The vast expanses of the sky-blue waters of Lake Taupo, topped by misty mountain horizons, evoke an extreme sense of calm.
And yet, deep underground, geological unrest is brewing, according to a new article in New Zealand Journal of Geology and Geophysics.
Lake Taupo is the largest freshwater lake in Australasia, located in the center of New Zealand’s northern island. Although the lake seems peaceful today, the lake has a turbulent origin story.
The waters of the lake are in a prehistoric caldera – a word based on the Spanish language meaning “cauldron” or “boiling cauldron” – formed during Earth’s most recent super-eruption, the Oruanui eruption, 25,400 years ago.
When magma is released from a supervolcano (defined as the release of at least 1,000 cubic kilometers of material in a single eruption) in an event such as the Oruanui eruption, the depleted magma vents collapse, the Earth’s surface sinks, and the landscape becomes a permanent caldera.
Over the past 12,000 years, the Taupo volcano has been active 25 times. Its latest eruption in 232 AD is described by the authors of a new article as “one of the most explosive eruptions on Earth in historical times.” Since then, the volcano has had at least four documented “episodes of turmoil”, causing devastating earthquakes and, in 1922, massive ground subsidence.
The researchers studied more recent supervolcano storms by analyzing 42 years of data collected from 22 locations scattered across the lake. And there is evidence that the supervolcano is still purring.
“In 1979 [researchers] started a new survey method that uses the surface of the lake to detect small changes, and since then there have been four surveys every year,” explained lead author and University of Wellington Victoria seismologist Finn Illsley-Kemp. This method involves using a sensor that measures vertical displacement. the bottom of the lake.
To ensure data reliability, these sensors are weighted to reduce wave effects, and multiple measurements are taken for each data point to determine degrees of variation and outliers. A back-up pressure gauge is also installed at each site as insurance against interference from other forces.
At the beginning of the project, measurements were taken from hand-held sensors installed at only six stations. Eight more stations were added between August 1982 and July 1983, during which time the value of these measurements began to emerge.
In early 1983, the system detected a rise or fall at various sites. Shortly thereafter, a swarm of earthquakes gently rocked the region, causing several faults to rupture, pushing the central Caiapo Fault Belt down and causing other sections at the southern end of the lake to rise.
The 1983 earthquake swarm was only the first of seven separate episodes of unrest recorded over the past 35 years.
By 1986, routine surveys were being carried out each year with additional sensors, with additional observations after earthquakes, creating a robust data set that became increasingly detailed over time.
The authors noticed that during periods of geological unrest, the northeastern end of the lake (closest to the center of the volcano and adjacent fault lines) tended to rise; the bottom of the lake near the center of the fault belt has sunk; and some subsidence occurred at the southern end of the lake.
“In the lake, not far from the reefs of Horomatangi, the volcano caused 160 mm [16 cm or 6.3 inches] uplift, while to the north of the lake tectonic faults caused 140 mm [5.5 inches] subsidence,” Ilsley-Kemp said.
He believes that this region, which has very few earthquakes compared to the surrounding areas, is the location of the Taupo magma reservoir, with deep rocks that are too hot and melted for earthquakes.
The researchers say the 16cm rise, which, while not catastrophic, is definitely enough to cause damage to buildings or pipes, may be due to magma moving closer to the surface during periods of unrest.
Illsley-Kemp said research shows that Taupo is an active and dynamic volcano, closely related to the surrounding tectonics.
The researchers believe that the northeastern tip of the volcano, which has the youngest vents, is more likely to be affected by the expansion of hot magma pushing the earth up. They think that the “submerging” center of the Taupo Fault and the subsidence at the southern end of the lake are likely caused by deep cooling of the magma (and hence compression), tectonic widening of the fault, or both.
Illsley-Kemp regularly assured people that although he was in a state of unrest, there was no evidence that the volcano would erupt anytime soon.
“However, Taupo is likely to erupt at some stage over the next few thousand years, so it is important that we track and understand these periods of unrest so that we can quickly identify any signs that may indicate an upcoming eruption,” he said. New Zealand Herald in a 2021 article.
Ultimately, this research is more about understanding the normal “behavior” of the caldera and what to look out for when things get heated up.
This study is published in New Zealand Journal of Geology, Geophysics and Geophysics.
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