southern expression of the ongoing rift of New Zealand as it is penetrated by the Havre Trough.  It is an active rift, a marginal basin, the central portion of which has been filled to a depth of several kilometers by a variety of silicic ignimbrites, lavas and volcanic sediments.

Four large rhyolitic volcanic centers, Taupo, Maroa, Rotorau and Okataina occupy the central part of the TVZ.  Output from these systems has been truly massive, erupting over 16,400 km3 of silicic ignimbrites and lavas over 45,000 km2.  These eruptions constructed an extensive plateau which extends beyond the depression.

Map of Taupo Volcanic Zone (TVZ) showing location of major volcanic centers.  Note Ruapehu at the far SW end of the zone.  Image courtesy Kosik, et al, 2019

Intermediate volcanics are located at the northern and southern end of the TVZ.  The southern region is referred to as the Tongariro Volcanic Center and includes at least five composite volcanoes including Ruapehu.  This center has multiple faults and lineaments.  Volcanic vent locations over the last 30,000 years appear to he controlled by the location of these basement structures.  Ruapehu contains about 40% of all intermediate volcanic material in the TVZ.

Crater Lake on top of Ruapehu looking to the north.  Neighboring volcanoes from closest to farthest are Mount Ngauruhoe (conical cone), Tongariro.  Lake Taupo lies beyond the two neighboring volcanoes to the north.  Image courtesy astronoo blog, 1997

Volcano

The andesitic Ruapehu has been active for over 250,000 years.  The massif has a volume over 110 km3.  It is surrounded by a 100 km3 ring of volcanic debris including a debris avalanche deposit on its north flank.  Total erupted volume is in the vicinity of 300 km3.

There were a series of subplinian eruptions 26,000 – 10,000 years ago.  Pyroclastic flows have been infrequent and generally on the small side.  There is a single historically active vent on the summit with another five (or more) vents on the summit and flank active during the Holocene.  Recent activity from the volcano has been frequent mild to moderate explosive eruptions from the Crater Lake vent.  This vent is relatively recent, perhaps as young as 3,000 years.

Aerial view of Ruapehu summit on Sept. 26, 2007.  Shows lahar and ash coverage from previous day’s eruption.  Eruption took place from Crater Lake and ejected ash and mud covering the summit area.  Large lahar swept down the Whangaehu glacier.  Image courtesy GeoNet, 2007 via Smithsonian GVP

Frequent lahars from phreatic eruptions are a continuing hazard for the ski area on its upper flanks and to residents and infrastructure in the lower river valleys.  Lahars are caused by melting snow due to eruptions and the occasional collapse of the wall containing the crater lake which flushes its contents down the slopes of the volcano.  Over time, this has eroded a significant cut in the southern flank of the volcano.

Ruapehu was built by at least four periods of intense activity separated by periods of erosion, sector collapse and low-level volcanic activity.  These periods produced four major lava-dominated formations.  There is no evidence of voluminous pyroclastic flows or large sector collapse depressions, though a smaller recent one does exist.

The intense eruptive periods were Te Herenga, 250 – 180,000, Wahianoa, 160 – 115,000, Mangawhero, 55 – 45,000 and 30 – 20,000 and Whakapapaiti, 15 – 2,000 years ago.

Mangawhero Falls on the flanks of Ruapehu in Tongariro National Park.  Falls come off the Mangawhero Formation, with a lava flow clearly visible as the top cap that the water is flowing off.  Image courtesy NZ Department of Conservation

The oldest formation is the Te Herenga Formation.  It is generally located on the northern slopes of Ruapehu.  It includes lava flows, tuffs and intrusive bodies overlain by younger deposits.  The greatest thickness known is around 300 m.  Total volume of these deposits is estimated on the order of 65 km3.

The Wahianoa Formation is exposed on the SE slopes of the volcano.  It is composed mainly of lava flows with tuff breccias.  The thickest exposure is 220 m.  Total volume of these deposits is estimated at 45 km3.

The newer Mangawhero Formation is extensive over most of the volcano.  It is less deeply eroded than the earlier formations and contains numerous fluvial cuts.  The largest exposed thickness is 120 m with an estimated volume of 35 km3.  Most of this formation was produced by subaerial effusion of lava flows.  There is a large quantity of hyaoclastites tuff breccias associated with these flows.  Eruptions took place from several centers in the summit region.  Many of these deposits show hydrothermal alteration.

Layered lava flow and underlying tephras / debris falls in Whakapapaiti Formation.  Image courtesy Spiritual Training Group on the Stanton Memorial Loop walk, Jan. 2017

The most recent Whakapapaiti Formation includes young post-glacial lava flows, hyaoclastite breccias and pyroclastics with only minor erosion.  Eruptions of these deposits took place from at least six vents near the summit.  There were also flank eruptions that created two lava fields and minor pyroclastics on the north slope of Ruapehu.  There was a rather large lava flow and minor pyroclastics erupted from a single vent on the southern slope.  There are welded airfall tuffs and vertical dikes from fissures associated with this vent.  This formation is neither thick nor voluminous, with a maximum exposed depth of 50 m and total volume less than 3 km3.  This formation has been deeply eroded in places.

The dominant erupted rock types for each formation were basaltic andesites and andesites.  There was a bit of dacite, and a single basalt flow so far identified.  These magmas were apparently dacitic or rhyolitic melts carrying mantle-derived crystals from at least two crustal sources.  They were sourced in the mantle and crust.  The plumbing system appears to be complex with magma stored in varying time scales in numerous dispersed smaller reservoirs.

Image and schematic of the Murimotu Formation, small flank collapse from the side of Ruapehu.  Image courtesy Conway, et al, 2016

A flank collapse around 9,540 years ago created the Murimotu Formation.  It was likely triggered by intrusion of dikes into the hydrothermally weakened Te Herenga formation.  The avalanche was confined to a valley while on the volcano and spread laterally when it hit the ring plain.  It did not appear to be a hot avalanche.  The flow overtopped drainage divides below the volcano.  The initial collapse was followed by at least three lahars.  It covers over 23 km2 and moved 0.2 km2 of material erupted 20 – 11,000 years ago.  The collapse followed a major eruptive period of the volcano.  The smallish collapse amphitheater was filled by the most extensive of the Whakapapaiti lava flows

Small phreatic eruption Feb 29, 1980 out of Crater Lake on Ruapehu.  Column of ash in the center surrounded by a ring of pyroclastic surges across the water.  View is from the NW.  This explosion was part of a series of small phreatic eruptions Dec 1979 – Apr 1980.  Image courtesy Peter Otway, New Zealand Geologic Society via Smithsonian GVP

The most recent 2,000 years of activity have been frequent, low volume mostly phreatic eruptions.  This is thought to be driven by the migration of small magma batches at shallow levels in the volcanic plumbing system.

Ruapehu has an active hydrothermal system, and thought to be essentially a open vent system.  This allows significant heat transfer into Crater Lake.  There may be multiple heat pipes within the volcanic system, meaning the upflow of thermal energy is more extensive than previously believed.

Video of 1995 – 11996 eruptions

Eruptions

Ruapehu is extremely active with nearly 60 eruptions since 1900.  The majority of these are VEI 1, with those prior to 1946 VEI 2.  I suspect this variation is more a product of more people looking at the volcano over the last 70 years than the eruptive violence ramping up.  Smaller eruptions were simply missed.  All eruptions took place from the most recently active Crater Lake vent.

Among that group of eruptions were VEI 3 eruptions in 1945 and 1966.

The aftermath of the 1945 eruption was particularly deadly, as the eruption emptied the crater lake and dammed its outlet with tephra.  Eight years later, the tephra dam collapsed, sending the contents of the lake downstream in the bed of the Whangaehu River.  The lahar took out the Tangiwai railway bridge across the river immediately before the arrival of a train.  The train plunged into the ravine, killing 151.  The railroad apparently knew of weakened piers before the lahar, which finished the job.  The engineer was unable to stop the train in time, losing six passenger cars into the ravine.

March 2007 lahar bearing down on the Tangiwai railway bridge crossing the Whangaehu River below Ruapehu.  This is a rebuilt bridge that washed out the railway bridge in 1945, killing 151.  The bridge is still subject to lahars.  Image courtesy Geoff Mackey

An eruption in June 1969 began around midnight with an explosion that may have produced a small pyroclastic flow along with the eruption plume.  The eruption caused lahars and hot mud flows that damaged ski buildings on the slopes of the volcano.  It expelled enough water from the lake to lower the water level 10 – 20 m and scattered boulders within 500 m of the crater rim.  Large blocks were scattered up to a km from the crater.  Lahars down neighboring river valleys killed a variety of fish.

Phreatic eruptions took place in 1971, 1975, 1977, 1980 – 1982.  The sequence 1980 – 1982 was particularly busy with frequent phreatic explosions, plumes, earthquakes and volcanic tremor.  Activity decreased significantly in late 1982 only to resume early in 1983 and last for most of the year.  This was mostly seen in changes in the crater lake water, tremor and volcanic earthquakes.  I was not able to find any reports of phreatic eruptions in 1983.  Phreatic eruptions resumed in 1985 but did not last out the year.  1988 also had a period of phreatic eruptions that did not last the entire year.  It was punctuated by a moderate eruption in Dec. 1988 that did not produce any juvenile material.  Phreatic eruptions took place in Jan – Feb and Aug 1989, Jan – Mar 1990.

1995 – 1996 Ruapehu eruption sequence.  Note the ash-covered snow cap on the volcano.  Image courtesy Waikato Times, Aug 2015.  Photo courtesy Lloyd Homer / GNS Science

The largest recent eruption took place 1995 – 1996.  Increased activity started Mar 1994 with elevated Crater Lake temperatures and minor phreatic eruptions.  There was a series of small eruptions over the next 9 months.  These in turn caused several lahars 18 – 25 Sept 1995.  The government issued hazard warnings to clear people off the mountain.  The eruption produced a 8 – 12 km plume disrupting regional air travel.  Ash in the water damaged generation equipment in a nearby hydroelectric power station.  This eruptive sequence led to the installation of what was called the first web cam late Sept. 1995.  A second phase of the sequence began June 1996

Because this eruption drained the crater lake, the threat of a major lahar due to a dump of the refilled crater lake grew for several years.  This eventually took place March 2007, dumping an estimated 1,400,000 m3 of volcanic slurry down the Whangaehu river valley.  The lahar made it all the way to the ocean, 140 km away.  Although this dump took place during the 2006 – 2007 eruption sequence, it is unclear from the reports whether it was caused by the eruption sequence.

Breach in tephra dam of Ruapehu Crater Lake.  Photo taken 3 days after dam was breached.  Note significant erosion downstream of the 2007 breach.  Image courtesy University of Hawaii at Manoa, School of Ocean and Earth Science and Technology (SOEST), 2015

The most recent eruption was a phreatic eruption in Sept. 2007 that injured a man near the crater.  It took place without warning, sent at least two lahars down the flanks of the volcano, and was accompanied with 7 minutes of volcanic tremor.  It sent a plume 4.6 km and covered the summit area snowpack with ash and mud.  Eruptive sequence began in Oct 2006 with an earthquake and a small phreatic eruption.  Lake water temperature also increased.

The volcano emitted a steam plume Jan. 2002 from the crater lake.  The 2001 activity was accompanied with volcanic earthquakes, tremor, and a change in crater lake color.

There were warnings of volcanic unrest in 2001, 2002, 2004, 2008, 2011, 2012, 2013, 2015, 2016, 2018 and 2019.

Lake temperatures and volcanic tremor were the primary indicators of unrest since 2011.  The slightly acidic lake frequently goes heating and cooling cycles and gets as warm as 46 C.

General tectonics of New Zealand showing subduction of the Hikurangi Plateau and the impact of the Havre Trough into North Island.  General subduction along the Kermadec trench is east to west.  Image courtesy Wiki

Tectonics

Volcanism in northern New Zealand is associated with westward subduction of the Pacific Plate under the Australian Plate.  The line of subduction is the Hikurangi – Kermadec Trench and extends from Tonga to New Zealand.  Convergence rate is less than 50 mm/year beneath North Island.  The Hikurangi Plateau, originally part of the Ontong – Java Plateau is currently subducting near North Island.

The Taupo Volcanic Zone (TVZ) is the southern extension of the Havre Trough into continental New Zealand.  It contains all quaternary volcanic centers in North Island and all the high temperature geothermal fields.  This extension has created a right structure, the Taupo Rift or Fault Belt.  Regional extension varies 19 mm/yr in the north to 7-8 mm/yr in the south.  This coupled with the relatively thin crust 15 – 20 km combine to make the TVZ one of the most active and productive silicic magmatic systems on Earth.

Simplified subduction regime beneath Ruapehu.  Image courtesy of Seismic Resilience blog

Ruapehu and its neighboring stratovolcanoes, Ohakune Craters, Hauhungatahi, Tongariro and Waimarino are in the southern end of the Taupo Volcanic Zone (TVZ).  The northern sector of the onshore TVZ is dominated by a series of rhyolitic caldera centers associated with voluminous rhyolitic ignimbrite eruptions over the last 2 Ma.  The southern sector has andesitic volcanoes that lie within a graben that is the southern margin of the southward propagating tip of the TVZ.

Faults within the TVZ tend to dip toward the eruptive centers.  There are multiple volcanic vents active over the last 10,000 years between Ruapehu and Ngauruhoe to the NE.

Tectonic setting of the Ruapehu vicinity.  TVZ to the north.  Image courtesy Pardo, et al, 2012

There is a narrow conductive dike imaged beneath the summit extending to the NE.  It is visible to a depth of at least 8 km below the summit and appears to be the conduit volcanic water and gasses migrate from depth to the surface.  It appears to be connected to a much larger, poorly resolved conductor to the NE of the volcano.  The three neighboring cones may be connected to a single deep feeder system.  There is no evidence for widespread magma accumulation in the mid-crust below the region.

Phreatic eruption in Sept 1995.  Water, ash, blocks and bombs thrown from Crater Lake.  This eruption created several lahars.  Image courtesy Dougall Gordon via Teara, 2006

Conclusions

Ruapehu continues to be an active, healthy stratovolcano, the southernmost expression of the currently active Tuapo Volcanic Zone (TVZ).  In the long view, that zone will continue to migrate SW into the region occupied by the volcano.  Logically, it is only a matter of time before the voluminous rhyolitic eruptive activity similarly migrates SW toward Ruapehu, though I did not run across anything that remotely suggests that.  Consider that last statement as the rankest of rank speculation.

On the other hand, Ruapehu continues to supply magma to the surface through its extensive and complex magma system.  Eruptions this last century have been frequent and explosive with significant involvement of water and ice in contact with magma.  This sort of activity on a volcano with a glacial ice cap creates frequent lahars, less than friendly visitors to active ski areas below the summit.

Ruapehu is a dangerous, active volcano.  Most likely threats to visitors on the mountain are lahars, though the occasional unannounced phreatic blast should never be discounted.

Undated photo of Ruapehu in eruption.  Photo taken looking west to east.  Neighboring volcanoes Mount Ngauruhoe (left of the road sign) and Tongariro (at the far left edge of the photo) are visible.  I suspect this is from the 1995 series of eruptions but cannot prove it.  Image courtesy India Today, May 2016

Additional information

https://www.mtruapehu.com/

https://volcano.si.edu/volcano.cfm?vn=241100

https://earthobservatory.nasa.gov/images/13064/mt-ruapehu-new-zealand

https://en.wikipedia.org/wiki/Mount_Ruapehu

https://academic.oup.com/gji/article/179/2/887/665997

http://researcharchive.vuw.ac.nz/handle/10063/743

https://academic.oup.com/petrology/article/53/10/2139/1480136

https://link.springer.com/article/10.1007/s004450050259

https://www.tandfonline.com/doi/pdf/10.1080/00288306.1989.10427555

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