What's Going on Under Norris Geyser Basin?

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If there were an Olympic medal for hydrothermal systems, Norris Geyser Basin would bring home the gold.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Dan Dzurisin, emeritus geologist with the U.S. Geological Survey.

 

Four radar interferograms (InSAR data) superimposed on digital terrain

Four radar interferograms (InSAR data) superimposed on digital terrain showing surface movement at Norris Geyser Basin and Yellowstone Caldera from 1996 to 2003. A color change from violet to blue to green to yellow to red, shown in the color bar in A, marks an increase in the distance from the satellite to points on the ground (mostly subsidence) of 28.3 mm (1.1 inch), and vice versa. White dots represent epicenters of earthquakes recorded during the time interval spanned by each interferogram. Black parallel dashes outline caldera rim. A, 1996–2000. Concentric color bands along the north caldera rim indicate surface uplift by as much as 8 cm (3.1 in) centered near Norris Geyser Basin. B, 2000–2001. Uplift near Norris and subsidence of the caldera floor continued at a lower rate. C, 2001–2002. Uplift near Norris and subsidence of the caldera floor continued to slow during this period. Arrow labeled NGB marks the location of Norris Geyser Basin. D, 2002–2003. Norris uplift and caldera floor subsidence had nearly stopped by 2003, but a second episode of Norris uplift started in mid-2013 (not shown).

(Public domain.)

If there were an Olympic medal for hydrothermal systems, Norris Geyser Basin would bring home the gold. Temperatures in its subsurface hydrothermal reservoir are the highest anywhere in Yellowstone National Park – more than 300C (570F). Norris is the most dynamic geyser basin in the Park and its more than 500 hydrothermal features are the most diverse. Among them is Steamboat Geyser, the world's tallest. Recently, Norris has been competing with Yellowstone Caldera for the title of fastest mover in the Park—and winning. A new study used ground deformation data collected by GPS and also via satellite by a technique called InSAR to conclude that an intrusion of magma deep under Norris two decades ago set the stage for much of what's happened there since, including the wild (by geologic standards) roller-coaster ride that might have ended, or at least paused starting in 2018.

From 1996 to 2004, an area more than 30 km (18 miles) across centered near Norris, just outside the north caldera rim, rose as much as 12 centimeters (4.7 inches) at an average rate of 1.5 cm/yr (0.6 in/yr). The area subsided about 7 cm (2.8 in) in the next several years, but uplift resumed in mid-2013. From late 2013 to early 2014, Norris rose at the highest rate ever measured in Yellowstone—more than 15 cm/yr (5.9 in/yr). For comparison, the highest uplift rate ever measured inside Yellowstone Caldera is about 7 cm/yr (2.8 in/yr) during 2004–2005.

The record-setting Norris uplift ended abruptly when a magnitude 4.9 earthquake, the largest in Yellowstone since the 1970s, struck the Norris area on March 30, 2014. Rapid uplift quickly reversed to subsidence that ended in 2015. But a third uplift episode, this one at an average rate of about 2 cm/yr (0.8 in/yr), began in early 2016 and continued through 2018. The net result was that Norris stood about 14 cm (5.5 in) higher in 2018 than it did in 1996. Since early 2019, the area has subsided about 2.5 cm (1 in), but Norris still stands 11–12 cm (4–5 in) taller than it did at the turn of the century.

Considering the area that's been moving is larger than the city of Chicago, those numbers are impressive. But what's more remarkable is the story that scientists have been able to piece together about what's been going on under the surface.

Modeling of InSAR and GPS data suggests the 1996–2004 uplift was caused by an intrusion of magma about 14 km (8.7 miles) beneath Norris. When magma intrudes the crust it cools, crystallizes, and releases gases that had been dissolved in the melt. Gas escape lowers pressure in the magma, causing the surface to subside. That's probably what was going on at Norris from 2005 to mid-2013. But rising gases can become trapped under an impermeable layer of rock, causing the kind of rapid uplift seen at Norris from late 2013 until the M 4.9 earthquake in March 2014. It seems likely the quake created microfractures that allowed gases to escape upward again, resulting in subsidence that ended in 2015. The third uplift episode from 2016 to 2018 suggests rising gases became trapped again, this time at a slightly shallower depth. Modeling indicates the source depth for the 2013–2014 uplift was about 3.2 km (2.0 miles) deep, compared to about 1.8 km (1.1 miles) for the 2016–2018 episode.

Should you worry? In a word, no. Activity like this is probably common at Norris and, at other times, under the caldera, too. What's different is that we now have the tools to detect and model Yellowstone's ups and downs better than ever before. For the first time, we've been able to track an entire episode of magma intrusion, degassing, and gas ascent to the near surface. For those in the know, like you, that's awesome—not alarming.

 

Vertical deformation near Norris Geyser Basin during 2003-2020

Vertical deformation near Norris Geyser Basin during 2003-2020 recorded by GPS. Positive change is uplift and negative is subsidence.

(Public domain.)