Fire and water: The making of the Grand Canyon

Natural History, Sept, 1997 by W.K. Hamblin, Laura Hamblin

"We have no difficulty as we float along, and I am able to observe the wonderful phenomena connected with this flood of lava. The canyon was doubtless filled to a height of 1,200 to 1,500 feet, perhaps by more than one flood. This would dam the water back; and in cutting through this great lava bed, a new channel has been formed, sometimes on one side, sometimes on the other.... What a conflict of water and fire there must have been here! Just imagine a river of molten rock running down over a river of melted snow. What a seething and boiling of waters; what clouds of steam rolled into the heavens!" John Wesley Powell first viewed the remnants of basalt lava adhering to the wars of the inner gorge in the western Grand Canyon more than a hundred years ago. Yet today this awesome record of relatively recent volcanic activity -- recent being 250,000 to more than one million years ago -- remains largely unknown. Each year, nearly five million tourists, approaching the canyon's northern and southern rims near the visitor centers, marvel at the two-billion-year geological record in the exposed rock and at the river's ability to erode a canyon one mile deep. Few are aware that ninety miles to the west -- in the Toroweap section -- lies an unparalleled scene of volcanic phenomena: a cycle of lava dam formation and destruction that is the most significant event in the recent history of the Grand Canyon. Were else in the world is there so much for a geologist to contemplate? Certainly, no record of volcanism is to be viewed in a more dramatic and inspiring setting.

Quiet and stark, the remote Toroweap area is accessible only by sixty-odd miles of rough gravel roads from Saint George or Kanab, Utah. Here, the walls of the inner gorge climb 3,000 feet straight up from the river to the Esplanade Platform, from which the outer rim rises another 2,000 feet. Within this inner sanctum, one can clearly see the types of forces that shape our planet. The walls of the deep canyon expose a sequence of Paleozoic strata a mile thick. Major faults are clear and distinct. Extinct volcanoes dot the landscape. Some perch on the rim of the inner gorge; remnants of others cling precariously to the walls like stranded rock climbers; eventually erosion will cause these remnants to slip off. Black falls of "frozen" lava cascade from the canyon rim to the river almost 5,000 feet below. Right on the rim of the inner gorge at the mouth of Toroweap Canyon, Vulcan's Throne, a 600-foot cinder cone, stands sentry over it all.

One remarkable manifestation of volcanism can be best appreciated from river level. It is Vulcan's Forge, a massive rock pipe through which lava once worked its way to the surface. Situated in the very middle of the river, it rises sixty to seventy feet above the water. But from the canyon rim, it appears to be only a black speck. Numerous dikes cut through canyon walls, marking the paths along which lava reached the surface. The most significant features, however, are the huge, vertical slabs of black basalt stacked side by side against the canyon walls. These are what is left of a series of ancient lava dams that once blocked the Colorado River and caused huge lakes to form in the Grand Canyon upstream. (Basalt is the most common and most fluid type of lava, and the one most often produced where tectonic plates split and move apart.)

A view from the air gives us a superb perspective from which to understand the recent volcanic activity in the western Grand Canyon. The canyon cuts across the southern margins of the Uinkaret field, a relatively small volcanic field in northern Arizona that has been the site of repeated eruptions during the last two to three million years. Numerous eruptions that occurred near the edge of the field have spilled lava into the Grand Canyon and tributary canyons, such as Toroweap, Whitmore, and Prospect. (Other volcanoes have erupted deep within the canyon.) Many of the early eruptions formed remarkable lava flows more than 3,000 feet high, roughly ten times higher than most of the great waterfalls of the world (compare Niagara Falls at 160 feet, Iguacu Falls at about 210 feet, or Victoria Falls at 350 to 400 feet). But the lava cascades tell only part of the story.

Between 1965 and 1992, with colleagues from the geology department at Brigham Young University, we studied this region along the river, eventually using helicopters to study features hidden from view at river level and enlisting a professional mountain climber to collect samples from high cliffs. Our studies show that during thirteen major periods of eruption within approximately one million years, more than 150 lava flows poured into the canyon. Each major eruption created a lava dam within the narrow confines of the inner gorge, causing the Colorado River to form a large lake extending far upstream.

From the sequence of basalts preserved in the inner gorge, we clearly see that four different types of dams formed during the major periods of volcanic activity. Unlike their human-made counterparts, these dams were created by long, thick lava flows that extended for miles downstream. The first lava dams -- more than one million years old -- were built from several huge, massive flows, each more than 500 feet thick and ten to twenty miles long. Reaching a cumulative height of as much as 2,000 feet, these probably were the highest dams ever in the Grand Canyon.