Looking upstream near Mile 186 at the lava cascade emanating from The Esplanade, 3000 feet above the Inner Gorge, down Whitmore Wash.  This lies directly above the Hurricane fault.  These lava flows periodically dammed the Colorado River on at least 26 occasions, beginning about 1 million years ago.

Aerial oblique view of Vulcan’s Throne and Lava Falls, near Mile 179.  Vulcans Throne is a 700 feet high cinder cone that erupted along the Toroweap fault about 10,000 years ago.  Lava Falls lies directly below.

I am indebted to discussions with Professor Kenneth Hamblin at Brigham Young University, who did the pioneering work on unraveling the sequence of lava dam emplacement and removal in the western Grand Canyon.  The following figures are taken from my article Evidence of Cataclysmic Erosional Events in the Grand Canyon of the Colorado River, Arizona, written by myself and Marv Pyles for the Second Conference on Scientific Research in the National Parks held in San Francisco in November 1979 (and published in 1980):

This is a sketch of the oldest lava flows exposed at Lava Falls near Mile 179, along the Toroweap fault.  These flows are about 1 million years old (1Ma).  There appeared to be about 150 feet of vertical offset along the fault since the flows were deposited.  This same scarp is only about 16 feet high on Vulcan’s Throne, 3000 feet above.

My conceptual sketch of how ancient Toroweap Valley would have appeared at the beginning of Lower Canyon Flow time, about 1.16 Ma. We opined that the Colorado River was probably about 300 feet higher than present, which was not the accepted belief at the time. Today, it is accepted that the Colorado River in this area is incising at a rate of about 0.4 mm/yr, or about 1.3 feet every 1000 years.

Conceptual sketches illustrating the emplacement of Lava Flows A and B at Toroweap, and their subsequent overtopping and planation by the proto Colorado River.  The remains of high energy sediments can be seen on top of B Flow on the right bank at Lava Falls.

This illustrates the emplacement of C and D flows at Toroweap.  A cobble layer lies upon a planated surface above C flow, similar to that observed above B Flow.

This shows E and F flows cresting at Toroweap, creating a composite lava dam about 1138 feet above river level.  The sediment (shown in orange) must have accumulated to at least the height of D flow to have deposited the cobbles on its planated surface.

Generalized section through an abandoned channel of the Colorado River near Mile 183, taken from Hamblin (1969) with some corrections I made in 1979.  Prof. Hamblin subsequently incorporated these and many other refinements in his final synopsis of the lava dams, published in GSA Memoir 183 in 1994.

Sketch depicting the hiatus between Lower and Middle Canyon Flows at Toroweap, when the lava dam was removed and two side canyons emanating from Toroweap Valley excavated through Flows A thru F.

Typical negative slope observed in retreat of basalt cliffs in the western Grand Canyon.  This is caused by the pervasive columnar jointing and spring sapping along intermittent cobble beds (shown in yellow).

Conceptual sketch of the initial stages of erosion of a long, low lava dam via overtopping.

Conceptual sketch of toe-headward removal of a long, low lava dam.  Note plunge pool scour of the underlying channel bed.

Large composite ash/flow lava dam showing snout of furthest flow downstream at far left, up to 75 miles downstream.  From a geologic perspective these lava dams were short-lived structures.  The highest dam at Toroweap appears to have reached about 2460 feet above the ancient river level, about 3-1/2 times higher than Hoover Dam!

Conceptual sketch illustrating the onset of erosion at the distal snout of a lava dam when the dam is initially overtopped.

Conceptual sketch of the waterfall and plunge pool enlarging itself as it retreats upstream.  The flows may actually have been removed as a series of toe-headward cuts, where successive flows lapped onto one another.  A modern-day corollary can be observed today in Iceland along the Mid-Atlantic Rift, where water drops over receding vertical falls.

Near the apex of the lava dam the overpour and plunge pool may have achieved their maximum dimensions. If the basalt was pervasively jointed, an enormous wedge failure could be expected, extending at about 50 degrees upward from the undercut toe, as sketched here.

Surface profile of Colorado River from Mile 145 to 200, with the mean gradient dashed in (between Lees Ferry and the Grand Wash Cliffs).  The channel profile upstream of Toroweap (Mile 179) exhibits an abnormally low gradient, suggesting hydraulic choking in vicinity of Lava Falls and Prospect Canyon, which has overwhelmed the channel with detrital debris.

Questions or comments on this page?
E-mail Dr. J David Rogers at rogersda@umr.edu.