February 5, 2013
image by LROC Featured Image (NASA/GSFC/Arizona State University)
Last Christmas Day the Lunar Reconnaissance Orbiter Camera team released a dramatic oblique view of Aristarchus (featured on LPOD last week). Down in the middle of the description was this color mosaic made from LRO Wide Angle Camera (WAC) images. The plum colored material is volcanic ash - pyroclastics- erupted from the Cobra Head vent that also fed Schröter's Valley. It is consistent that the vent of the Moon's largest sinuous rille also has the largest pyroclastic deposit. Superposed on the deep plum is a grayer more homogeneous purple that is ejecta from Aristarchus. That the ash is older than the surrounding Oceanus Procellarum lavas is demonstrated by the sharp boundary along the west/left side of the Aristarchus Plateau. Ash covered a much broader area but the lavas covered it up to the relatively abrupt 250-350 m rise (thank you QuickMap Path) rise of the plateau. But not all surrounding lavas are the same age. The thin passage of lava between the north end of the plateau and the string-like Agricola Mountains is veneered by plumy ash showing that those lavas preceded the eruptions. Of course, there may have been different periods of pyroclastic eruptions but I assume that because there is no wind that the ash would spread equally in all directions. An interesting question is how much time elapsed between the ash deposits and the surrounding lava eruptions? It is widely accepted that at least some of the Southern Procellarum lavas came from the Cobra Head eruptions. A model for what happened on the Moon may be the Kileaua Iki, Hawaii eruption in 1959. A pyroclastic eruption built a step cone around the vent and strong eruptions blasted ash widely around the countryside. After the magma had degassed (and the ash phase ended), lavas poured from the same vent and made a lava flow field. Such cycles repeated. Perhaps on the Moon the plum colored ash spewed from a vent, with some ash falling back to built the 2 km tall Cobra Head mountain, and after the magma degassed, fluid lavas flowed out of the vent, cutting Schröter's Valley, and debouching and spreading across the surface that became what we call Oceanus Procellarum. In this scenario the Procellarum lavas might be only days or weeks younger than the ash. If, as in Hawaii, there were cycles of explosive and then fluid eruptions, then the lavas by the Agricola Mountains may have been emplaced between ashy phases.
Rükl plate 18
21st Century Atlas chart 20.
Yesterday's LPOD: Where We Have Been
Tomorrow's LPOD: Near Polar Rubble