2012 Eruptions at Te Maari Craters, Tongariro
Caption: Tongariro eruption, 2012. Photo: Craig Miller, GNS/EQC
The eruptions of Te Maari craters, Tongariro on 6 August and 21 November generated a substantial response involving many scientists at home and abroad. The Natural Hazards Research Platform ensured a coordinated response from New Zealand’s best research teams.
Most of our volcanology research is in the Geological Hazards theme, but across the Platform there are many people working on different aspects of volcanoes, including atmospheric monitoring and forecasting (NIWA) and emergency management (Societal Resilience theme). We monitor the volcanoes through the EQC-funded GeoNet project and research their past behaviour with Platform funding in close collaboration with nearly all New Zealand universities. Under the Guidelines to the National Civil Defence and Emergency Management plan, we are tasked with providing specific advice on geological hazards and risks. The aim of the Platform is to coordinate quality science from across New Zealand so that authoritative information on volcanic hazards is delivered to responding agencies with clear, consistent messages put before the public.
Tongariro eruption debrief meeting. Photo: Steven Sherburn, GNS/EQC.
Tongariro has an interesting geological history. It was formed by eruptions from 12 vents over 275,000 years ago. The youngest vent is Ngauruhoe, now thought to be ca. 4,500 years old. At least three large eruptions occurred between ca. 10,000 and 15,000 years ago with widespread ashfall. Past eruptive activity is made up predominantly of lava flows and ashfall with some lahars, flank collapses and pyroclastic density currents. Historical eruptions have been documented and the best records are from 1892 and 1896-7.
Tongariro had been dormant for over a century in July 2012, when activity increased beneath its northern slopes. Significant changes in the Te Maari fumarole discharge had occurred since May, suggesting gases from the deeper, higher temperature system were emerging and making their way to the surface. Small tectonic earthquakes are not uncommon in this area, but the signals recorded in July had low frequency resonant characteristics, commonly thought to be related to fluid movement(either magma or gases). Seismo-acoustic studies revealed that immediately prior to the eruption, a sequence of small micro-earthquakes and persistent tremor occurred for about four minutes. The micro-earthquakes had low frequency energy consistent with a subsurface volcanic process, while the pre-eruption tremor had a wider range of frequencies. The main eruption was marked by strong seismic signal that lasted for about one minute.
Ash emissions as detected by NOAA 18 satellite on 7 Aug 2012. Blue colour indicates volcanic ash. Data: NIWA.
During this time, scientists at NIWA and MetService monitored the atmosphere. NOAA satellite infrared observations can be used to discriminate between water vapour and ash cloud. Combining the infrared signatures of the ash cloud with vertical temperature profiles from NIWA weather model NZLAM indicated that the initial plume reached an altitude of approximately 12 km. MetService weather radar also detected the eruption but is limited to ash particles larger than about 500 µm and these only reached altitude of about 7 km. As atmospheric winds carried the plume eastward it decreased in altitude; by the time it passed over the Hawke’s Bay it had lowered to near 4 km. By the afternoon of 7 August ash was still visible in MODIS satellite imagery far out to sea off Hawke’s Bay. Particle trajectory analyses based on NZLAM forecast output also verified well against observed ash fall and gas observations, where sulphurous smells, sometimes mistaken for blocked sewers, were reported at many locations from Hunterville to Wellington and even 500 km away at North Brighton Beach in Canterbury.
Potential volcanic hazards during an eruption
There are a number of hazards that can occur from an eruption on Tongariro. Based on the past geological record, we can determine which hazards are the most likely and what impacts they might have. When an explosion occurs, the initial part of the eruption fires ballistic projectiles away from the vent area. These rocks tend to be hot, dense and are commonly up to a metre or so across. They result in impact craters and can cause severe injury and building damage.
Shane Cronin & Jonathan Procter led the Massey University response. Photo: Massey University.
A second common phenomenon from explosive eruptions is a pyroclastic density current. These are fast moving, warm to very hot clouds of ash and gas travelling at tens to hundreds of kilometres per hour; they could lead to bush/forest fires and the more energetic flows lead to complete vegetation and building destruction. A less immediate impact is from ash fall. This tends to be thicker closer to a vent, but even in small amounts (less than a few millimetres thickness) can result in infrastructure damage, agriculture impacts and building damage. Ashfall tends to have a chronic, cumulative effect and can travel 100 km and more.
Debris flow on 6 Aug 2012. Photo: Steven Sherburn, GNS/EQC
Remobilisation of ashfall or other deposits results in lahars. These are mixtures of ash, rock and water that can be formed from lake breakouts, by rainfall washing material from the flanks of volcanoes or by snow melt. They tend to stay predominantly in valleys and cause a hazard to people and infrastructure in valleys. Past Tongariro eruptions have also produced lava flows, but these tend to be slow moving and although can give rise to bush fires and destroy everything in their path, the risk to people is low. The August eruption displaced 320,000 cubic metres of material from the flanks of the vent area in the form of a landslide, generating a 2.5 km debris flow from source that blocked a valley. The resulting dam and the lake behind the dam presented a changing hazardscape. Computer simulations and their results were combined with new topography to help us predict landscape changes over time.
From these events we developed a new phenomena map (LINK ♦). This map incorporates flow hazards modelling from Massey University and includes expert opinion based on current activity and past geological record. This map represents the coordinated effort between science, councils, New Zealand Transport Agency (NZTA) and Civil Defence and Emergency Management (CDEM) groups.
Current Understanding
Analyses of gas, ash and rocks collected from Te Maari now suggest that the eruption was driven primarily by magmatic gas pressurising the near surface hydrothermal system. We believe there is magma somewhere under the volcano, but from the data we are unable to determine exactly where. So although it seems that magma did not reach the surface in either the August or November eruptions, the involvement of magma in future eruptions cannot be ruled out.
Key lessons learned
Communication amongst the science teams generally worked well and the use of an open wiki and email lists enabled immediate sharing of data, photos and discussion points. Regular updates proved critical, ensuring everyone was aware of the situation. Dynamic hazard and risk assessments were also valuable to key end users, particularly when making decisions about public and staff safety in areas close to the vents.
Overall, consistent messaging is important for providing quality information to responders, but also for joint information statements from CDEM, scientists, landowners and other key parties. This is made easier by the long-term relationships that have been built up over the years via the Central Plateau Volcanic Advisory Group, led by regional CDEM groups, with key science organisation involvement.
This story originally appeared in Natural Hazards 2012.
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