With the decline of heavy impacts at the end of the Noachian, volcanism became the primary geologic process on Mars, producing vast plains of flood basalts and broad volcanic constructs (highland paterae). By Hesperian times, all of the large shield volcanoes on Mars, including Olympus Mons, had begun to form. Volcanic outgassing released large amounts of sulfur dioxide (SO2) and hydrogen sulfide (H2S) into the atmosphere, causing a transition in the style of weathering from dominantly phyllosilicate (clay) to sulfate mineralogy. Liquid water became more localized in extent and turned more acidic as it interacted with SO2 and H2S to form sulfuric acid.