Erosive Effects of Vacuum Bubbles (Sidebar 3)

Excerpt from Grand Canyon: Monument to Catastrophe

...Together, Hopi, Canyonlands, and Vernal Lakes would occupy about one-fourth of the drainage basin above Grand Canyon (an area of over 30,000 square miles). The volume of water contained in these lakes is estimated to have been about three times the volume of Lake Michigan (over 3,000 cubic miles).⁶³ The drainage of these lakes through Grand Canyon would have caused the significant downcutting through the plateaus of northern Arizona. As catastrophic erosion of the dam proceeded, large landslides from side canyons, and sapping, caused by outflow of water from wet strata, would enlarge the Canyon. The drainage of these three lakes through the Canyon would keep it free of obstructions.

Figure 6

The breached dam theory for the origin of Grand Canyon can be integrated into a Biblical Flood model. Late in the Flood, and in the immediate post-Flood, the Kaibab Upwarp was formed. This upwarp blocked the drainages of the Colorado Plateau. In the post-Flood period, possibly hundreds of years after the Flood, enough water had built up in these lakes, that dam failure could occur. Forests and animals appear to have been living on the plateau when the dam was breached. This breaching event may explain the unusual distribution of the tassel-eared squirrels on the Kaibab and Coconino Plateaus (see chapter 8).

Rapid Erosion of Bedrock
How much erosion could occur as these three lakes, containing 3,000 cubic miles of water, breached their dams? Could significant erosion of hundreds of cubic miles of both strata and crystalline basement rock occur within Grand Canyon? Many people, no doubt, will question the breached dam theory because of the inability to conceive of such massive erosion in so short a period of time. Can solid rock be eroded rapidly?

The question concerning the magnitude and speed of erosion of solid rock has already been the subject of an intense geologic debate, which lasted over 40 years (1923-1962). The flood controversy for the Channeled Scabland of eastern Washington centered on whether catastrophic water flows could create enough scouring and irregular plucking of basalt bedrock to form what are today dry, deeply incised channels.⁶⁴

J. Harlen Bretz proposed, in 1923, that channels hundreds of feet deep were eroded by a catastrophic flood. The controversy became especially heated, after Bretz⁶⁵ provided evidence, in 1932, that Grand Coulee, of Washington State, (Figure 5.11) was eroded by catastrophic drainage of Lake Missoula, in Montana. He proposed that Grand Coulee, a 50-mile-long trench from one to six miles wide and up to 900 feet deep, was eroded catastrophically! That would involve rapid erosion of many cubic miles of basalt from Grand Coulee.

Figure 5.11 Click image for larger view (47K)
Grand Coulee in Washington State. The severely eroded landscape is viewed toward the northeast, in this high-altitude, oblique, aerial photo. Water from the ice-blocked channel of the Columbia River was diverted southwestward, as a flood through the Upper Grand Coulee. The Flood eroded the trench now occupied by Bank Lake (the two-mile-wide lake in top center of photo). The flood proceeded southwest, forming the severely scoured landscape of the Lower Grand Coulee (lower half of photo). The western edge of the flood-scoured zone is the 1000-foot-high cliff on the west side of Park Lake (lower left corner of photo). The boundary between the upper and lower Coulee is Dry falls—a broad vertical escarpment (center of photo) up to 350 feet high and five times the width of Niagara Falls. Water from this flood was estimated to have had a depth of over 200 feet, and velocity of over 60 feet per second. Imagine how puny Niagara Falls would appear next to Dry Falls in its prime! (Photo copyright by Leonard Delano.)

There were many critics of the Lake Missoula Flood between 1930 and 1960. Even some of the most famous geologists of that time remained solidly uniformitarian, and refused to believe in massive-flood erosion in eastern Washington. That controversy formed one of the most interesting episodes in the history of geology. V. R. Baker wrote:

The role of floods in the erosion of stream channels has been one of the most controversial topics in fluvial geomorphology. . . . Indeed, the famous Spokane flood debate, concerning the effects of the greatest known freshwater floods on the planet,. . . . centered on the issue of the erosive capability of running water. . . . Those who disbelieved the flood theory of J Harlen Bretz did so out of their experience that rivers did not behave as Bretz proposed. Subsequent work showed that their experience, not Bretz's theory, was inadequate.66

The debate concerning the magnitude and speed of flood erosion of bedrock was resolved, largely by investigations of cavitation and plucking, two processes which dominate erosion during catastrophic floods. Figure 5.23 shows these processes. Cavitation, the rock-pulverizing process associated with fluid flows greater than 30 feet per second (20 miles per hour), occurs as the fluid detaches from irregularities in the bedrock channel. The detachment of the fluid produces vacuum cavities ("bubbles"), which implode. The cavitation process inflicts explosive, hammerlike blows on the bedrock surface, with pressures ranging as high as 30,000 atmospheres (440,000 pounds per square inch).⁶⁷ The extreme pressure which can be delivered by cavitation is many times greater than the compressive strength of rocks. Therefore, rocks are literally converted to powder by the cavitation process. Because of its obvious catastrophic effects, engineers take deliberate steps to avoid the conditions which cause cavitation.

Plucking is the second erosive process which causes extremely rapid erosion in bedrock channels. High-velocity flows are able to rip loose large blocks of bedrock along bedrock-joint surfaces (Figure 5.23). Once a large block of bedrock is dislodged, the high-velocity flow is able to move and abrade it.

Perhaps the most energetic phenomenon associated with macroturbulent flow is the production of a "kolk," the underwater equivalent of a tornado.⁶⁸ The vortex of water (figure 5.23) producing a kolk has very low pressure beneath the flowing water. The suction power of the kolk exerts intense hydraulic lifting forces and can remove or pluck slabs of bedrock from the channel.

A number of features eroded by the Lake Missoula Flood, as well as those eroded by modern catastrophic floods, have been attributed to macroturbulent flow processes.⁶⁹ Close study of some of the hard-rock erosion features which formed recently at Mount St. Helens also gives us a heightened appreciation of plucking as a potent geologic process (see figure 5.14). Evidence for rapid breaching of natural lava dams in western Grand Canyon is noteworthy and is the subject of ongoing studies.⁷⁰

Figure 5.14 Click image for larger view (50K)
Loowit Canyon, north of Mount St. Helens, which was eroded through solid rock after the summer of 1980. Note the small stream in the canyon and the waterfall in upper left. Depth of Canyon is 100 feet. (Photo by Steven A. Austin.)

Reference

Austin, Steven A. editor. 1994. Grand Canyon: Monument to Catastrophe, Institute for Creation Research, Santee, CA. p.104, 106.