The expanse of Lake Taupō’s azure waters, crowned by hazy, mountainous horizons, evoke an extreme sense of calm.
And yet, deep beneath the ground, geological unrest is brewing, according to a new paper in the New Zealand Journal of Geology and Geophysics.
Lake Taupō is the largest freshwater lake in Australasia and is located in the center of New Zealand’s North Island. And while it appears peaceful today, the lake has a violent origin story.
The lake’s water is contained within a prehistoric caldera — a word based on Spanish for “cauldron” or “boiling pot” — that formed during Earth’s most recent supereruption, the Oruanui Eruption, 25,400 years ago.
When an event such as the Oruanui eruption releases magma from a supervolcano (defined as releasing at least 1,000 cubic kilometers of material in a single eruption), the depleted magma collapses, the Earth’s surface sinks, and the landscape is permanently transformed into a caldera.
In the last 12,000 years, the Taupō volcano has been active 25 times. Its most recent eruption in AD 232 is described by the new paper’s authors as “one of the most explosive eruptions on Earth in historical times.” Since then, the volcano has experienced at least four documented “troubles” that caused devastating earthquakes and a massive subsidence in 1922.
It’s the supervolcano’s more recent turbulent periods that the researchers have been studying, analyzing up to 42 years of data collected from 22 locations around and across the lake. And there is evidence that the supervolcano is still rumbling.
“In 1979 [researchers] began with a novel surveying technique that uses the lake surface to detect small changes, and since then four surveys have been conducted each year,” explained lead author and Victoria University of Wellington seismologist Finn Illsley-Kemp. This technique involves the use of a gauge that measures the vertical displacement of the lake bottom.
To ensure the data is reliable, these gauges are weighted to reduce the effects of waves, and multiple measurements are taken for each data point to detect levels of variation and outliers. A reserve gauge is also installed at each location as insurance against interference from other forces.
At the beginning of the project, the measurements were recorded by hand-held measuring devices that were set up at only six stations. Eight more stations were added between August 1982 and July 1983, and during that time the value of these measurements began to show.
In early 1983, the system recognized rising or falling values at various locations. Not long after, an earthquake swarm gently shook the region, leading to the rupture of several faults that pushed down the central Kaiapo fault belt and raised other areas on the southern end of the lake.
The 1983 earthquake swarms were just the first of seven discrete episodes of civil unrest recorded over the past 35 years.
Routine surveys with additional sensors and additional post-earthquake observations were conducted every year until 1986, creating a robust dataset that became more detailed over time.
The authors noted that the northeast end of the lake (closest to the center of the volcano and adjacent fault lines) tends to rise during times of geological turmoil; the lake bed near the center of the fault belt subsided; and at the south end of the lake there was a slight subsidence.
“Inside the lake, near the Horomatangi reefs, the volcano caused 160mm [16 cm or 6.3 inches] uplift, while north of the lake the tectonic disturbances have caused 140 mm [5.5 inches] of lowering,” said Illsley-Kemp.
He believes that this region, which has very few earthquakes compared to the surrounding areas, is the site of Taupō’s magma reservoir, with deep rock too hot and molten for earthquakes to occur.
The researchers say the 16 cm uplift — which, while not catastrophic, is definitely enough to cause some damage to buildings or pipes — may be due to magma moving closer to the surface during times of turmoil.
Illsley-Kemp said the research shows Taupō is an active and dynamic volcano that is closely related to the surrounding tectonics.
The researchers believe the northeast end of the volcano – which has the youngest vents – will be more affected by the expansion of hot magma pushing the ground up. They believe that the “sinking” center of the Taupō Fault and the subsidence at the southern end of the lake are likely due to deep cooling of the magma (and thus shrinking), tectonic widening of a rift, or both.
Illsley-Kemp has regularly assured people that although it is in a state of turmoil, there are no signs the volcano will erupt anytime soon.
“However, Taupō will most likely erupt at some point in the next few thousand years – and so it’s important that we monitor and understand these periods of unrest so that we can quickly spot any signs that might indicate an impending eruption,” he told the New Zealand Herald in a 2021 article.
Ultimately, this research is more about understanding the caldera’s normal “behaviour” and what to look for when it gets hotter.
This study is published in New Zealand Journal of Geology and Geophysics and Geophysics.