We measured the surface area of minerals in two 21 m cores from soil to bedrock to test hypotheses on the formation of the surface area of weathered rock. A sharp increase in high-surface-area secondary minerals extended from 3 m to the ground surface. Half the total surface area was from corroded iron minerals, which form in the presence of oxygen, even though corroded iron comprised less than 0.1 % of the rock. Element removal by rock dissolution started at 7 m but did not form new minerals.