Steve Austin presents an argument for the age of the earth in his 1994 book "Grand Canyon: A Monument to Catastrophe" on pages 87-89. Starting with an observed sediments carried by the Colorado River of 168 million tons per year, he shows, (correctly) that this represents the erosion .015 cubic miles per year. He then correctly notes that the volume eroded out of the Grand Canyon is approximately 1000 cubic miles. Dividing the two numbers he incorrectly obtains an age of 67,000 years for the time it would take to erode the Grand Canyon. Beyond the fact that this is ten times too old to fit into his young-earth scenario, Austin argues that the evolutionists are wrong to believe in millions of years for the canyon to form.
Where does Steve go wrong? Well, it is in assuming that ALL the sediment carried by the Colorado River comes from the Canyon itself. Without his reader's knowing it, Steve is saying that absolutely no erosion is occurring in Wyoming, Utah and Colorado, which, of course, is a bizarre claim to make if one were to make it explicitly. But by not mentioning this implicit assumption in his calculation, those unfamiliar with the geologic facts will be fooled into thinking Steve has proven a short erosion time for the Grand Canyon.
The Colorado River extends 1450 miles from the mouth to the source in Colorado. Only 500 miles of this length are to be found in the Grand Canyon. The drainage area of the Colorado River is 640,000 square kilometers but the Canyon has an area of only 13,000 square kilometers. Scott Mclennan (reference below) reports that the Colorado River, prior to the building of the Glen Canyon Dam, carried 121 million tons of sediment each year. This is slightly less than the study cited by Austin so we will use his larger value of 168 million tons per year so that no one can claim we are shopping for values favorable to us.
The drainage area of the Colorado is 640,000 sq. km. and the area of the canyon is only 13,000 sq km. 800 km x 16 km wide = 12,800 sq. km. Thus, on a linear weighting, one can expect that only 2 percent of the sediment actually comes from the canyon, the rest comes from the rest of the area. But let's be fair to Steve and say that 30 percent of the sediment comes from the canyon. That means that 168,000,000*.3 =50,400,000 tons per year are moved by the river OUT OF THE CANYON AREA with the rest coming from the rest of the drainage basin. To a first approximation there are 1000 kg/ton which means that 50.4 billion kg of material is moved down the river, or given a density of 2500 kg/m, we have 20.1 million cubic meters moving down the river each year. Dividing by the area of the Canyon ( which is 13,000,000,000 square meters) we have a lowering of the Canyon surface by 20.1 x 10^6/1.3 x 10^10= 1.5 x 10^-3 m per year--hardly indicating a young earth. This is about 1.5 mm per year of excavation out of the Canyon.
So how long will it take the canyon to be excavated at this rate?
20.1 million cubic meters sediment per year = .0048 cubic mile sediment/year
1000 cubic miles/ .0048 cubic mile sediment/year = 207,000 years
Only 207,000 years? Yes, but remember, we have favored Steve greatly by allowing 30% of all erosion to occur in 2% of the area and there is NO evidence that this is the case. If one scales the determines the canyon erosion by its proportional area it would take three million years to erode the canyon, a value much more aligned with the geologic evidence.
But one question arises here. Why do young-earth creationists think that a 67,000 year age (or a 207,000 year age) for the Grand Canyon indicate a young earth? I have been on this earth less than 100 years. Does my age indicate that the earth is really only 100 years old? Does the silting up of the Colorado River behind the Glen Canyon Dam (a process which has been going on since the dam was close less than 100 years ago) indicate that the earth is only 100 years old or less? Of course, this is a silly line of logic and would be roundly condemned if stated explicitly. But that is what Steve is doing with his calculation. He is claiming that something which began long after the earth was formed, limits the age of the earth.
And what he doesn't tell people is that there are conclusive evidence of long ages prior to the Canyon's erosion in the sediments of the canyon. This evidence consists of burrows in the sediment (http://www.psiaz.com/Schur/azpaleo/nacofm.html http://www.geocities.com/earthhistory/grand2.htm, footprints on sedimentary layer after layer (Lockley and Hunt,1995, p. 57), and cave erosion and collapse all occurring prior to the beginning of the canyon erosion. This last evidence is particularly interesting.
The Red-wall limestone contains collapsed caves which collapsed at a time when the entire Grand Canyon area was covered with an additional couple of hundred feet of sediment. When the cave collapsed the sediment above it fell into the cave void, filling it. This is what happens in Florida with a sink hole--a cave below the surface collapsed and whatever was above the collapse falls into the hole. At the Grand Canyon Wenrich and Huntoon (1989, p. 212) write of these pipes which occur at a rate of 6 per square mile:
"The breccia pipes formed as sedimentary strata collapsed into dissolution caverns in the underlying Mississippian Redwall Limestone. Upward stoping through the upper Paleozoic and lower Mesozoic strata, involving units as high as the Triassic Chinle Formation."
The Chinle is a Triassic bed which lies above the Moenkopi which in turn lies above the Triassic Shinarump. Just north of the Canyon the Shinarump and Moenkopi are 1900 feet thick. Thus it is possible that when the caves collapsed, the Grand Canyon was covered with as much as an additional 2000 feet of sediment which was nearly totally removed. There is a small remnant of it on the SE side of the Canyon at Cedar Mountain. Assuming that we removed this covering layer at the same rate as the Colorado River today removes sediment how long would it take? The Triassic strata has been removed over at least 20,000 square kilometers and was at least 2000 feet thick. This means that nearly 3000 cubic miles of sediment have been removed BEFORE THE CANYON EROSION BEGAN. Using the 2 percent rule we last used above, 3000/.0048 = at least 625,000 years of erosion. Of course, Steve never tells anyone about that erosional event.
It takes time for the sediment to be deposited, burrows be dug, more sediment be deposited more burrows and animal tracks to be made and then for caves to erode in solid rock and then to collapse and then for the Triassic strata to be almost completely eroded from the Canyon area AND ONLY THEN does the Canyon erosion begin.
One general comment, if Steve admits that it takes several tens of thousands of years to dig out the canyon (by his own calculation) he must then allow for other buried canyons which are found on seismic data.
Thus the age of the entire canyon sequence must be older than the length of time it takes to erode the canyon. Of course, the young-earthers can't accept this evidence. It is a shame that Christian apologetics relies on such sloppy logic to support a young-earth.
Steven A. Austin, editor, Grand Canyon: A Monument to Catastrophe, (Santee: Inst. for Creation Research, 1994)
M. Lockley and Adrian P. Hunt, Dinosaur Tracks, (New York: Columbia University Press, 1995),
Scott M. Mclennan "Weathering and Global Denudation", Journal of Geology , 101:2, p. 296
Karen J. Wenrich and Peter W. Huntoon, "Breccia Pipes and Associated mineralization in the Grand Canyon Region, Northern Arizona,"Geology of the Grand Canyon, Northern Arizona, 28th Int. Geol. Congress, Field Trip Guide Book, (Washington: AGU, 1989), p. 212