History

The Grand Canyon May Have Appeared During the Time of the Dinosaurs

Apatite says so

By Stefan Anitei, Science Editor

16th of April 2008, 07:05 GMT

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It is the symbol of the American Southwest and western movies. But how old is the Grand Canyon? A new research suggests that it could have started to form during the dinosaur era.

The over a mile (1.6 km) deep canyon in Arizona is the work of the Colorado River digging through ancient rock layers

when the plateau of the canyon was uplifted. This started millions of years ago. One research published one month ago dated this to have occurred 16 - 17 million years ago. A new study carried out at Caltech published in the May issue of the Geological Society of America Bulletin has found a much older age: 55 million years ago, and it started with younger rock layers than those exposed at the surface of the canyon today.

The team focused on the analysis of the mineral apatite from the canyon's ancient sandstone walls. Apatite comprises the radioactive elements uranium and thorium, which break down into helium atoms. The ratio of these elements reveals the age of the apatite grains. These crystals emerge deep in the Earth crust, where temperatures are much higher than at the surface: each mile of depth means a temperature rise of 72� F (26� C for 1 km). Apatite having temperatures over 158 F (70� C) releases the helium, while under this temperature, the helium is trapped. Thus, the analysis of the apatite grains can reveal the last time the rock layer was found deep underground.

"The dates the team got from samples taken from the bottom of the Upper Granite Gorge and the top of the surrounding plateau show that the two different rock sequences were both about 131 F (55 C) from 55 to 28 million years ago, then cooled to near-surface temperatures about 15 million years ago," said co-author Brian Wernicke.

The results point that the canyon must have started at least 55 million years ago in younger rock layers, now eroded, atop the southwestern plateaus.

"Because both canyon and plateau samples resided near the same depth since 55 million years ago, a canyon of about the same dimensions as today must have existed at least that far back, and possibly as far back as the time of the last dinosaurs at the end of the Cretaceous period 65 million years ago," said Wernicke.

An accelerated erosion seems to have started 28 to 15 million years ago, decreasing the height of the already-existing canyon and the surrounding plateaus from the upper Mesozoic-era strata to the older Paleozoic rocks, as it is now. Previous theories say the upper rock layers were eroded away before the formation of the canyon.

TAGS:

canyon | rock | river

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