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Huge Pyroclastic Cones Surrounding Cobra Head, and Implications for the Eruptive History on the Aristarchus Plateau

Author: 
Erica Jawin
Topic: 
Geology
Delivered As: 
Oral
Abstract Text: 

The Aristarchus plateau is viewed as one of the most complex volcanic regions on the Moon. The most prominent features on the Aristarchus plateau include Aristarchus crater and Vallis Schröteri, the widest and deepest sinuous rille on the Moon. The Aristarchus Plateau is composed of several major volcanological features: Cobra Head; the primary sinuous rille (Vallis Schröteri); a smaller, nested inner rille; a chain of cone-like mounds that extend from Cobra Head south toward Herodotus and Aristarchus craters, and the largest dark mantle deposit (DMD) identified on the Moon (~40,000 km2). The cone unit appears to be genetically related to Cobra Head on the basis of stratigraphy, and was likely emplaced contemporaneously with Cobra Head. We present the following paradigm as a means of explaining these volcanic features (Cobra Head, Valles Schröteri, cone-like unit, and the DMD) as part of the same long-duration fire fountain eruption with variable conditions through the eruptive fountain.

The optical density is variable within the eruptive plume during a fire fountain eruption; the inner, optically thick portion of the plume inhibits radiative cooling. The particles present in this portion of the plume will be erupted and land while maintaining magmatic temperatures, and can coalesce into a lava pond. Turbulent flow within this lava pond and in the lava flowing out of the pond will thermally erode a broad depression into the substrate. In the case of Aristarchus, this process acted to form Cobra Head, as well as a sinuous rille originating from the lava pond, creating Vallis Schröteri. In the outer part of an eruptive plume, lower optical densities lead to rapid radiative cooling and inhibited crystallization. In the Aristarchus eruption, this quenched glassy material would have been dispersed across the plateau, creating the DMD identified in visible and spectral data.

In the intermediate region of the eruptive plume (between the optically dense and optically thin regions), cooler pyroclasts will form clots of material but remain molten in their interior, as seen in terrestrial cinder cones. In terrestrial cinder cones, clots of material can accumulate to form spatter which form rough, irregular features. The proximity of the cone-like feature at Cobra Head suggests that it could have formed in an analogous process, through an accumulation of partially molten pyroclast clots in the transition zone of the eruptive fountain. The linear nature of the chain of cones could suggest that as the dike propagated towards the surface, several vents formed in the initial stages of the eruption, forming small spatter cones. As the eruption progressed, the smaller vents closed and a single, central vent (Cobra Head) persisted for the duration of the eruption. The largest cone represents the accumulated spatter through the remaining duration of the eruption. In this interpretation the Aristarchus DMD, cones, Cobra Head, and Vallis Schröteri all formed from a single fire fountain eruption. This analysis allows constraints to be placed on the conditions of the eruptive fountain, which facilitates quantitative observations such as volume flux of the eruption.

Co-Authors: 
Scott Rowland, Jacob Bleacher, Brent Garry, Patrick Whelley
SSERVI Identifier: 
NESF2016-072

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