Origin of Lunar Maria

Abstract

Lunar maria are not simply large lunar craters. Most craters have raised rims; most maria are bordered partly or wholly by broad zones of downwarping. A satisfactory theory for the origin of maria must provide a genetic link between the origin of the depression and the origin of the material which floors it. This paper advances the concept that the maria are the result of giant meteorite impacts with resultant magma formation, circumferential slumping, nuee-ardente eruption, and pooling in topographic low areas. Slumped rims of craters occur on craters with a diameter of 20 ± 5 km or larger. Terrestrial slumps can be shown to be caused by removal of lateral support and movement of the mass initiated by yielding near the base of the free face. Accepting the theory that most lunar craters were formed by impact, then craters over 20 ± 5 km in diameter, but not smaller, penetrated to depths where temperatures were such as to permit the rock to deform rapidly in the lower part of the crater wall. Slumps thus may provide a clue to the thermal gradient in the outer part of the Moon at the time of slumping. Craters 100 km or more in diameter appear to have slumped rims of such magnitude that magma formation and flow is indicated. Frothing of the magma (formation of nuees ardentes) in the low gravity and hard vacuum on the Moon seems likely, as has been demonstrated by laboratory experiments. Mare-scale eruptions indicate ignimbrites of thousands of cubic kilometers (same order of size as the largest terrestrial ignimbrites) and slumping of the St. Lawrence type. This concept implies that the typical mare is floored by a stratigraphic sequence consisting of: (a) the fall-back breccia and partly molten material of the first eruptive phase, (b) an ignimbrite representing the normal nuee-ardente phase of the same eruption, (c) ignimbrites representing younger eruptions elsewhere on the Moon, and (d) minor lava flows. Lunar ignimbrites explain differences in elevation of adjacent flat regions separated by mountains or crater rims; eliminate the necessity of individual feeders to furnish molten material to floor each crater as well as the surrounding plains by the same age and type of material; and allow the preservation, with reduced relief, of pre-existing topography (“ghost craters” and other compaction features common in ignimbrite terrains).