Depressurizing a New Zealand Volcano

Today we are going back a few years in time, to August 6th, 2012. On this day, in New Zealand, a volcano that had been mostly dormant for the last century, suddenly came alive.  This event–followed by another on Novermber 21st of the same year–became known as the Te Maari eruptions of Tongariro.
To get some idea of where we are, let’s back up to New Zealand itself. Mostly known in modern times for the set of Lord of the Rings movies, New Zealand has two islands, named rather unoriginally North Island and South Island. Nearly direct center on the North Island is the area called Taupo (the name of both a town and a lake): here we find the Taupo Volcanic Zone. At the southern end of that, we have the Tongariro Volcanic Center, which is where the action discussed today took place. The Tongariro Volcanic Center hosts multiple volcanoes, including Mount Tongariro, Mount Ruapehu, and Mount Ngauruhoe.
The eruption discussed today. Labels are present. Image credit: NASA Earth Observatory.
The eruption discussed today. Labels are present. Image credit: NASA Earth Observatory.
The Taupo Volcanic Zone is an open magmatic system, meaning it can be influenced by outside sources. In particular, Tongariro is hydrothermal (a system of heat and water, as seen by the roots of the word) closed in by rocks, resting below craters and vents. Prior to the explosion on August 6th, there were a few earthquakes (which is not uncommon in New Zealand), with an epicenter 1 to 1.5 kilometers (0.6 to 0.9 miles) below the surface, near the volcanoes–a possible sign of an impending eruption. Like other open magmatic systems, there usually if not much for warning. Evidence found after the explosion suggests that magma flowed  into the hydrothermal system slowly, raising the temperature and pressure. A landslide occurred a few minutes before the eruption–decompressing the chamber enough for its pressurized contents to come bursting out at the Upper Te Maari crater. The main effects for the people on the ground were steam and ash that interrupted travel, especially by air.
This brings us to today’s article, “Depressurization of a hydrothermal system following the August and November 2012 Te Maari eurpstions of Tongariro, New Zealand“, written by I. L. Hamling, C. A. Williams, and S. Hreinsdottir. It was published in 2016 in the American Geophysical Union’s journal Geophysical Research Letters.
Using a technique called interferomerty to measure the surface,  our authors found that the results of data sent back from the Italian Space Agency’s SkyMed satellite showed that there was subsidence. Subsidence is the sinking of the ground. In this case, it was around 22 millimeters per year (0.79 inches per year) consistently after the August 6th event. It was centered in a 2 kilometer by 2 kilometer (1.2 by 1.2 mile) region near the Te Maari eruption.
Limited data was available from other sites, and suggest that there were two sources for the subsidence; one 500 meters from the surface (547 yards), the other 2.5 kilometers (1.6 miles). Due to the limited data, however, this deeper source is much more questionable. More research will give a better understanding of how this and other volcanoes behave, and what to expect in the future.
Bibliography
Betz, Laura. Mount Tongariro Erupts. NASA Earth Observatory. 8 August 2012. 
The source of the image used in this post, plus some more information on the event.
Hamling, I. J., C. A. Williams, and S. Hreinsdóttir (2016), Depressurization of a hydrothermal system following the August and November 2012 Te Maari eruptions of Tongariro, New Zealand, Geophys. Res. Lett., 43,168175, doi:10.1002/2015GL067264.
Today’s featured article.
Marshak, Stephen. Earth: Portrait of a Planet. 3rd ed. W.W. Norton & Company. Print
A geology textbook used for reference o=about subsidence.
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