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Earth Notes: The Colorado River and Earth’s Mass Extinctions

Grand Canyon Trail of Time - Coconino Sandstone 2111
NPS photo by Michael Quinn
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Grand Canyon National Park
Detail of Fossil Footprint Exhibit. The Trail of Time is an interpretive walking trail that focuses on Grand Canyon's vistas and rocks, encouraging visitors to ponder, explore, and understand the magnitude of geologic time and the stories told by canyon's rock layers and landscapes. Walking the trail is intended to give park visitors a visceral appreciation for the magnitude of geologic time.

The Colorado River is young by geologic standards—only five or six million years. But the channel it cuts through the Colorado Plateau exposes rocks that are much older. At the bottom of the Grand Canyon, river runners can spot ancient bedrock that formed nearly two billion years ago. In that vast span of time, the Earth experienced five mass extinctions. But the signs of those cataclysms are mostly hidden.

Take the Earth’s most famous extinction event: the asteroid that killed the dinosaurs. It struck on the Yucatán Peninsula in Mexico 66 million years ago and scattered debris all over the planet. Temperatures spiked and forests on the Colorado Plateau burst into flame. Dust blocked out the sun, leaving the world in darkness. When the dust settled, the world had changed. Ferns covered the Colorado Plateau where the forests used to be, and small mammals replaced the dinosaurs.

Nearly all signs of this catastrophe have been washed away from the Colorado River watershed. But at the very top of the river, one can see sediments laid down in the Cretaceous period just before the dinosaurs met their doom. As the river flows downhill, it also digs deeper into time, to earlier extinction events that took out trilobites, sea lilies, and prehistoric fish covered in armored plates.

The river’s journey is a reminder that the Earth is always changing, even when the signs of change are locked away in stone.

Planetary scientist David Kring will speak about the Colorado River and Earth’s mass extinction events tonight at Flagstaff’s Lowell Observatory, as part of Colorado River Days.

This Earth Note was written by Melissa Sevigny and produced by KNAU and the Sustainable Communities Program at Northern Arizona University.

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Melissa joined KNAU's team in 2015 to report on science, health, and the environment. Her work has appeared nationally on NPR and been featured on Science Friday. She grew up in Tucson, Arizona, where she fell in love with the ecology and geology of the Sonoran desert.
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