Stirring New Zealand Supervolcano Moves Ground Above It

A supervolcano hidden underneath a New Zealand lake is shaking the ground so much that the lakebed is starting to deform.

Despite not having erupted for hundreds of years, the supervolcano is still rumbling, according to new research published in the New Zealand Journal of Geology and Geophysics. Lake Taupō, which is the largest freshwater lake in New Zealand, is situated on the country's north island. The water sits within a giant caldera of a supervolcano, 10km (6 miles) above the magma chamber, that has erupted 25 times in the last 12,000 years, most recently in the year 232 AD. This last eruption was "one of the Earth's most explosive eruptions in historic times," according to the authors of the paper, and in the 1800 years since, has had several periods of unrest.

A supervolcano is defined as a volcano that has had an eruption of magnitude 8 on the Volcano Explosivity Index, i.e. that the measured deposits for that eruption is greater than 240 cubic miles. Other supervolcanoes include Yellowstone, Long Valley in eastern California, and Toba in Indonesia.

lava lake
Stock image of a volcanic lava lake, in Afar, Ethiopia. Lake Taupō sits in the caldera of an ancient supervolcano. iStock / Getty Images Plus

In the paper, the authors described geological surveys that have been done around the lake for decades and analyzed the dataset, showing how the volcano has been acting over the last half century. They found that the rumbling of the supervolcano has caused the lakebed to deform up and down, due to magma movement and tectonic activity.

"The dataset confirms that vertical deformation is occurring in the lakebed. Long periods of slow deformation, dominated by subsidence totalling 140 mm [5.5 inches] in the Taupō Fault Belt at the northern end of the lake, and to a lesser degree at the southern end, are interrupted by uplift episodes now reaching 160 mm [6.3 inches] in the north-eastern sector containing the most recent active vents. We suggest that the subsidence is primarily tectonic while inflation episodes are driven by upward migration of magma to shallow levels during periods of unrest."

Six inches of movement isn't enormous, but it is enough to potentially cause damage to infrastructure. The movement differs across the lakebed: the bed at the north-eastern end of the lake—closest to the volcano's center and the adjoining fault lines—mostly rose, where the bed at the south of the lake subsided. According to the researchers, the rising ground level is due to the expansion of hot magma pushing the ground upwards, with the subsidence being due to the cooling and shrinking of the magma. The movement is also indicative of how active the volcano is and, therefore, how likely it is to erupt.

"Taupō will most likely erupt at some stage over the next few thousand years - and so it's important that we monitor and understand these unrest periods so that we can quickly identify any signs which might indicate a forthcoming eruption," co-author Finn Illsley-Kemp told the New Zealand Herald in 2021.