Yellowstone's Breathing Ground: How Satellites Caught the Caldera Rising, 1997–2003

The ground at Yellowstone is never still. Like other calderas, it rises for years or decades, then sinks for a while, then rises again. Between 1923 and 1985 scientists measured more than two feet of uplift, followed by a shorter spell of subsidence. Until the 1990s all this movement was thought to happen in the centre of the caldera, around its two resurgent domes. Then satellites found the north of the caldera rising.

Ground movement from satellite passes between 1996 and 2000: 125 millimeters (about 5 inches) of uplift centred about 10 kilometers south of Norris. Each full cycle of colour is about 28 millimeters. Image from the USGS Yellowstone Volcano Observatory.

A new bulge

In March 2006 the journal Nature published a study by Charles Wicks of the Yellowstone Volcano Observatory and colleagues describing uplift in the northern caldera. It began in late 1997 or early 1998 and ended in late 2002 or early 2003, raising an area about 35 by 40 kilometers by about 125 millimeters (a little under 5 inches). Afterwards that area, including the Norris Geyser Basin, stopped moving, and uplift returned to the centre of the caldera.

Two eyes in the sky

Scientists track the ground from space in two complementary ways.

• InSAR (Interferometric Synthetic Aperture Radar) bounces radar pulses off the ground from a satellite. Comparing passes made at different times, a year or so apart, produces interferograms that map rise and fall in great detail over a wide area.
• GPS stations fixed to the ground listen to navigation satellites and record movement continuously, north-south and east-west as well as up and down, giving the day-to-day picture at each site.

InSAR needs two or more passes to measure changes in ground height. Image from the USGS Yellowstone Volcano Observatory.

The LKWY GPS station at the north end of Yellowstone Lake, 1997–2006. While the northern caldera rose, it sank; later it rose too. Image from the USGS Yellowstone Volcano Observatory.

GPS stations (yellow) and planned ones (red), with the 2.1, 1.3 and 0.64 million-year-old calderas outlined in orange and the two resurgent domes in yellow. Image from the USGS Yellowstone Volcano Observatory.

What pushes the ground up?

By fitting mathematical models to the GPS and InSAR data, scientists estimate how deep and how large the source of deformation is. They agree that some fluid moved in, swelling the crust within its upper 15 kilometers. Wicks and colleagues proposed that a small amount of basaltic magma flowed out of the main caldera, letting it sink, and north along a gently dipping sill about 12 to 15 kilometers down, lifting the ground there. Others have favoured hot water or gas rather than magma. Yellowstone probably stays hot because magma is injected repeatedly deep below, but injections at depth do not mean an eruption: many never reach the surface, though they can swell the ground and increase steam and hot water.

Up and down through time

Leveling surveys recorded uplift of about 18 centimeters from 1976 to 1984, no movement in 1984–1985, then about 14 centimeters of subsidence. Image from the USGS Yellowstone Volcano Observatory.

• Around Yellowstone Lake and the Yellowstone River, field studies show many episodes of rise and fall over the last 15,000 years, some of several meters.
• Leveling surveys found about 180 millimeters of uplift in the northeast caldera from 1976 to 1984, none in 1984–1985, then about 140 millimeters of subsidence. GPS, added in 1987, confirmed the pattern.
• InSAR showed about 60 millimeters of subsidence from 1992 to 1995, then 30 millimeters of uplift from 1995 to 1997, before the northern bulge.

A link to Norris?

At the same time, Norris Geyser Basin, the hottest and most active part of the park, saw more geyser activity and rising ground temperatures. Such periodic "disturbances" are thought to come mainly from changes in pressure or rock permeability in the hydrothermal system. The uplift centred 10 kilometers south of Norris may have changed those, helping to set off Steamboat Geyser and heat up the Back Basin.

Sources

• USGS Yellowstone Volcano Observatory, "Satellite Technologies Detect Uplift in the Yellowstone Caldera," on the study by Wicks and others in Nature, 2006; rewritten in hubnx's own words. Images from the USGS.