The Geologic Evolution of the Moon

Abstract

This paper is a synthesis of pre- and post-Apollo 11 studies to produce an outline of the moon's geologic evolution from three lines of evidence: (1) relative ages of lunar landforms and rock types, (2) absolute ages of returned lunar samples, and (3) petrography, chemistry, and isotopic ratios of lunar rocks and soils. It is assumed that the ray craters, circular mare basins, and most intermediate circular landforms are primarily of impact origin, although many other landforms are volcanic or of hybrid origin. The moon's evolution is divided into four main stages, each including several distinct but overlapping events or processes: Stage I (4.7 b.y. ago)-origin of moon by unknown means, followed immediately by heating of outer 500 km to at least 1,000° C. Stage II (4.6-3.7 b.y. before present)-first differentiation to form highland crust, by eruption of high-alumina basaltic magma and minor quantities of differentiation products such as anorthosite and felsite; infall of small circum-terrestrial bodies to form old highland craters; formation of circular mare basins by infall of large circum-terrestrial bodies; formation of Archimedes-type craters; highland vulcanism. Stage III (3.7-3.2 b.y. before present)-second differentiation of moon, by generation and eruption of iron-rich basaltic magmas to form maria. Stage IV (3.2 b.y. ago to present)-sporadic impacts by asteroid belt meteoroids and comet nuclei to form ray craters; local vulcanism and gas venting from deep sources; minor faulting; continual slow landscape degradation by small impacts and other agents. The moon now appears relatively cold and rigid to depths of about 500 km. Major unsolved problems include the composition and origin of the highland crust, the role of vulcanism in formation of ray craters, the nature of the lunar interior, the cause of asymmetric mare distribution, and the relation of the moon to the earth.