Volcanic Unrest at Mt. Tongariro

Low-level activity which began at Mt. Tongariro on July 13th has continued, and GNS Science has increased its monitoring of the volcano. Seismic activity and volcanic gas emissions confirm a low level of unrest.

Wednesday 1st August 2012

Mt Tongariro lies at the northern end of the distinctive volcanic structure in the centre of New Zealand’s North Island that comprises mounts Ruapehu (2797 m), Ngauruhoe (2287 m) and Tongariro. At 1967 metres, Tongariro is the lowest structure of the three, and has also been the least active in modern times. It erupted ash between 1855 and 1897 and GNS Science reports unconfirmed activity in 1926-27.

Volcanism has determined the appearance of the volcanic plateau, technically known as the Taupo Volcanic Zone which stretches from the central North Island through to the Bay of Plenty and, undersea, north to the Kermadec Islands. There are numerous volcanoes between the central North Island and the Bay of Plenty coast, including the powerful caldera (volcanic crater) known as Lake Taupo which lies 40 km northwest of Tongariro.

The northern slopes of Mt. Tongariro consist of numerous craters and vents. Ketetahi hot springs lie to the north of the volcano, and Te Mari craters are situated about two kilometers east of the springs. Te Mari craters are the last vents to have been active on Tongariro.

Seismic activity commenced on the northern side of Mt Tongariro on July 13th and has continued intermittently since. The activity is clustered between Emerald Crater and Te Mari craters at depths between 2 and 7 kilometres. The activity encouraged GNS Science to visit the volcano to conduct tests in addition to its regular monitoring. Selected springs and fumaroles have been sampled, a Global Position System instrument has been installed to monitor ground movement, and gases have been analysed. Gas sampling shows that volcanic gas levels are at levels above those normally measured at Tongariro. There is always a mix of volcanic and hydrothermal gases rising to the surface at Tongariro, but recent samples show a marked increase, confirming the unrest indicated by the earthquakes.

Of the 99 earthquakes recently recorded at Tongariro, most have occurred during three bursts of activity on July 13th, July 20th and July 29th. GNS Science reports that 3-10 earthquake events are being recorded each day, but the events are very small and unlikely to be felt. Historically, seismic data shows that small volcanic earthquakes do occur at Tongariro, but usually at the rate of about two events per year.

The Volcanic Alert Level for Mt. Tongariro was changed from 0 to 1 on July 20th. At Alert Level 0 a volcano is regarded as in its dormant or quiescent state, with typical background surface activity; seismicity, deformation and heat flow at low levels. At Alert Level 1 a volcano is considered to be showing signs of unrest, having departed from its typical activity level.

The scale runs to level 5, with level 3 indicating a significant local eruption, 4 a hazardous local eruption and level 5 a large hazardous eruption.

GNS Science has also raised the Aviation Colour Code from Green to Yellow in the vicinity of Mt. Tongariro. This four-level scale identifies aviation hazards near a volcano and ranges from green through yellow and orange to red. The change to Yellow at Mt. Tongariro indicates to aviation that a change has occurred, but ash is not being emitted.

GNS Science hosts a page summarising the status of Mt. Tongariro on the GeoNet website. The map shows the tight cluster of earthquakes occurring near Emerald Lake on the northern side of Mt. Tongariro. The larger lake partially visible in the top right corner of the map is Lake Rotoaira, not Lake Taupo which is much further north-west at this scale.

The seismograph on this page is located to the west of Mt. Tongariro, and seems to show little of the activity currently occurring on the northern flank of the volcano. The seismograph situated at Oturere to the east of Mt Ngauruhoe (and a few km south-east of Tongariro) appears to have recorded the bursts of activity at Tongariro in recent days.

Three of New Zealand’s four currently most active volcanic areas are now at Alert Level 1. They are Mt. Ruapehu, White Island and Mt. Tongariro. Raoul Island in the Kermadecs remains at Alert Level 0.

[Compiled from GNS Science alert bulletins published on the GeoNet website.]

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