Nearly three-quarters of the world's copper production comes from large deposits that form about 2 kilometers (1.2 miles) beneath the Earth's surface, known as porphyry copper deposits. Over the course of millions to tens of millions of years, they are exposed by erosion and can then be mined.
Brian Yanites of the University of Idaho and Stephen Kesler of the University of Michigan examined data on the age and number of exposed porphyry copper deposits worldwide. When they compared those data to the climate in each region, they noticed a pattern: The youngest deposits are in areas of high rainfall, such as the tropics, where erosion was rapid. Deposits are older in dry areas that have low rates of erosion.