Some canyons are formed by slow erosion over millions of years, as rivers obstinately eat away at the bedrock. But, in regions where the rock has built-in weaknesses, such as Iceland’s heavily jointed basalt, canyon formation can happen in sudden bursts: Powerful floods can swiftly create canyons by plucking out giant blocks and casting them downstream. How important those types of episodic, catastrophic events are in shaping the landscape has been poorly understood, as such an erosive signature can be difficult to trace through millennia. However, by combining an analysis of the landscape with helium isotope dating, which measures when the newly eroded rocks were exposed to cosmic rays in Earth’s atmosphere, scientists have now traced the step-wise history of the formation of Iceland’s Jökulsárgljúfur canyon, they report online today in the Proceedings of the National Academy of Sciences. The canyon, they found, was formed during three extreme flood events on the Jökulsá á Fjöllum River (which flows into the Dettifoss waterfall, pictured, reputed to be the most powerful in Europe). These flood events were fed by meltwater from the Vatnajökull glacier, which sits atop several volcanoes, including Bárðarbunga. The floods occurred roughly 9000 years, 5000 years, and 2000 years ago; they lasted only days, but each dramatically deepened and widened the canyon by cutting away more than 2 kilometers of bedrock.