Mount Ruapehu

The 1945 eruption emptied Crater Lake and dammed the outlet with tephra. The crater slowly refilled with water, until on 24 December 1953 the tephra dam collapsed causing a lahar in the Whangaehu River. The lahar caused 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.

It was already known that the river had partially undermined one of the bridge piers and the lahar finished the job, causing the bridge to collapse. Although warned of the collapsed bridge, the train driver was unable to stop the train in time and six of the carriages fell into the river.

Spectacular eruptions occurred during 1995 and 1996. Ruapehu had been showing signs of increased activity since late November 1994, with elevated Crater Lake temperatures and a series of eruptions that increased in intensity over about nine months. Several lahars were observed, both in the Whangaehu River and other areas of the mountain, between 18 September and 25 September 1995, indicating Crater Lake was being emptied by the eruptions.

The Department of Conservation immediately issued hazard warnings and advised people to keep off the mountain, thus ending the ski season. The eruption cloud disrupted air travel, occasionally closing airports and the central North Island airspace. Black sand-like ash fell on surrounding farmland and stock had to be moved. The ash also entered streams and was washed into the pen stocks and turbines of the Rangipo power station causing rapid corrosion on the turbine blades which had to be rebuilt. Episodic eruptions continued until the end of November 1995.

Within hours of a major eruption during the night being reported on 25 September 1995, news media were trying to get live video of the eruption and amateur photographers had published eruption images on the World Wide Web. A web camera, dubbed the world's first "Volcano Cam", was set up. Since then Ruapehu has been monitored by at least one and sometimes several volcano cams.

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.

Another, smaller, eruption phase began on the morning of 17 June 1996. Despite a series of small eruptions that spread thin layers of ash across both Whakapapa and Turoa ski areas, the ski fields opened for the 1996 season. Turoa closed on 29 September – earlier than usual, while Whakapapa stayed open until Labour Weekend.

After the 1996 eruption it was recognised that a catastrophic lahar could again occur when Crater Lake bursts the volcanic ash dam blocking the lake outlet. This is the same mechanism that caused the 1953 lahar. 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).

Ruapehu erupted at 10.30pm on 4 October 2006. The small eruption created a volcanic earthquake at a magnitude of 2.8, sending a water plume 200 m into the air and 6 metres (20 ft) tall waves crashing into the wall of the crater.[5][6]

Fresh lahar channels scar Ruapehu's eastern slopes, 2007.

On 18 March 2007, the tephra dam which had been holding back Crater Lake burst, sending a lahar down the mountain. An estimated 1.4 million cubic metres of mud, rock, and water travelled down the Whangaehu river. The Department of Conservation had received warning signals from ERLAWS at around 10:30 that morning and closed all major roads in the area, preventing thousands of motorists from travelling, and shut down the main rail system for the North Island. The river banks held and no spillovers occurred. There was no serious damage and no injuries. One family was trapped for around 24 hours after the lahar swept away the access route to their home.[7]

At about 8:20 p.m. on 25 September 2007, a hydrothermal eruption occurred without warning.[8][9] One man, a 22-year-old primary school teacher, had a leg crushed by a rock during the eruption and a rescue operation was mounted to rescue him from the Dome Shelter near the crater. The rock crashed into the Dome Shelter, landed on the man and was too heavy for his companion to lift off.

Two lahars that travelled down the mountain activated warning signals from the lahar warning system and prompted the evacuation of some ski lodges on the mountain and the closure of roads in the area. The eruption was accompanied by a 7-minute-long earthquake, measuring 2.9 on the Richter Scale.

On 2 May 2008, a level-1 warning was issued after GNS scientists who were monitoring the lake found irregular signs of volcanic activity. This included increased chemical changes, gases and temperature. The warning was used to let visitors know the potential of an explosion was higher than usual and could happen at any time. This could also be a follow-up to the previous year's 2007 eruption.[10]

A GeoNet New Zealand Bulletin was released on 21 July 2008 stating that "the current phase of volcano unrest appears to be over, however Ruapehu remains an active volcano. Future eruptions may occur without warning."[11] However the level has remained at 1, where it has been since 1997.

On 5 April 2011, Geonet changed Mount Ruapehu's Volcanic Aviation Colour Code from Green to Yellow (elevated unrest above the known background). Ruapehu's Volcanic Alert Level remained at level 1 (signs of volcanic unrest). The Volcanic Aviation Colour Code was changed because a sustained period of high water temperatures in the Crater Lake (approximately 38–39 °C) was observed and because changes were observed in volcanic gas output, seismic activity and Crater Lake water chemistry.[12]

On 16 November 2012, GNS Science volcanologists warned that pressure was likely building up beneath Crater Lake, and that an eruption in the coming weeks or months was more likely than normal.[13] The aviation colour code was raised from green to yellow.[14]

On 29 April 2016, Geonet issued a volcanic alert reporting that Crater Lake's temperature had risen to 40 °C (104 °F), up from 25 °C (77 °F) in mid-April. An earthquake swarm was also recorded under Crater Lake. On 11 May the volcanic alert level was raised to level 2 and the aviation colour code was changed from green to yellow.[15][16]

On 10 April 2019, Geonet issued a new alert, stating that the lake temperature had been cycling between 15 and 45 °C (59–113 °F) over the course of a year for the last decade,[17][18] but that this trend had been broken in September 2018, with the lake staying above 30 °C (86 °F) all summer.[17][18] For the 2 weeks leading up to 10 April, the lake had warmed further, at a rate of around 0.5 °C (0.90 °F) per day, reaching 42 °C (108 °F) on 9 April, a temperature exceeded by only 1% of temperature measurements in the preceding decade, most recently in May 2016.[17][18] While GeoNet retained the level 1 alert state, they added "it is a useful reminder that eruptions can occur with little or no warning".[18] On the 29th of April the crater lake temperature dropped to 39 °C (102 °F).[19]

On 26 Aug 2019 Crater Lake temperature rose to 27 °C (81 °F) from 14 °C (57 °F).[20]