To stand on the South Rim and gaze into the Grand Canyon is to behold an awesome immensity of time. The Colorado River has incised a 450km-long chasm that in some places is 29km wide and 1.6km deep. Visitors to Grand Canyon National Park will learn that scientists believe the canyon is about 6m years old, relatively young by geological standards.

Now a few scientists want to call time out. The canyon is not 6m years old, they say, but more like 70m years old. That would push the Grand Canyon back into the Cretaceous, the era of dinosaurs.

This debate pits a young scientist against some veteran colleagues who are decades older, and centres on an iconic feature of the American landscape. New science is being applied to very old rocks, in service of the basic question of what it is, exactly, that we are looking at when we're staring into the Grand Canyon.

"Our data detects a major canyon sitting there about 70m years ago," said Rebecca Flowers, 36, a geologist at the University of Colorado and the lead author of a paper published online by the journal Science. "We know it's going to be controversial."

Correct: her research is being met with a chilly reception from those who champion the Young Canyon.

"It is simply ludicrous," said Karl Karlstrom, 61, a professor of geology at the University of New Mexico who has made more than 50 river trips through the canyon and helped create the Trail of Time exhibit at Grand Canyon National Park, which promulgates the estimate of 6m years for the canyon's age.

"We can't put a canyon where they want to put it at the time they want to put it," said Richard Young, a geologist at the State University of New York at Geneseo who has been studying the Grand Canyon for four decades.

Wondrous though it is, the Grand Canyon doesn't seem terribly mysterious at first glance. It's a gash in the landscape with a river at the bottom. The causality seems obvious.

But Flowers and her fellow Old Canyon theorists say that what we see today in northern Arizona was originally carved in large degree by two rivers – neither of which was the Colorado.

The western part of the canyon, they say, was largely incised about 70m years ago by what has been dubbed the California river, which drained a mountain range to the west and flowed to the east, the opposite direction of today's Colorado river. The eastern part of the canyon, they say, was gouged later, around 55m years ago, by a different river.

In this scenario, the Colorado river is a bit of an opportunist, taking advantage of the pre-existing canyons.

The debate to some extent pivots on the semantic question of whether "an Ancient Grand Canyon" (as the Science paper calls it) is the same thing as the Grand Canyon of today. The Flowers paper says the depth of the ancient canyon was within a "few hundred" metres – roughly 300 metres – of today's canyon.

Karlstrom warns that the Old Canyon theory threatens to confuse the park's 5 million annual visitors: "To them, it seems like dinosaurs might have lived with humans (like the Flintstones) and that geologists do not know if Grand Canyon was carved by the Colorado River or not (it was)," he wrote in response to the new paper.

Flowers began advancing the Old Canyon scenario in 2008, and the idea has been championed by Brian Wernicke, a geologist at Caltech.

"I see all the data as aligning very nicely for an Old Canyon model," Wernicke said.

The new research combines old-fashioned fieldwork with a new laboratory technique called thermochronology. Scientists infer the temperature of ancient rocks by studying crystals that record the radioactive decay of uranium and thorium into helium.

If the temperature of the crystals was hot, as would be expected in rock buried deep in the Earth, they will show no hint of helium. But if the rock was cooler – as you'd expect if it was relatively close to the surface – there will be abundant signs of helium.

Scientists interviewed for this article believe the technique is a robust method for reconstructing ancient landscapes. But some say Flowers and Wernicke have pushed their interpretation too far.

The consensus estimate for the age of the Grand Canyon is based in part on well-dated gravel deposits at the western mouth of the canyon where the river exits the Colorado plateau. The river incises the canyon at a known rate – about 150 metres per million years, or about the thickness of a piece of paper annually, Karlstrom said. The erosion rate suggests a younger canyon, Karlstrom said. And an old, dry canyon abandoned by an ancient river would have quickly been filled with sediment, turning into little more than a ditch, he said.

Young, meanwhile, has an objection based on boulders found on the south side of today's canyon. They eroded more than 24m years ago from the cliff face of the Shivwits Plateau at the canyon's North Rim. In the years since, the cliff has receded to the north, and the Grand Canyon has formed as the Colorado flowed along the base of the cliff.

In Young's view, there can't have been a huge canyon in that spot 70m years ago, because the boulder and gravel from the Shivwits cliff would have had to jump that canyon like Evel Knievel.

Young believes the new research is recording the cooling of rocks due to the gradual recession northward of the cliff, not due to the carving of a deep chasm.

Joel Pederson, an associate professor of geology at Utah State, applauds the new paper, saying, "They are looking at a really awesome precursor canyon that the Colorado River later in time took advantage of." But that precursor canyon was not the Grand Canyon, Pederson said. How old is the Grand Canyon? "It is 6m years old," he said.

Geologists craft narratives around enigmatic landscapes. Erosion has exposed the stratigraphic record of the Grand Canyon's ancient rocks, but it's intrinsically difficult to know when that erosion happened.

"Erosion's always been the toughest problem in geology," Wernicke said, "because what you're trying to study is all gone now."As for why we should care about when and how the canyon formed, Wernicke said: "It's a fundamental question of human curiosity. It's about as basic a scientific thing as one can imagine."

• This article appeared in Guardian Weekly, which incorporates material from the Washington Post