Three caldera-shaped accidents: Volcanic calderas,meteoric scars and lunar cirques

A caldera is a large volcanic depression, more or less circular, the diameter of which is many times greater than those of the included volcanic vents. Calderas must be separated from feetono-volcanic depressions, which have an irregular shape. Volcanic calderas are produced by engulfment. The scars, produced by the impact of meteorites on the earth, are circular or elliptical depressions. Lunar cirques are nearly all circular; some of them have a polygonal, and then sometimes an hexagonal shape. On the surface of the moon elliptical depressions are wholly absent. Moreover, on the moon it is a rigid law that when intersections of ring plains do occur, the smaller cirque is entire, the rim of the middlemost being interrupted by the smallest, whereas the biggest is interrupted by the middle one. This phenomenon would be in accordance with a volcanic origin and a decreasing volcanic activity whereas it is incompatible with an impact of meteorites.