A great mud boom 400 million years ago changed features such as the shape of rivers and the planet’s biodiversity, writes Laura Poppick for Knowable Magazine and The Atlantic.
Since the 1960s, geologists have noticed that rivers that flowed before plants arrived on land often look different in the geological record than those that formed after continents greened. The earliest rivers resembled the ones that today tumble along the gravelly coast of Alaska, says Taylor Perron, a geologist at the Massachusetts Institute of Technology who wrote about the factors that control landscape formation in the 2017 Annual Review of Earth and Planetary Sciences.
Those gravelly Alaskan rivers have many channels that braid across sand banks, continually slumping and forming more channels as they periodically overflow—like rivulets at the edge of a beach. Without anything anchoring these riverbanks in place, they continuously collapse to form new channels. But for ancient rivers elsewhere, the arrival of plants kept similar erosion at bay—and mud added to the riverbanks’ cohesion—so rivers were less likely to slump into those braided forms. Instead, they developed single channels that meandered through the landscape in cohesive “S” shapes, like parts of the Mississippi and Amazon rivers do today. In this sense, the arrival of plants “is one of the best natural experiments in landscapes that has ever happened on Earth,” Perron says.
Story Image: Before plants colonized land, many rivers braided across barren landscapes and dumped large loads of sediment into the sea, as the Copper River in Alaska does today. (Credit: NASA Earth Observatory, Robert Simmon and Jesse Allen)