John Wesley Powell wrote in 1895: “...what a conflict of water and fire there must have been [in western Grand Canyon]! Just imagine a river of molten rock running down over a river of melted snow.”
Over 110 years later, a synthesis of new and existing dates on these lava flows shows that many are significantly younger than initially thought and all are less than 725 thousand years old. The geochronology data indicates four major episodes when lava flows either erupted into the canyon or flowed over the rim into it: 725-475 thousand years ago (ka), 400-275 ka, 225-150 ka, and 150-75 ka.
These flows formed lava dams in western Grand Canyon that had dramatic impact on the Colorado River.
This paper* presents light detection and ranging (lidar) data to establish the elevations of the tops and bottoms of basalt flow remnants along the river corridor. These data show the original extent of now-dissected intra-canyon flows and aid in correlation of flow remnants.
From 725 to 475 ka, volcanism built a high edifice within Grand Canyon in the area of the Toroweap fault, with dike-cored cinder cones on both rims and within the canyon itself. These large-volume eruptions helped drive the far-traveled basalt flows which flowed down-canyon over 120 km. A second episode of volcanism, from 400 to 275 ka, built a 215-m-high dam along the Hurricane fault, about 15 km downstream.
The ca. 200 and 100 ka flows (previously mapped as Gray Ledge) were smaller flows and lava cascades that entered the canyon from the north rim between the Toroweap and Hurricane faults.
The combined results suggest a new model for the spatial and temporal distribution of volcanism in Grand Canyon in which composite lava dams and edifices were generally leaky in proximal areas.
Available data suggest that the demise of volcanic edifices may have involved either large outburst-flood events or normal fluvial deposition at times when the river was established on top of basalt flows. These data highlight complex interactions of volcanism and fluvial processes in this classic locality.
This research, authored by Ryan Crow (University of New Mexico) et al. was published in the February issue of Geosphere, published by the Geological Society of America.
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