Plinian eruptions and passive collapse events as the mechanisms of formation of Martian pit chain craters.

Scott, Evelyn D. and Wilson, Lionel (2002) Plinian eruptions and passive collapse events as the mechanisms of formation of Martian pit chain craters. Journal of Geophysical Research: Planets, 107 (E4). p. 5020. ISSN 2169-9100

Abstract

Three distinct and intersecting sets of graben exist near the Martian volcano Alba Patera. One set formed as a response to the evolution of the Alba Patera edifice. A second set includes mainly linear structures widely interpreted to have formed in response to very large-scale regional stresses. The members of the third set contain numerous pit craters, and we propose that these craters formed as a result of volcanic activity above long regional dikes emanating from a source to the south within the Tharsis region. The pit craters are of two morphologies: small (<2 km), regular-shaped ones, usually completely contained within a graben but occasionally unbounded, and larger (4–10 km), irregular ones which commonly coalesce with one another. We argue that these two types of crater formed by different but related mechanisms. The smaller ones developed as the magma in a stalled dike degassed and volatiles were lost to the atmosphere via cracks and fractures in the overlying crustal rocks, which then collapsed into the potential void space at the top of the dike, producing the pit craters. The larger craters formed subsequent to short-lived explosive eruptions sourced from the underlying dike when the dike walls imploded and collapsed, destroying evidence of any vent structures. There are no visible products of these eruptions because the ambient Martian atmospheric conditions caused the explosive eruptions to be very vigorous, dispersing the products over very wide areas. Evidence indicates that both formation mechanisms occurred within very short distances over the same dike. Preexisting local tectonic stresses appear to have influenced the locations of the pit craters, and groundwater may have been involved in determining their locations and the style of eruptive activity.