Shhh! Mt Taranaki is Sleeping
Photo: Mt Taranaki. Brad Scott, GNS Science.
Quantifying the Hazard from Re-Awakening Volcanoes.
Excellent soils, abundant water, mineral resources and natural beauty have drawn humans to volcanoes throughout history. Many volcanoes are constantly active, with ‘business as usual’ eruptions expected by the people that live around them. But even these regular performers produce the odd unwelcome surprise. A massive eruption of Mt Merapi (Indonesia) in late 2010 killed over 380 people, despite evacuation of almost 250,000 people. This scale of eruption had not been experienced since the early 1890s, long enough for the memories (and fear) of such events to fade.
In New Zealand, eruptions are much less frequent. Decades (Mt Ruapehu), centuries (Mt Taranaki), or even millennia (Taupo) may occur without eruption, encouraging complacency. Such volcanoes can ‘re-awaken’ at any time. Taranaki is a perfect example. Despite it not having erupted since the mid 1800s, its proximity to dairy, natural gas and oil production, means that an understanding of its hazards is crucial.
To assess hazard we must first define “when” an eruption will occur. Over the past several years we have developed a highly detailed record of Mt Taranaki eruptions. By examining soil sequences and drill-core sections from lakes and swamps around the volcano, volcanic ash layers have been counted, chemically fingerprinted and dated. From novel statistical and geochemical approaches developed by us, we estimate the volcano has erupted over 160 times in the last 36,000 years.
From past work we know that Taranaki’s activity occurs on a regular cycle, with periods of rapid-versus slow-recurring eruptions. We also know that both the size of the eruptions and their magma chemistry changes between different parts of these eruption cycles. This strongly affects the type of eruption, and the consequent hazards faced: slow-moving lava flows versus explosions and widespread ash.
◊ Caption right: Estimates of eruption likelihood since the last known eruption at Mt Taranaki (in probability of event per year). The new model with geochemical control is shown by the solid line and our former 2009 hazard model is the dashed line. The likelihood of eruption increases over time of resting.
In latest research involving PhD student Rebecca Green, we have used our understanding of the chemical cycling at Mt. Taranaki to better forecast the ‘rest period’ before the next eruption. The specifics of this process rest on the chemistry of some of the smallest components of magma: the iron-titanium bearing oxides known as titanomagnetites. In general, if aluminium content varies in the titanomagnetite of a given ash deposit, we know that several different magma batches were mixed deep within the volcano before eruption, correlating with longer rest times and larger eruptions. If titanium content varies, it tells us that the magma has rested within a few kilometres of the surface, resulting in more frequent yet smaller eruptions.
The improvement in our understanding of the volcanic processes has led to an increase in the forecasted probability of Taranaki erupting: The new data and modelling increase the estimate of an eruption in 2014 from 1.6 to 3.1 percent.
There is still much more to be uncovered about magmatic processes driving eruptions. These ongoing studies lead the way for international volcanology and can be applied to other settings, such as Mt. Merapi.
Major contributors to this research have been: Shane Cronin, Mark Bebbington, Ian Smith, Richard Price, Michael Turner, Thomas Platz, Ting Wang, Bob Stewart, Vince Neall, Rebecca Green and Magret Damaschke.
- By Shane Cronin and colleagues at Massey University
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Last updated 7 Nov 2014