An introduction to the geology of the moon

Abstract

An introduction to the study of the moon is presented from a geological point of view. Lunar physiography, stratigraphy and tectonics are described and discussed. The most important physiographic features are craters, terrae, maria, rilles, wrinkle ridges, rays, haloes, mountain ranges and alpine valleys. The surface of the moon is generally smooth or gently sloping. Steep slopes are common, but occupy a small part of the total area. A very porous layer covering essentially all of the exposed surface to an unknown depth is structurally very complex and rough at a scale of less than one centimetre. This layer is perhaps the result of ‘space weathering’; that is, bombardment by micrometeorites and subatomic particles. Geologic processes acting on the moon include the just-mentioned ‘space weathering’, meteoritic impact, volcanism, transportation and sedimentation accomplished by ejection from impact craters in a ballistic trajectory, possible electrostatic transportation and sedimentation, tectonism, and perhaps others still unknown. The stratigraphy of the moon has been determined by applying the law of superposition to lunar terranes. The stratigraphic column, beginning with the lowest unit, consists of the Pre-Imbrian, Imbrian, Procellarian, Eratosthenian and Copernican Systems. By measuring the number of craters per unit area it is possible to determine the relative ages of exposed terranes. Absolute dating and even interplanetary correlation with the earth's geological column have been attempted. Lunar tectonics are complicated by the lack of an easy method for measuring the equipotential surface, defined on the earth by the oceanic surface. The moon apparently shows a bulge which points toward the earth, but whether this bulge is in isostatic equilibrium or not is still unknown. Endogeneous features on the surface of the moon include rilles, wrinkle ridges, faults, aligned craters, domes, and perhaps isolated volcanic craters. The future of lunar exploration is discussed. The Ranger, Surveyor and Apollo Programmes of the U.S.A. are presented.