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                    A Fresh Look at Western Grand Canyon
                          Lava Dams: Introduction
     _________________________________________________________________

   If you ever get a chance to camp at Toroweap Overlook, go stand on the
   edge of the Esplanade and look down at Lava Falls Rapid and all those
   lava flows and dams that remain frozen to the canyon walls (Figure 1).
   If you stare hard and long enough, you'll expect to see the lava flow
   just west of Vulcan's Throne start moving again, flowing down Toroweap
   Valley, and into the Colorado River some 2000 feet below. You'll begin
   to imagine what it would have been like to stand at that same spot
   hundreds of thousands of years ago and watch the hot lava flow into
   the Colorado River.
   For many years, people have wondered how these lava dams were formed
   and destroyed and on what time scales these events occurred. Through a
   series of articles, we'll present to you new ideas on how those lava
   flows and the Colorado River may have interacted.
   During the past two million years, significant volumes of basalt were
   extruded from vents in the Uinkaret volcanic field (Hamblin, 1994).
   Many of these flows cascaded over the rim, mainly on the north side of
   the canyon, and into the canyon, particularly in the vicinity of
   present-day Lava Falls and Whitmore Rapids. There are more than 150
   flows present in this volcanic field, and Hamblin (1994) identified
   the remnants of at least thirteen different lava dams. Hamblin
   proposed that most lava dams occurred between 10,000 and 1.8 million
   years ago, and that western Grand Canyon lava dams took several days
   to several thousand years to form. He hypothesized that the dams were
   stable, could have lasted up to forty thousand years, and that deep,
   long-lived lakes backed all the way up to Moab in one case. The lakes
   then filled with both water and sediment, and the lava dams were
   gradually eroded through headward erosion, similar to erosion at the
   base of Niagara Falls, as water flowed on top of the sediments and
   down the face of the dam. In addition, Hamblin (1994) identified
   unusually coarse river gravels with huge foresets--preserved riverbed
   ripples--in a deposit overlying the remnant of a basalt flow at river
   mile 188 (river left) indicative of a large-scale flood, but he
   attributed the deposits to failure of a landslide dam upstream.
   Lucchitta et al. (2000) proposed that major accumulation of
   basalt-rich gravels in western Grand Canyon represents extremely
   vigorous erosion of a lava dam as a result of overtopping, headward
   erosion and plunge-pool action.
   New studies of those basalt-rich river gravels (Figure 2) suggest that
   the gravels were emplaced by the rapid and catastrophic failure of
   lava dams (Fenton et al., in press; 2002). Whether any of the lava
   dams lasted long enough to allow the deposition of lake deposits in
   their upstream reservoirs is uncertain, as deposits from deep-water
   lakes linked to lava dams have not yet been verified in Grand Canyon
   (Kaufmann et al., 2002).
   The chemical composition and different ages of the deposits lead us to
   believe that at least five of these failures occurred not long after
   the dams were formed. Among the geologic evidence of these floods are
   large basalt boulders up to 115 feet in diameter and perched high
   above the modern Colorado River. Rocks in the flood deposits are
   mostly basalt; essentially these deposits are the rock that formed the
   dams.

   We propose that some of the dams were inherently unstable, too
   unstable to create long-lasting reservoirs that would leave lake
   deposits behind. We hypothesize that basalt poured over the rim of
   western Grand Canyon and into the gorge cut by the Colorado River. The
   lava eventually "froze" in place following the initial hydroexplosive
   interaction with the Colorado River, creating a dam whose base and
   abutments rested on loose talus slopes and unconsolidated river
   sediments. While the dam was forming, interaction of the lava and
   water caused the explosive fragmentation of basalt glass and zones of
   hydrothermal fracturing. These structurally weaker zones formed both
   at the base and higher in the dam as the reservoir filled as quickly
   as the lava piled up. At sufficient hydraulic gradients, water stored
   in the reservoir flowed, or piped, through the now porous dam. The
   piping created larger and larger conduits, eventually allowing water
   to entrain sediment and dam material, ultimately causing the complete
   collapse of the lava dam and the rapid draining of the lake behind it.
   Preliminary data indicate that one of these floods was the largest
   ever to run through Grand Canyon and it ranks among the largest known
   in the continental United States.
   Until recently, the timing of landscape development in western Grand
   Canyon has been mainly based on Hamblin's (1994) interpretation of
   lava dams near the Uinkaret volcanic field and age-dating of those
   lavas. Most of the dating of the Uinkaret volcanic field was
   undertaken in the 1960s and 1970s, and even at the time problems were
   known to exist with the application of the technique to these lavas.
   In future articles, we will discuss age dating of these lavas--both
   old and new--and detail our studies on catastrophic dam failures and
   flood discharges. Stay tuned.
   Cassie Fenton & Bob Webb

   References:

   Fenton, C.R., Poreda, R.J., Nash, B.P., Webb, R.H., and Cerling, T.E.,
   Geochemical discrimination of five Pleistocene lava-dam outburst-flood
   deposits, western Grand Canyon, Arizona, Journal of Geology, in press.
   Fenton, C.R., Webb, R.H., Cerling, T.E., Poreda, R.J., and Nash, B.P.,
   2002, Cosmogenic 3He Ages and Geochemical Discrimination of Lava-Dam
   Out-burst-Flood Deposits in Western Grand Canyon, Arizona, in House,
   K. et al., eds., Paleoflood Hydrology, American Geophysical Union, p.
   191-215.
   Hamblin, W.K., Late Cenozoic lava dams in the western Grand Canyon,
   135 pp., Geol. Soc. Amer. Memoir 183, 139 pp., 1994.
   Kaufmann, D., O'Brien, G., Mead, J.I., Bright, J., and Umhoefer, P.,
   2002. Late Quaternary spring-fed deposits in the eastern Grand Canyon
   and their implications for deep lava-dammed lakes, Quat. Res, 58, p.
   329-340.
   Lucchitta, I., G.H. Curtis, M.E. Davis, S.W. Davis, and B. Turrin,
   Cyclic aggradation and downcutting, fluvial response to volcanic
   activity, and calibration of soil-carbonate stages in the western
   Grand Canyon, Arizona, Quat. Res., 53, 23-33, 2000.

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