To an unfamiliar person in this respected field, the Grand Canyon may simply appear to be a giant hole in the ground. But for most scientists, the American Southwest's one of a kind landmark is increasingly, at an astounding rate, looking like multiple ancient canyons of different ages, compiled together by erosion that occurred nearly six million years ago, and ultimately sculpted into its current form and beauty.
A recent study published in the journal Nature Geoscience, combined with more than a century's worth of research in the field, is assisting researchers in the deciphering of a geological event that started when dinosaurs roamed its very own turf. For almost 150 years, scientists have been debating how and when the Grand Canyon formed, says Karl Karlstrom, a geologist at the University of New Mexico in Albuquerque. In recent years they've nearly completely split into two ways of thinking: those suggesting a "young canyon" theory in which the Colorado River alone shaped most of the gorge in the past, relatively recent, five million years or so. Then there are those implying an "old canyon" theory in which multiple ancient rivers carved ancestral canyons along more or less the same route as a series of carvings. Recent research by Karlstrom and his colleagues express the concept that what actually occurred lies somewhere between these two extremes. To help estimate ancient erosion rates, the team turned to thermochronology. Thermochronology is the study of how a rock's temperature has changed through its history. Since temperature rises as depth in the Earth's crust increases, a rock's thermal history gives insight to when, and how rapidly, the terrain above it washed away due to erosion. This is especially important in accurate dating of the rocks due to the inconsistency of carbon dating.
In the recent study, the scientists used several techniques to analyze samples of phosphate-bearing minerals taken from four of the five major subsections of the Grand Canyon, both from river level and from the canyon rim, which on average lies nearly 1.5 kilometers (a mile) above the river down below. One of the techniques is called apatite fission-track dating. This entitles the tedious yet necessary action of counting the number of paths carved through apatite crystals by high-speed alpha particles (helium nuclei) emitted during radioactive decay. Because the atoms in these crystals tend to shift and heal such defects at temperatures above 230°F (110°C), the number of paths remaining in minerals today gives researchers a sense of how long ago the rocks cooled below that threshold. Similarly, by analyzing the amount of radiogenic helium trapped in apatite crystals, the researchers are then able to estimate relatively how long ago the minerals cooled to a temperature below 86°F (30°C). Lastly, by measuring the length of fission tracks still left in apatite crystals, their team figured out just how long those minerals being tested had remained at baseline temperatures.
Similarly to multiple earlier researchers, Karlstrom and his colleagues discovered that unlike parts of the canyon formed at different times. The Hurricane segment, one of the oldest segments, was named after a famous geological fault, can be found in the western portion of the canyon. The data suggest that this section of the gorge had been carved to about half its current depth between 70 million and 55 million years ago.But the researchers posit that erosion hadn't started etching a section that geologists have dubbed "Eastern Grand Canyon," immediately downstream of where the Little Colorado River joins the Colorado, until some 25 million years ago. And the team's analyses suggest that the westernmost and easternmost segments of the canyon were largely carved in the past five million or six million years.
Karlstrom and his colleagues argue that while the Hurricane and Eastern Grand Canyon segments were originally sculpted by different rivers, the Colorado took over the job in the past six million years, joining the disparate canyons and carving them wider and deeper. "Overall, I think they've done a really good job," says Peter Reiners, a geochemist at the University of Arizona in Tucson. "This new model isn't just a compromise of all previous notions; it's recognition that a big river can have a complicated history." However, not everyone is so easily swayed to believe in this kind of data collecting. Brian Wernicke, a geoscientist at the California Institute of Technology in Pasadena, reveals that interpreting thermochronology data, especially fission-track data in terrain where erosion carves downward as well as sideways, is most simply put, difficult. "The new model seems to be much too complicated," he says. However, personally, I feel thermochron