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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. 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.
This allowed us to investigate the accuracy of the K-Ar ages that have been reported in Grand Canyon studies since the 1960s.
New 40Ar/39Ar studies of western Grand Canyon lavas (Pederson et al., 2002; Luchitta et al., 2000; Mc Intosh et al., 2002) show the oldest lava flow, the Black Ledge flow, is approximately 600,000 to 650,000 years old. 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.
The youngest flow (Esplanade Dam) dated with Ar/Ar or K-Ar techniques is 110,000 years old (Dalrymple and Hamblin, 1998).
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.
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.