Geometry shows why river rocks are round

Geologists know that rocks that line riverbeds tend to be smaller and rounder further downstream. Abrasion causes rocks to grind down and become rounder, but does the grinding reduce the size of rocks or is it that smaller rocks are simply more easily transported? 

A new model suggests that abrasion plays a key role in upholding these patterns, but it does so in a distinctive, two-phase process. First, abrasion makes a rock round. Then, only when the rock is smooth, does abrasion act to make it smaller in diameter.

“It was a rather remarkable and simple result that helps to solve an outstanding problem in geology,” says Douglas Jerolmack, associate professor of Earth and Environmental Sciences at the University of Pennsylvania.

Not only does the model help explain the process of erosion and sediment travel in rivers, but it could also help geologists answer questions about a river’s history, such as how long it has flowed and how far they have actually tumbled to reach this point. 

Text: http://www.futurity.org/geometry-shows-river-rocks-round/                                                                                                                                                                              Photo: GCE guest, Gene G.