Into the Great Unknown: Vulcan the Fire God says "You Call That Little Piece of Concrete a Dam?"

Dam engineers sure love their dam creations. The Colorado River, being the only river of note draining the Colorado Plateau, was a target of their fevered dreams, and major projects have "tamed" the river, most notably at Hoover Dam/Lake Mead, and Glen Canyon Dam/Lake Powell (above). Those who administer the giant concrete plugs love to cite the statistics: Hoover holds back 28 million acre feet, Powell 24 million, Mead is 112 miles long when full, Powell is 186 miles. It can't be denied that the concrete monsters have had a huge effect on the ecosystems of the river. Glen Canyon is entirely submerged. The river downstream runs cold all year, and surges high and low in response to electrical production needs. Native fish and flora struggle to survive in the new regime.

We were at the end of our thirteenth and starting the fourteenth day of our journey into the Great Unknown, a rafting trip down the Colorado River from Lees Ferry to Diamond Creek. I had taken an involuntary swim through Lava Falls Rapid that afternoon, but with the swim having been a far less terrifying experience than the first flip back at Crystal, I was feeling okay. Passing through Lava Falls represents to many the climax of the trip, and the last two or three days are sort of a winding down of the journey, with few large rapids.

For me though, the last two days were some of the most astounding because we had reached the site of one of the most extraordinary geological stories in the entire history of the Grand Canyon. Visitors to the main tourist areas on the north and south rims of the canyon never see the rocks that lined the canyon walls around us, and are often surprised to find they exist at all: miles and miles of basaltic lava flows!

The edge of the Colorado Plateau is punctuated by a series of north trending extensional ("normal") fault zones. They represent the boundary zone between the thick crust of the Colorado Plateau, and the thin extended crust of the Basin and Range Province that reaches across Arizona, Nevada and eastern California. When the crust stretches and breaks, pressure is released in the Earth's mantle below, allowing partial melting of the hot pliable rock. The resulting magma follows the fault zones to the surface. Between 1.8 million and just 1,300 years ago, at least 150 eruptions took place in the vicinity of the western Grand Canyon, covering 600 square miles, forming the Uinkaret Volcanic Field.

Most importantly, at least 13 of these flows spilled over the edge of the canyon and filled the canyon bottom. Vulcan, the fire god, had built his own version of Bureau of Reclamation dams. They weren't small dams. They were hundreds of feet high, and one topped out at least 2,500 feet (Glen Canyon Dam is 710 feet tall). It was the remnants of these lava flows and lava dams that surrounded us as we floated down the river. It was the first time I had seen these rocks. I was mesmerized (yes, we geologists are a strange lot).

What's even more incredible are the lakes that formed behind the dam. The largest dam formed a lake that backed the river up into Utah. If it happened today, the lava dam lake would inundate Lake Powell. It would make for a long hard rafting journey, but the rapid at the end would have been memorable...

Even more mysterious would be how the lakes met their end. It's still the subject of some research, but evidence suggests that at least five of the lava dams failed catastrophically, collapsing and ending the lake in days rather than years. What kind of evidence? The most compelling would be river deposits containing basalt boulders 115 feet across. How do you move boulders that big?

The amount of water unleashed on the lower canyon by such a failure is almost unimaginable. A modest 'fake' flood produced by releasing water from Glen Canyon Dam up the river might involve flows of 40-45,000 cubic feet per second. The largest historically recorded flood (in 1884) produced flows of about 300,000 cfs. A researcher has found evidence of a flood of 400,000 cfs around 4,000 years ago. Estimates of major floods during the Pleistocene ice ages range in the vicinity of a million cubic feet per second.

The collapse of a 1,500 foot tall lava dam may have produced a flood of 15 million cubic feet per second. That's more than 30 times larger than the biggest flood ever recorded on the Mississippi River. That's how you move 115 foot boulders.

Pictures of gigantic floods filled my imagination as we drifted past lava flow after lava flow. At first, the most vivid outcrops were the flows that had spilled over the rim in the vicinity of Lava Falls and Whitmore Wash. As we floated downstream, the basalt flows tracked along the river, forming low cliffs that went on for miles. The longest flows traveled more than sixty miles down the river bed.

In places the lava flows were thick enough to develop columnar jointing, similar to places like Devils Postpile in California or the Giant's Causeway in Ireland. The columns form when the lava flow pools and then contracts while it cools. The contraction causes the vertical fractures to develop, and they characteristically form hexagonal columns or sometimes rosettes. All in all, the day had been fascinating.

Lava wasn't the only feature of the day. At Whitmore Wash we had a chance to hike up to some interesting pictographs on a sandstone panel a few hundred feet above the river.

 The view up the river was fantastic...

We camped at the very creatively named 202 Mile Camp. While I was cooking, my nephew came up to report that the bank of the river was collapsing. I wandered down to have a look and found that an underwater slide was causing large slabs of sand to be pulled towards the river, forming a large arcuate landslide scarp. After seeing a gigantic normal fault to begin the day, it was interesting to see a small-scale version of the same kind of faulting along the riverbank. The little collapsed block in the center would be called a graben.

The slide ultimately ate up a lot of the shoreline, more than 30 feet, and it was clear that a lot of sand was being lost to the deeper part of the river channel. Ever since the floodgates of Glen Canyon Dam closed in 1963, sand has been disappearing along all the shorelines of the river. There have been a few attempts to produce artificial floods that have temporarily moved sand back onto the beaches, but without the sand that is now trapped in Lake Powell, the beaches are going to continue to disappear.

At the end of the day, the moon made an appearance. It was the first we had seen of it on pretty much the entire trip. I had enjoyed seeing the Milky Way each night, and the moon would have obscured many of the stars, but it was nice to see the beautiful crescent setting over the basalt cliffs.

With the last of the twilight, I hit the sack, realizing we were down to our final two days on the river. Our takeout at Diamond Creek was only 24 miles downstream.

Time Almost Not Beyond Imagining: Recent Volcanism on the Colorado Plateau Part 2

photo by Doc Searls via http://en.wikipedia.org/wiki/File:VulcanUinkart.jpg

Photo by J.R. Balsley, taken in 1950, courtesy of the USGS

We continue with my long-running exploration of the geological history of the Colorado Plateau...

Once a region has risen high above sea level, and rivers are established to convey sediment, one might assume that the eventual result will be the complete removal of the rocks and the erosion of the region to a near sea level peneplain, at least until another cycle of uplift begins. Certainly that was the attitude of geomorphologists at the end of nineteenth century and the beginning of the twentieth century. Relationships exposed in the Grand Canyon seemed to bear this out to an extent, with the exposure of two major unconformities that suggested just such erosional smoothing of the landscape. It was all so cyclical, and so uniformitarian. As the decades passed, it became clear that crustal evolution was not nearly so neat and tidy, and that the geological history of any region was cluttered with all manner of untidy complications.

In the far west section of the Grand Canyon, faults related to the Basin and Range extension slice through the plateau with vertical offsets of hundreds, even thousands of feet. In addition to scrambling the landscape, they also provided a convenient pathway to the surface for magmas generated deep below in the upper mantle. Some of the resulting lava flows are easily seen near St. George and Zion National Park where they are a spectacular addition to the scenery. But, one of the most profound places I have never seen (I had to borrow the photos) contains one of the most bizarre parts of the Grand Canyon story. It's where lava flows invaded the realm of the Colorado River: the Uinkaret Volcanic Field.

Lava flows cover some 600 square miles in the Uinkaret field, and some of the flows spilled over the canyon rim and reached the Colorado River. Major John Wesley Powell, on his epic journey down the river in 1869 recognized the evidence of what must have been a spectacular sight:

"August 25, 1869 Great quantities of lava are seen on either side; and then we come to an abrupt cataract. Just over the fall a cinder cone, or extinct volcano, stands on the very brink of the canyon. What a conflict of water and fire there must have been here! Just imagine a river of molten rock running down into a river of melted snow. What a seething and boiling of the waters; what clouds of steam rolled into the heavens. "

Geologists who have been able to make more comprehensive studies of the region have come to a more startling conclusion. Lava flows didn't just reach the river. They dammed the river. Among the 150 individual lava flows in the Uinkaret from 1.8 million to just 1,300 years ago, at least 13 spilled over the cliffs into the river gorge, accumulating enough bulk to dam the stream. These were not modest dams: one of them was more than 1,500 feet high, enough to form a lake reaching hundreds of miles upstream into Utah. Such lakes would have been easily visible from the rim of Grand Canyon National Park. The past existence of the dams was recognized early, in the late 1800's. What was not so well understood is what happened to them. It was assumed that the dams were simply eroded away over several thousands of years.

Anyone who has looked closely at a lava flow will realize that they are not as solid and monolithic as one might think. The cooling lava fractures and splinters, and in water will form loose globular masses called 'pillows', or simply explode into sand-sized particles. In other words, not the first choice of dam-building material that a dam engineer might choose.

Deposits exist high on the walls of the western Grand Canyon which contain large boulders of basalt derived from the lava flows. One of them is 115 feet across. How do you move boulders of this size? The most current research is suggesting that at least five of the dams failed catastrophically; once they were breached by the river, they fell apart in a matter of days or weeks. The amount of water unleashed on the lower canyon is almost unimaginable.

A modest 'fake' flood produced by releasing water from Glen Canyon Dam up the river might involve 40-45,000 cubic feet per second flows. The largest historically recorded flood (in 1884) produced flows of about 300,000 cfs. A researcher has found evidence of a flood of 400,000 cfs around 4,000 years ago (my memory may be failing me, but I think it involved flood debris in a cave high above the river which also contained human artifacts). Estimates of major floods during the Pleistocene ice ages range in the vicinity of a million cubic feet per second.

The collapse of a 1,500 foot tall lava dam may have produced a flood of 15 million cubic feet per second. That's more than 30 times larger than the biggest flood ever recorded on the Mississippi River.

That's how you move 115 foot boulders...and carve a 2,000 foot deep gorge in the last 700,000 years or so.

Fenton, C.R., Webb, R.H., Cerling, T.E., Poreda, R.J., and Nash, B.P., 2002, Cosmogenic 3He Ages and Geochemical Discrimination of Lava-Dam Out-burst-Flood Deposits in Western Grand Canyon, Arizona, in House, K. et al., eds., Paleoflood Hydrology, American Geophysical Union, p. 191–215.

Hamblin, W.K., Late Cenozoic lava dams in the western Grand Canyon, 135 pp., Geol. Soc. Amer. Memoir 183, 139 pp., 1994.