Explained: Why Taupō Volcano's alert level was raised and what the unrest means

Taupō's volcano alert level was raised to 1 for the first time following minor volcanic unrest - but what does that really mean and what can happen during this alert?

Although this is the first time GeoNet has raised the alert to level 1, it's not the first volcanic unrest at Taupō. There have been 17 previous episodes of unrest over the past 150 years. 

The volcano has not erupted over the past 1800 years before written records were kept and the last eruption at Taupō volcano was in 232 AD ± 10 years. 

Why was the alert level raised at Taupō Volcano?

Volcanic alert level 1 means minor unrest, GNS Science volcanology team leader Nico Fournier said, and in the volcanic alert level system, it is the lowest level of unrest at a given volcano.

Volcanic unrest has continued at the lake for months, which is when magma or magma-heated hot water and steam force their way through the ground under a volcano, producing earthquakes and ground movement.

GeoNet has located a swarm of almost 700 small earthquakes beneath the lake since the start of May, including a 4.2 magnitude shake on September 10.

The movement under the active volcano has also caused the land around the lake to uplift.

"One of the key things is over the past few years, we've really started to understand better what volcanic alert level one means at Lake Taupō volcano," Fournier said.

"With the current level of activity that we've recorded, the earthquakes, and the ground deformation, we feel it is appropriate to set the alert level at 1."

What does unrest at Lake Taupō mean?

Fournier said Lake Taupō, like most large volcanoes, goes through some quiet periods and then periods with more unrest. With those periods of unrest comes more "symptoms".

Volcanic alert levels one and two cover minor and moderate to heightened volcanic unrest. Taupō is currently at minor unrest.

Hazards linked to unrest occur on and near the volcano and may include steam eruptions, volcanic gases, earthquakes, landslides, uplift, subsidence, changes to hot springs, and/or lahars (mudflows).

While volcanic alert levels three to five measure the intensity of eruptions, an eruption could occur at any level and volcano alerts may not move in sequence since activity can change rapidly.

Could there be an eruption at Lake Taupō?

There have been large eruptions in the past at Lake Taupō Volcano, including 30,000 years ago that created the huge crater now filled with the lake.

And then later on, 1800 years ago, another big eruption created another of those calderas inside the bigger one, Fournier said.

But the vast majority of eruptions at Lake Taupō are quite small, he added.

"Over the last 29 eruptions, only two of them were actually large. All the other eruptions were a very small size, probably similar to what happened to Ruapehu in 1995, 1996."

What about earthquakes around Lake Taupō?

Since May 2022, GeoNet has recorded about 600 earthquakes in the lake area and some of them were big enough to be felt by people, Fournier said.

But the vast majority of those earthquakes were actually quite small and wouldn't be felt, but they would be recorded by GNS's instrumentation.

While also being a large volcano, Lake Taupō is also crossed by a number of really big faults that cross the whole region, which is what GNS calls the Taupō Volcanic Zones.

"When we do have earthquakes occurring inside the lake, they are either occurring along some of those faults or they are occurring around the body of magma, those molten rocks that we know exist under the surface at depth since the last eruption," Fournier said.

"So a key thing for us as we try to understand what those earthquakes actually mean during those periods of earthquakes is to try to understand where they occur and what process they're actually related to."