LAUSR

Abstract Mons Hansteen is a relatively high-albedo, well-known lunar ``red spot'' located on the southern margin of Oceanus Procellarum (2.3°S, 50.2°W). It is an arrowhead-shaped (∼ 25 km on a side), two-layer mesa with a small cone-shaped massif on its north edge formed by three morphologically and compositionally distinct geologic units. These units were emplaced in three phases over nearly 200 million years. The oldest (∼3.74 Ga), Hilly–Dissected unit, composed of high-silica, and low-FeO content materials formed a low, steep sided mesa. The materials of this unit erupted mainly from vents along northeast- and northwest-trending sets of fractures. The Pitted unit, which comprises the upper-tier mesa, is composed of high-silica and even lower-FeO content materials. This material was erupted at ∼ 3.5 Ga from numerous closely spaced vents (i.e., pits) formed along closely spaced northeast-southwest-trending sets of fractures. At nearly the same time, eruptions of lower silica and higher FeO materials occurred on the north flank of Mons Hansteen at the intersection of two major fractures to produce the North Massif unit. The eruptions that created the North Massif units also produced materials that thinly blanketed small areas of the Hilly-Dissected and Pitted units on the north flank of Mons Hansteen. Also at nearly the same time (i.e., ∼ 3.5 Ga), basalt flows formed the surrounding mare. Each unit of Mons Hansteen appears to be mantled by locally derived ash, which only modestly contaminated the other units. The morphology of Mons Hansteen (especially the Pitted unit) suggests a style of volcanism where only a relatively small amount of material is explosively erupted from numerous, closely spaced vents.