The Hesperian is a geologic system and time period on the planet Mars characterized by widespread volcanic activity and catastrophic flooding that carved immense outflow channels across the surface. The Hesperian is an intermediate and transitional period of Martian history. During the Hesperian, Mars changed from the wetter and perhaps warmer world of the Noachian to the dry, cold, and dusty planet seen today. The absolute age of the Hesperian Period is uncertain. The beginning of the period followed the end of the Late Heavy Bombardment and probably corresponds to the start of the lunar Late Imbrian period, around 3700 million years ago (Mya). The end of the Hesperian Period is much more uncertain and could range anywhere from 3200 to 2000 Mya, with 3000 Mya being frequently cited. The Hesperian Period is roughly coincident with the Earth's early Archean Eon. 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. By the beginning of the Late Hesperian the atmosphere had probably thinned to its present density. As the planet cooled, groundwater stored in the upper crust (megaregolith) began to freeze, forming a thick cryosphere overlying a deeper zone of liquid water. Subsequent volcanic or tectonic activity occasionally fractured the cryosphere, releasing enormous quantities of deep groundwater to the surface and carving huge outflow channels. Much of this water flowed into the northern hemisphere where it probably pooled to form large transient lakes or an ice covered ocean. Description and name origin The Hesperian System and Period is named after Hesperia Planum, a moderately cratered highland region northeast of the Hellas basin. The type area of the Hesperian System is in the Mare Tyrrhenum quadrangle (MC-22) around . The region consists of rolling, wind-streaked plains with abundant wrinkle ridges resembling those on the lunar maria. These "ridged plains" are interpreted to be basaltic lava flows (flood basalts)