Braun, Mike

Yellowstone

Yellowstone, our nation's first national park is a picturesque combination of boundless wildlife and majestic scenery. Recently, the region was ravaged by massive wildfires, but the geological record details an even more horrific past. Catastrophic eruptions from the Yellowstone volcanic system spewed lava and ash which blanketed much of the western half of North America. Modern geophysical investigations have revealed that these seemingly serene lands are now alive and rumbling. Lurking in the heart of this now tranquil wilderness is a geological beast waiting to be unleashed again.

The Yellowstone region, lies at the center of the Parabola of Doom, and is a region of highly focused modern tectonic activity. Major earthquakes such as Hebgen lake (Mw 7.3) in 1959 and Borah Peak (Mw 7.3) in 1983 triggered landslides and floods which radically transformed the region. Aftershocks from Borah Peak reached magnitude 5.8. In the last decade, there have been nearly 5000 earthquakes of magnitude 0 and higher within the Yellowstone region. This background seismicity has been punctuated by three large shallow earthquakes around the rim of the caldera: a) 1994 Mw 4.9, depth 5.7 km, ~20 km east of Hebgen lake, b) 1994, Mw 4.8, depth 3.2 km, within the caldera ~15 km northwest of Yellowstone lake, and c) 1995, Mw 5.1, depth 1.9 km, ~20 km south of Yellowstone lake. (Figure 1 - From [Wicks et al, 1998]) This seismic activity is expressed at the surface by significant vertical deformation. The Hebgen lake caused more than 6 m of elevation change, followed by an additional ~30 cm of postseismic rebound.

Since the1920's, leveling, trilateration, and tilt surveys in the Yellowstone region have also shown substantial and rapid surface deformation. Comparison of leveling surveys from 1923 and 1975-77, indicates the caldera rose ~75 cm (~14 mm/yr). [Pelton & Smith, 1979] After 1975, repeated annual surveys showed the caldera still inflating at a rate of ~22 mm/yr until about 1985. [Holdahl & Dzurisin, 1991] From 1985 until 1995 the caldera subsided at ~19 mm/yr. Unfortunately, these types of measurements are costly due to the intensive amounts of time and manpower, and as a result are temporally sporadic and spatially limited.

Now with satellite-based altimetry methods, predominantly interferometric synthetic aperture radar (InSAR), maps of surface deformation can easily be made, with millimeter vertical resolution over hundreds of square kilometers. Recent InSAR data show the caldera to be even more active than previously determined, with two large domes surging back and forth. (Figure 2- From [Dzurisin et al., 1999]) From 1992 to 1993, the Sour Creek dome in the northeast of the caldera subsided ~3 cm, then stopped. From 1993 to 1995, the center of deformation shifted to the Mallard Lake dome in the southwest portion of the caldera, which subsided ~4 cm. From 1995 to 1996, the caldera began to inflate in the Sour Creek area (~2 cm). InSAR data spanning 1995 to 1997 shows the whole caldera inflating as much as 3cm. [Dzurisin et al., 1999] The coupled resurgence of these silicic domes, in conjunction with the pervasive shallow seismicity, has been attributed to large scale fluid migration beneath the caldera. The spatial distribution and temporal variation of the surface deformation can be adequately modeled by two interconnected sill bodies approximately 8 km below the caldera. [Wicks et al, 1998]

Regional tilt measurements are consistent with the uplift rates as determined by InSAR. Typically tilt measurements are difficult, since they are spatially localized and limited in precision due to the small length scales over which the measurements are made. However using modified tide gauges around the shore of Yellowstone lake, [Kleinman & Otway, 1992] have overcome both problems. Using the whole lake at a tilt meter Kleinman and Otway can observe rotations with a precision of 0.5-1.0 microradians.

Despite its serene surface, Yellowstone lake itself is a hotbed of geothermal activity. Recent geophysical surveys of the lake bottom show intense heat flow anomalies. The lake accounts for approximately 20% of the total heat flux from the region. In some places the lake radiates 20,000 times the background heat flow (~2 W/m2). [Karajick, 2001] In the past this buildup of heat resulted in catastrophic explosions. Massive blowouts from Mary's Bay (11 Ka) and Indian Point (3 Ka) created large craters in the lakebed and tsunami-like deposits around the shore. Since 1997, many smaller craters have been discovered, still radiating heat. "[The lake] is like a pressure cooker: Lift the lid, and it will blow you away," says Robert Smith of the University of Utah in Salt Lake City.

The Yellowstone caldera has a well documented history of volcanic eruptions of biblical proportions. The first major eruption occurred 2 Ma ago, releasing 2500 km3 of lava and ash. After ~700,000 years passed, Yellowstone erupted again, releasing another 1000 km3 of material. Another 600,000 years passed, and Yellowstone erupted for a third time. That last major eruption was 630,000 years ago, and like the second eruption released more than 1000 times the volume of material as the Mt. St. Helens eruption of 1980. Pervasive seismicity around the caldera, rapid resurgence of the recent volcanic constructs, and massive hydrothermal explosions indicate migration of magma and other fluids in the subsurface. Catastrophic eruption from the Yellowstone caldera, although not imminent, is inevitable. As we idly pass through our earthly lives, we are forced to consider who will warn us of the coming Armageddon. It is the geophysicists who detect the dark rumblings from below. Mankind must remain ever vigilant and prepare for the impending disasters that lie waiting in the heart of the Parabola of Doom. REPENT!

Bibliography:

Dzurisin, D., Wicks, C.W., Thatcher, W.R., Renewed uplift at the Yellowstone caldera measured by leveling surveys and radar interferometry, Bull. Volcanol., v 61, p. 349-355, 1999.

Holdahl, S. R., Dzurisin, D., Time-dependent models of vertical deformation for the Yellowstone-Hebgen Lake region, 1923-1987, J. Geophys. Res. , v. 96, p. 2465-2483, 1991.

Karajick, K., Thermal Features Bubble in Yellowstone Lake, Science, v. 292, p. 1479-1480, 2001

Kleinman, J.W., Otway, P.M., Lake-level monitoring as a tool for studies of crustal deformation, Monitoring volcanoes; Techniques and strategies used by the staff of the Cascades Volcano Observatory, 1980-90,USGS Survey Bulletin, ed. Swanson, D.A., p 159-174, 1992.

Pelton, J.R., Smith, R.B., Recent crustal uplift in Yellowstone National Park, Science, v. 206, p. 1179-1182, 1979.

Wicks, C.W., Thatcher, W.R., Dzurisin, D., Migration of fluids beneath Yellowstone Caldera inferred from satellite radar interferometry, Science, 282 (5388), p. 458-462, 1998.