http://SaturnianCosmology.Org/ mirrored file For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== www.gcrg.org *A Fresh Look at Western Grand Canyon Lava Dams: Dating the Rocks* ------------------------------------------------------------------------ At one point or another, anyone who has worked in Grand Canyon has had these questions posed to them: ?How old is Grand Canyon? How long did it take to form?? The answers to these questions are often full of ages produced by the geoscientists who have studied rocks and minerals of the Canyon for decades. For over thirty years, geologists have used the position and age of lava flows in western Grand Canyon, Toroweap Dam in particular, to constrain the timing of incision of the Colorado River through the Grand Canyon (McKee et al., 1968). Before our work in western Grand Canyon began in the mid-1990s, nearly all the age constraints on lava flows relied on a technique called potassium-argon dating, also known as K-Ar ages. K-Ar ages rely on the radioactive decay of potassium to argon gas. Radioactive potassium is a small part of all the potassium that make up minerals within western Grand Canyon lavas. Over time, as the potassium decays to argon, the gas accumulates and is stored within the minerals. Because the decay rate of potassium is well known, geochronologists use the amounts of potassium and argon present in rocks today to determine the age of the rock. This technique was used by geochronologists to study the ages of lava flows in western Grand Canyon during the 1960s and 1970s (Damon et al., 1967; Dalrymple and Hamblin, 1998). In using K-Ar dating, geochronologists assumed that the lava had no argon gas in it as it erupted out of the earth?s interior and flowed out onto the Uinkaret Plateau. As soon as the lava cooled and formed rock, the potassium in it started to turn into argon. The scientists soon noted, however, that there appeared to be ?extra? argon gas in the lava, most likely brought up from the earth?s interior trapped in microscopic ?bubbles? in minerals brought up with the lava. Because the lava was bringing up and storing extra argon before it cooled into rock, measurements made by scientists made it appear that the lava had been rock for a much longer period of time than actually had transpired. K-Ar ages of western Grand Canyon lava flows range from 10,000 years (Vulcan?s Throne) to 1.8 million years (Prospect Dam). Argon-argon dating, a newer technique also known as 40Ar/39Ar dating, is very similar to K-Ar dating, but 40Ar/39Ar dating is more accurate and can account for most?but not always all?of this extra gas brought up from the earth?s interior. New 40Ar/39Ar studies of western Grand Canyon lavas (Pederson et al., 2002; Luchitta et al., 2000; McIntosh et al., 2002) show the oldest lava flow, the Black Ledge flow, is approximately 600,000 to 650,000 years old. The youngest flow (Esplanade Dam) dated with Ar/Ar or K-Ar techniques is 110,000 years old (Dalrymple and Hamblin, 1998). In 1995, we began in earnest to re-evaluate the ages of the lava flows and dams in western Grand Canyon using a new technique called cosmogenic dating. This allowed us to investigate the accuracy of the K-Ar ages that have been reported in Grand Canyon studies since the 1960s. In a nutshell, cosmogenic dating is like dating the suntan of a rock, or dating how long the surface of a rock has been exposed to cosmic rays coming in through the earth?s atmosphere from outer space. These cosmic rays are simply high-energy protons that have enough energy to move through the earth?s magnetic field and into the earth?s atmosphere. These protons cause a cascading shower of nuclear interactions in both the atmosphere and in the surface of rocks on the ground. The longer a rock has been exposed to these cosmic rays, the older the cosmogenic age of the rock. In the case of western Grand Canyon, the olivine crystals within basalt are dense enough to trap helium gas that is spalled off oxygen, magnesium, aluminum, and silicon?just about all the elements that make up olivine?and the buildup of that helium (3He) indicates that amount of time that the rocks have been ?sunbathing.? Which of these three age-dating techniques you use can radically change the interpretation of the age of western Grand Canyon. The Toroweap Dam (Figure 1) is a lava flow exposed on river right just upstream from Lava Falls Rapid (rm 179). This structure has a K-Ar age of 1.2 million years (McKee et al., 1968) and is widely cited within the scientific literature dealing with the age of Grand Canyon. This lava flow has been incised and eroded by the Colorado River, exposing the base and interior of the lava flow where it overlies an ancient reworked debris-flow and old Colorado River gravel deposits. The base of the lava flow, thus, the top of the gravels, is about 100 feet above present-day river level, and for several decades, geologists have used the position and age of this dam to say that, after initial incision of the Grand Canyon began, the river downcut to within 100 feet of its current level by 1.2 million years ago (Hamblin, 1994). The active Toroweap fault runs north-south across the river, offsetting by different amounts lava flows, river gravel, and hillslope deposits of different ages, including the Toroweap and Prospect Dams (Figures 1 and 2 respectively). These offsets occur over time during large-scale earthquakes (magnitude seven or higher on the Richter scale). The last earthquake strong enough to rupture the earth?s surface along this fault occurred about 3,000 years ago (Jackson, 1990; Webb et al., 1999), and it has been inferred that this fault has been moving for the past million years or so (Jackson, 1990; Huntoon, 1977; McKee et al., 1968). Both the Toroweap and Prospect lava dams have approximately 145 feet of displacement along the fault, but interestingly enough, the flows have K-Ar ages that differ by 600,000 years. Our research, which uses cosmogenic 3He dating rather than K-Ar dating of rocks, shows that the Toroweap fault has been moving at the same rate for at least the past 400,000 years (Fenton et al., 2001), and because the offsets in the Toroweap and Prospect lava dams were essentially the same, that the ages of the lava flows should be the same as well. Cosmogenic ages of lava flows on the Uinkaret Plateau range in age from 1,000 to 400,000 years and for the most part, agree with Ar/Ar ages of the same flows. In some cases, the K-Ar ages agree with the cosmogenic ages, as well, however, the K-Ar ages are usually two to three times older than ages provided by the other methods. In cosmogenic dating, we assumed little to no erosion of the surfaces we dated, but we did notice that the older lava flows, particularly the Prospect Dam, had cosmogenic ages that were younger than 40Ar/39Ar ages of the same flow (Fenton et al., 2004). The Prospect Dam has a cosmogenic age of approximately 395,000 years, one K-Ar and several 40Ar/39Ar ages of approximately 500,000 years, and one K-Ar age of 1.8 million years. The 500,000 year-old age is likely most accurate, whereas the 1.8 million year age is likely affected by extra argon gas, and the 395,000 year age affected by erosion of the rock?s surface. So, as you can see, all of these dating techniques have their own sources of error. Because the offsets in the flows on either side of the canyon are about the same, this strongly suggests that the Prospect Dam and Toroweap lava flows should also be about the same age. We draw this conclusion despite the uncertainties in the age dating. The take-home message from all of this is that every age reported in scientific literature, as well as every dating technique used to produce these ages, have their own uncertainties and sources of error. No age is, in fact, ?set in stone,? but by using a variety of dating techniques to date the same lava flows or deposits, geologists produce ages that begin to converge on a ?true age? of the lava flows or deposits. So, when people ask you ?how old is the Grand Canyon?? or ?how long did it take to form??, gently explain to them that science has helped constrain the age of the big ditch to five to six million years old, depending upon whether you believe in creation science or not, but that the really interesting thing is that the age of the canyon beneath Toroweap Point is much younger than previously believed. Our work, combined with other researchers, suggests that this part of the canyon formed less than a million years ago and the river at Lava Falls arrived within 100 feet of its current depth about 500,000 years ago. By anyone?s idea of speed, that?s fast downcutting. /Cassie Fenton & Bob Webb/ *References:* Dalrymple, G.B., and Hamblin, W.K., 1998. K-Ar ages of Pleistocene lava dams in the Grand Canyon in Arizona. Proceedings of the National Academy of Sciences, v. 95, p. 9,744-9,749. Damon, P.E., Laughlin, A.W., and Percious, J.K., 1967. The problem of excess Argon-40 in volcanic rocks, in Proc. Symp. Rad. Dating and Methods of Low-Level Counting, Monaco 2-10 March, 1967: International Atomic Energy Agency: 24 p. 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, AZ, Journal of Geology, v. 112, p. 91-110. Fenton, C.R., Webb, R.H., Pearthree, P.A., Cerling, T.E., and Poreda, R.J., 2001. Displacement rates on the Toroweap and Hurricane faults: Implications for Quaternary downcutting in Grand Canyon. Geology 29: 1,035-1,038. Hamblin, W.K., 1994, Late Cenozoic lava dams in the western Grand Canyon, 135 pp., Geological Society of America Memoir 183, 139 pp. Huntoon, P.W., 1977, Holocene faulting in the western Grand Canyon, Arizona, Geological Society of America Bulletin, v. 88, p. 1619?1622. Jackson, G.W., 1990, Tectonic geomorphology of the Toroweap fault, western Grand Canyon, Arizona: Implications for transgression of faulting on the Colorado Plateau. Arizona Geological Survey Open-File Report 90-4, p. 1?66. 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, Quaternary Research, v. 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, Quaternary Research, v. 53, 23-33, 2000. McIntosh, W.C., Peters, L., Karlstrom, K.E., and Pederson, J.L., 2002. New 40Ar-39Ar dates on basalts in Grand Canyon: Constraints on rates of Quaternary river incision and slip on the Toroweap fault and implications for lava dams: Geological Society of America Abstracts with. Programs, Rocky Mountain Section. McKee, E.D. and Hamblin, W.K., and Damon, P.E., 1968. K-Ar age of lava dams in Grand Canyon. Geol. Soc. Amer. Bull. 79: 133-136. Pederson, J., Karlstrom, K., Sharp, W., and McIntosh, W., 2002. Differential incision of the Grand Canyon related to Quaternary faulting ? Constraints from U-series and Ar/Ar dating. Geology, v. 30, p. 739-742. Webb, R.H., Melis, T.S., Griffiths, P.G., Elliott, J.G., Cerling, T.E., Poreda, R.J., Wise, T.W., and Pizzuto, J., 1999, Lava Falls Rapid in Grand Canyon: Effects of Late Holocene debris flows on the Colorado River. U. S. Geol. Surv. Prof. Paper 1591, 90 pp. + map.