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Grand Canyon 20 million years old, study says

March 6, 2008
Courtesy Science
and World Science staff

New ev­i­dence from cave forma­t­ions through­out the Grand Can­yon sug­gests water began to carve the vast chasm at least 17 mil­lion years ago, re­search­ers say. That’s about 11 mil­lion years old­er than some ev­i­dence had in­di­cat­ed.

Marble Canyon at Vasey’s Paradise in the eastern Grand Canyon. (Courtesy Viktor Polyak)
The “in­ci­sion his­to­ry” of the can­yon, the im­mense gorge of the Co­lo­rado Riv­er in north­ern Ari­zona, has been dis­put­ed for more than a cen­t­ury. 

That is in part be­cause some of the more com­mon meth­ods for dat­ing the ge­o­log­i­cal event don’t reach back more than about a mil­lion years ago. 

In new re­search, Vic­tor Pol­yak and col­leagues at the Uni­ver­s­ity of New Mex­i­co used im­prove­ments in a tech­nique known as ura­n­i­um-lead iso­tope dat­ing to meas­ure the ages of for­m­a­t­ions in the can­yon known as mam­mil­lar­ies or “cave clouds.” These are de­po­sits of the salt car­bon­ate that form at or near the wa­ter ta­ble lev­el. 

As­sum­ing these pro­vide a rec­ord of a drop­ping wa­ter ta­ble as the can­yon deep­ened, the re­search­ers found that the can­yon is old­est on its west­ern end and opened up steadily to the east through head­ward ero­sion. 

The can­yon’s east­ern part was cut at a much faster rate than the west­ern part, and the can­yon was probably com­pletely sliced through 5 to 6 mil­lion years ago, the sci­en­tists said. Their study ap­pears in the March 6 is­sue of the re­search jour­nal Sci­ence.

An ear­li­er es­ti­mate, that the can­yon is 6 mil­lion years old, was based on “in­di­rect ge­o­log­i­cal ev­i­dence for the initia­t­ion of the through-flowing Col­o­rad­o River,” wrote sci­en­tists with Uni­ver­s­ity Col­lege Lon­don and Uni­ver­s­ity of East An­glia, U.K., in an ac­com­pa­ny­ing com­men­tary in the jour­nal.

How­ev­er, many re­search­ers have doubted this shorter time frame, added the com­men­ta­tors, who weren’t in­volved in the stu­dy. That’s be­cause the 6 mil­lion years did­n’t fit with the rate at which the wa­ter was es­ti­mated to have cut the rock. “Many ge­ol­o­gists have long sus­pect­ed” a long­er time frame such as 20 mil­lion years, they added, but the new study “uses an in­gen­ious com­bina­t­ion of meth­ods to dem­on­strate it firmly for the first time.”

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New evidence from cave formations throughout the Grand Canyon suggests that the vast chasm began to open at least 17 million years ago, researchers say. That’s about 11 million years older than some indirect evidence had indicated, according to scientists. The canyon’s “incision history” has been disputed for more than 100 years, in part because some of the more common methods for dating the geological event don’t reach back more than about a million years ago. In new research, Victor Polyak and colleagues at the University of New Mexico used improvements in a technique known as uranium-lead isotope dating to measure the ages of formations in the canyon known as mammillaries or “cave clouds.” These are deposits of the salt carbonate that form at or near the water table level. Assuming these provide a record of a dropping water table as the canyon deepened, the researchers found that the canyon is oldest on its western end and opened up steadily to the east through headward erosion. The canyon’s eastern part was cut at a much faster rate than the western part, and the canyon was probably completely cut through 5 to 6 million years ago, the scientists said. Their study appears in the March 6 issue of the research journal Science. An earlier estimate, that the canyon was 6 million years old, was based on “indirect geological evidence for the initiation of the through-flowing Colorado River,” wrote scientists with University College London and University of East Anglia, U.K., in an accompanying commentary in the journal. However, many researchers have doubted this shorter time frame, added the commentators, who weren’t involved in the study. That’s because the 6 million years because it didn’t fit with the rate at which the water was estimated to have cut the rock. “Many geologists have long suspected” a longer time frame such as 20 million years, they added, but the new study “uses an ingenious combination of methods to demonstrate it firmly for the first time.”