Satellite Technologies Detect Uplift in the Yellowstone Caldera
The ground surface can rise for decades, and then switch to a period of subsidence. At Yellowstone, scientists measured over two feet of uplift during the period 1923-1985, followed by a shorter period of subsidence. Until the 1990s, it was thought that all the ups and downs occurred in the central part of the Yellowstone caldera, near its two resurgent domes.
March 2006, the journal Nature published a paper by YVO scientist Charles Wicks and colleagues about a period of uplift in the northern part of the Yellowstone caldera (Wicks, et al., 2006). It describes an uplift of 12 cm (~5 inches) that occurred between 1997 and 2003, at a time when other changes were occurring in the vicinity. Since that time, the area, including the Norris Geyser Basin, has stopped moving, while uplift has returned to the central part of the caldera and the resurgent domes.
This article discusses the 1997-2003 uplift, how we continue to monitor the ever-changing Yellowstone caldera, and what may have caused this and other uplifts.
Using InSAR and GPS to Look at Ground Deformation
We use two primary techniques to measure ground movement from space, Interferometric Synthetic Aperture Radar (InSAR) and Global Position System (GPS). These techniques are highly complementary, with one giving a view of a wide area over a long period of time and the other providing a detailed time series for specific localities.
InSAR provides infrequent (annual to semi-annual) but highly detailed images of the amount of movement of the ground relative to a satellite. The technique is very useful for monitoring ground movements or ground deformation at volcanoes because detailed data can be collected for a relatively large area from space. By aiming a pulse of radar energy at the ground and then recording information about the return signal, such as the time traveled by the radar waves, InSAR quickly provides data for an entire area. By returning to the same area at different times, we can gain information about relative ground movements over time. The InSAR data are processed into images, called interferograms. Ground movement, either up or down, is shown by the changes in color on the processed images (see the 1996-2000 InSAR image of the Yellowstone caldera at the top of this page). InSAR data can be combined to produce very high-resolution images that compare changes from one year to the next or multiple passes can be combined to show changes over a few years of time. Although InSAR is a powerful new technique, it is still expensive and often difficult to get time on a satellite for data collection. All of the data shown on this page is from the European Space Agency's ERS-2 satellite. There are currently no U.S. satellites collecting radar data that can be used for InSAR, which makes collecting data even more difficult.
GPS provides detailed time series information about a specific locality. Monitoring ground movement with GPS requires an instrument on the ground (a ground-based receiver) that "listens to" a network of Earth-orbiting satellites. The satellites continuously transmit information to ground-based receivers that are a more precise version of those found in some cars and hand-held units. Some of the benefits of GPS are the ability to continuously monitor a site, for example to take a measurement every few minutes, and to record information about movement along the surface of the earth (in a north-south or east-west direction instead of just a vertical direction). Because we only obtain InSAR images every year or so, our day-to-day understanding of ground movement at Yellowstone comes from the GPS stations (antennas firmly attached to the ground) located around Yellowstone National Park (see the map of GPS station locations below). At the present time (January 2006), we have 12 stations at Yellowstone, six of them were installed in summer 2005 by the Plate Boundary Observatory (PBO)/EarthScope project. The network currently reveals an entirely new uplift within the central part of the Yellowstone caldera that can be seen in the GPS data for the WLWY, HVWY, OFWY2 and LKWY stations.
What causes uplift?
Once the GPS and InSAR data reveal a pattern of ground movement, scientists try to re-create similar patterns using mathematical models. This allows them to estimate possible depths and volumes of the deformation source. Scientists agree that a fluid has moved into the region, causing an increase in volume within the upper 15 km of the crust. Wicks et al. (2006) propose the idea that a small amount of basaltic magma (molten rock) has moved out of the main caldera (causing the ground to subside) and into the area north of the caldera (causing the ground to move upwards). They discern that the magma moved along a northward-dipping sill (sub-horizontal crack) located about 12-15 km beneath the ground surface. In the past, some scientists have preferred models where deformation at Yellowstone is caused by movement of hydrothermal fluid (hot water) or gas, rather than magma. For example, Waite et al. (2002) discussed a swarm of earthquakes that occurred in the western part of the caldera, that coincided with a change from uplift to subsidence in the caldera. They suggested the swarm was caused by movement of hydrothermal fluids (hot waters and gases) from inside the caldera to the northwest. Both types of fluids (magmatic and hydrothermal) may occasionally cause the observed deformation at Yellowstone. Scientists will continue to collect information that can help discriminate among the various possibilities.
Yellowstone is one of the largest hydrothermal systems in the world and the Norris Geyser Basin is the hottest, most active, area in the park. Yellowstone most likely stays thermally active due to repeated injections of magma miles beneath the surface of the earth. But multiple injections of magma at depth does not mean there will be a magmatic eruption at the surface. There are many examples of magma injections within the crust that never reach the surface. Although there is no evidence that the fluid (or magma) will move to the surface, intrusions at depth can cause disturbances on the surface such as ground swelling and increased discharge of steam and hot water.
How does the uplift from 1997 to 2003 compare to earlier activity?
Over the history of the Yellowstone caldera we see evidence for multiple cycles of uplift and subsidence. Scientists have been studying the timing of these movements using a variety of methods. For information about pre-historic times we use field studies from around the Yellowstone Lake and Yellowstone River. These studies show that the Yellowstone caldera floor has experienced many episodes of uplift and subsidence over the last 15,000 years (Pierce, et al., 2002), some with several meters (> 10 feet) of up and down movement!
Early monitoring studies using leveling surveys show the northeast caldera floor experienced about 180 mm (7 inches) of uplift from 1976 through 1984, no measurable deformation in 1984-1985, and then subsidence of about 140 mm (5.5 inches) from 1985 through 1995 (see figure Dzurisin, et al., 1995). Starting in 1987, GPS was added to the Yellowstone monitoring system and corroborated the results from the leveling surveys (Meertens, et al., 1991).
An InSAR study from 1992 through 1997 (just prior to the study we are focusing on in this article) showed subsidence of about 60 mm (2 inches) from 1992-1995 in the northeast and then northwest half of the caldera. From 1995-1997 the same areas showed uplift of 30 mm (1 inch) (Wicks, et al., 1998).
Are there any connections between the uplift and the thermal disturbances?
A previous article on the thermal disturbances in the Norris Geyser Basin describes recently increased geyser activity and increased temperatures in some areas around Norris. Some of the increased thermal activity is similar to the periodic "disturbances" that have long-been recognized at Norris. Scientists have generally concluded that the leading contributor to the disturbances is variation in the pressure within the hydrothermal system or perhaps the permeability of the rocks hosting the fluids (permeability is a measure of the ability of fluids to migrate through rocks). The recent uplift centered 10 km (6.2 mi) south of the Norris Geyser Basin could have changed the pressure or permeability of the hydrothermal system, contributing to the increased activity at Steamboat Geyser and increased ground temperatures within the Back Basin at Norris. We look forward to future opportunities to study possible connections between uplift and thermal disturbances.
A summary of our findings
- The Yellowstone caldera appears to be in a constant state of movement and with new monitoring techniques we were able to observe uplift in the northern part of the Yellowstone Caldera, in a region where it had not been previously recorded.
- The uplift began in late 1997 or early 1998 and ended in late 2002 or early 2003. The total uplift during this time was approximately 125 mm (a little under 5 inches).
- The uplift was likely due to movement of fluid beneath the northern part of the Yellowstone caldera. The fluid may have been magma, hot water or gas and the source was likely shallower than about 15 km (approximately 9 miles).
- The increased thermal activity at the Norris Geyser Basin, observed in 2003, may be partly related to the uplift detected by InSAR and GPS. Whatever caused the uplift may have also affected the pressure of deep fluids or the permeability of the rocks near Norris.
- Currently, deformation has returned to the central caldera, with a maximum of about 8 cm of uplift recorded over the past year.
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