Volcanic hazard scenarios: Mount Taranaki, New Zealand


Over the last 5 000 years, Mount Taranaki volcano in the westernmost part of New Zealand's North Island produced at least 16 Plinian-scale explosive eruptions. These eruptions had magnitudes of 4 to 5, eruptive styles, and contrasting basaltic to andesitic chemical compositions comparable to the eruptions of Etna, 122 BC; Vesuvius, AD79; Tarawera, 1886; Pelée, 1902; Colima, 1910; Mount Saint Helens, 1980; Merapi, 2010; and Calbuco, 2015.

In a study published by Geological Society of America Bulletin, it was modeled how an eruption at the volcano might play out. Rafael Torres-Orozco and colleagues combined both geological mapping and lithostratigraphic analyses to define the possible hazard scenarios in case of Taranaki's explosive reawakening.

Its most recent big eruption happened around 1655 AD. Today, the mountain is considered sleeping and likely to erupt again, with significant potential hazards from lahars, pyroclastic flows, lava flows, debris avalanches, floods, and ashfalls.

According to estimates, more than 85 000 people live within 30 km (18.6 miles) of the mountain and 40 000 in high-priority evacuation areas – and there were threats to the region's dairy farming and petrochemical industries, including reticulated supply to the North Island.

Results of their work indicate that, during a future Plinian event, bursting of both long-lasting, large-volume lava domes and transient, small-volume lava plugs from Taranaki's andesitic summit crater would be typical, and these would produce different types of pyroclastic density currents (PDCs) flowing down the volcano flanks mainly due to gravity.

"This study used painstaking, fine-scale observations of deposits on the upper volcano flanks, mainly covering the last 5 000 years of eruption," said Cronin, who currently directs one of New Zealand's national science challenges, dedicated to natural hazards, according to NZ Herald.

"This is the first time such modeling of scenarios has been carried out at Taranaki – or indeed to such detail in all of New Zealand volcanoes."

"We have looked at broad scenarios of different scale for economic studies, or we have designed one-off scenarios for testing emergency response, but this is the first time we are able to make our own scenarios from such detailed volcano-deposit studies."

The most deadly "blast-type" PDCs would first explode and expand laterally, and then would flow downstream, reaching urban areas located at up to 18 km (11.2 miles) distance from the crater. Eruptive columns following or accompanying PDCs are ubiquitous to every scenario. These columns would inject ash and gas into the atmosphere and could disperse 10 cm (3.94 inches) thick layers of volcanic material over the most populated areas at 20 – 30 km (12.4 – 18.6 miles) from the crater. In the scenario of eruptions produced from vents different to the summit-crater, these would be expected to be basaltic and lack major pyroclastic density currents.

These scenarios highlight the major role that PDCs must have in evaluating the hazards scenario of Taranaki and of other similar volcanoes. The scenarios can be tailored to different sites around the world by localized studies, and can also be used to plan emergency management.

Volcanic hazard scenario Mount Taranaki

Figure caption: Volcanic hazard scenarios for Plinian eruptions at Mount Taranaki's summit crater and Fanthams Peak vent. A-F: Scenario I. Close-conduits and conduit decompression by vent unroofing and dome collapse. G-K: Scenario II. Transient open and clogged conduits by repeated plugging-and-bursting of gas-depleted or chilled magma. L-O: Scenario III. Rapid progression into steady phases by open conduits. P: Possible upper conduit dynamics for each scenario based on data and interpretations of Torres-Orozco et al. (2017a, 2017b). Image credit: R. Torres-Orozco et al., GSA Bulletin, 2018.

The classically shaped symmetrical peak of Mt. Taranaki stands in isolation to the west of the Central North Island volcanoes. At 2 518 m (8 261 feet), it is the second highest peak in the North Island after Ruapehu, and is New Zealand's largest mainland volcanic cone by volume.

It's a stratovolcano – also called a composite cone volcano – made of layers of mostly andesite lava flows and pyroclastic deposits, or tephra. The summit crater is filled with ice and snow and has a lava dome in the center. Volcanic debris from lahars and landslides covers the plains around the volcano.

Past huge landslides have reached as far as 40 km (24.85 miles) from the cone, with lava reaching 7 km (48 miles) and pyroclastic flows 15 km (9.32 miles) from the vent. Volcanic ash has been weathered and mixed with the soil to produce rich, fertile farmland.

Taranaki is the youngest, largest and only active volcano in a chain that includes the Kaitake and Pouakai Ranges, Paritutu and the Sugar Loaves. All these are now eroded remains of what were once large volcanoes.

Taranaki began erupting about 130 000 years ago, with large eruptions occurring on average every 500 years and smaller eruptions about 90 years apart.

Featured image: Kyle Bland / Flickr

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