Islands in the rain

Researchers use volcanic islands to measure how rainfall sets the pace of landscape formation. If you've ever stood on a hill during a rainstorm, you've probably witnessed landscape evolution, at least on a small scale: rivulets of water streaming down a slope, cutting deeper trenches in the earth when the rain turns heavier. It's a simple phenomenon that scientists have long believed applies to large-scale landforms as well - that is, rivers cut faster into mountains that receive heavier precipitation. It's thought that if rainfall patterns influence how rivers cut into rock, over time, the cumulative erosion and its effects on rock deformation can ultimately control how entire mountain ranges take shape. However, this seemingly intuitive theory - that precipitation influences how quickly landscapes erode - has been difficult to verify, because many other factors, such as rock strength and tectonic-plate motions, can also influence erosion rates. These photographs were both taken in June 2012, about 30 kilometers (about 20 miles) apart. This greener image was taken in the Hanalei River basin on the north side of the Hawaiian island of Kauai.