Abstract

The geomorphic response to volcanic incursions is spectacularly documented in western Grand Canyon, where numerous Quaternary lava flows dammed the Colorado River. This paper uses new 40Ar/39Ar ages, geochemistry, paleomagnetism, and field relationships to suggest 17 damming events, requiring major revision to previously published intracanyon flow sequences. From ca. 850 to 400 ka and at ca. 320 ka, numerous lava dams formed near the modern-day Lava Falls area. Starting around 250 ka, major volcanism shifted to the Whitmore Wash area, where additional dams formed. From ca. 200 to 100 ka, cascades flowed over the north rim in areas between Lava Falls and Whitmore Wash to form the youngest set of lava dams.

Field observations and new dam reconstructions require a new model for how the Colorado River interacted with ephemeral lava dams in Grand Canyon. Specifically, the structure of lava dams, the position, character, and provenance of basaltic gravels within and above dams, and cooling structures in intracanyon flows suggest that unstable upstream dam portions failed quickly, while stable downstream dam segments were dismantled by the Colorado River more slowly. Time scales of dam removal are hard to assess, but we infer that lava dams that are overlain by monomictic basalt gravels were removed by the river in tens of years to centuries. In contrast, dams overlain by far-traveled gravel may have persisted for millennia.

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