Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Michael Poland, geophysicist with the U.S. Geological Survey and Scientist-in-Charge of the Yellowstone Volcano Observatory.

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Although Yellowstone is distinct in many ways—for example, it has the largest concentration of geysers in the world—it is not completely unique.  Other large caldera systems exist all around the planet, and many are “restless” and have had geologically recent eruptions.  Taupō, in New Zealand, is one such system and is a good analog for Yellowstone in many ways.

Taupō, located in the center of New Zealand’s North Island, is short for “Taupō-nui-a–Tia,” which means “the great cloak of Tia.”  In Māori traditions, the early explorer and chief Tia observed some cliffs along the shores of Lake Taupō that resembled his coat, resulting in the name for the area.

Like Yellowstone, Taupō has experienced repeated large explosive eruptions interspersed with smaller eruptions of ash and lava.  At Yellowstone, volcanic activity is driven by a hotspot—a region of melting within the Earth that is stationary relative to the tectonic plates that make up the crust.  In contrast, Taupō’s volcanic activity is fed by subduction of one plate beneath another, which generates melt.  Both volcanic systems are underlain by a large basaltic magma system that heats the overlying crust and sustains a shallower rhyolitic magma chamber, which is responsible for large explosive eruptions and lava flows.  At both Taupō and Yellowstone the rhyolite magma reservoir is mostly solid, based on seismic imaging, although the proportion of melt at Taupō (20–30%) is higher than that at Yellowstone (5–15%).

Eruptions first began from the Taupō system about 300,000 years ago, culminating with the Oruanui eruption 25,500 years ago.  That eruption, which caused collapse of the surface and created a caldera that is now largely filled by Lake Taupō, was even larger than the eruption that formed Yellowstone Caldera 631,000 years ago!  After the Oruanui eruption, 28 additional but smaller eruptions of ash and lava occurred, 25 of which took place in the past 12,000 years.  The most recent eruption took place 1800 years ago (prior to the arrival of humans in New Zealand), and was huge in its own right, comparable in size to the 1815 eruption of Tambora, Indonesia.  The explosion was followed by a lava eruption beneath the lake.  During this period of exceptional activity at Taupō, Yellowstone was taking a snooze—its most recent eruption was 70,000 years ago.

In addition to a violent history, Taupō and Yellowstone share vigorous hydrothermal systems that have resulted in geysers and hot springs, and even small steam explosions.  Both volcanic regions also experience unrest in the form of earthquakes and ground deformation.  At Yellowstone, there are typically 1500–2500 earthquakes per year, and the ground rises and falls at rates of about 2–3 cm (1 in) per year.  Much of the unrest is attributed to groundwater accumulating and migrating beneath the surface and interacting with crustal faults.  At Taupō, earthquake swarms are also common and, like at Yellowstone, are often caused by water moving around in the subsurface near crustal faults.  Ground deformation at Taupō can be more intense than at Yellowstone, with rates of more than 10 cm (4 inches) per year.  While at Yellowstone it is difficult to relate any unrest to magma, some episodes of unrest at Taupō may be driven by magmatic activity.

The 2019 unrest at Taupō is a good example.  During that year, volcanologists in New Zealand were able to locate over 7000 small earthquakes that occurred as parts of seven discrete swarms. Although the first two swarms had characteristics associated with tectonic seismicity—movement along existing faults—and subsurface water migration, the subsequent five swarms appear to have been associated with magma movement.  Of particular note was a M5.3 earthquake that occurred in September 2019 and that was followed by over 1,100 earthquakes in the ensuing 72 hours.  While most earthquakes occur by rock slipping along a fracture, these unusual earthquakes had a character that suggests the fractures were being pried open by the injection of magma deep beneath the volcano.  Over the course of the year, a small amount—about 1 cm (less than 1 inch)—of inflation of the volcano was also noted.

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These results emphasize the importance of monitoring caldera systems like Taupō and Yellowstone, to distinguish activity driven by magma from that driven by tectonic faulting and subsurface water migration.  At Yellowstone, monitoring consists of dozens of seismic and deformation stations, along with satellite and ground-based surveys of thermal and gas emissions.  Monitoring at Taupō is more challenging, since most of the caldera is filled by a lake—in fact, Lake Taupō is about twice the size of Yellowstone Lake!  But lessons learned at each volcano can be applied to the other—what we learn from Taupō can teach us more about how Yellowstone might work, and vice versa.  In some ways, Taupō can be considered a much younger sibling to Yellowstone.  While Yellowstone has been around for a while and is a mature, calm system, Taupō is still an energetic teenager.

For more about the 2019 unrest at Taupō, see the recent articles at https://doi.org/10.1029/2021GC009803 and https://eos.org/science-updates/implications-of-a-supervolcanos-seismicity.  And for information about Taupō geology and unrest in general, check out https://doi.org/10.1007/s00445-015-0956-5 and https://www.tandfonline.com/doi/full/10.1080/00288306.2020.1792515.