Eruption of magmatic foams on the Moon: Formation in the waning stages of dike emplacement events as an explanation of “irregular mare patches”

Article history: Received 23 September 2016 Received in revised form 31 January 2017 Accepted 7 February 2017 Available online 9 February 2017 Volcanic eruptions on theMoon take place in conditions of low gravity and negligible atmospheric pressure, very different from those on Earth. These differences lead to characteristic lunar versions of hawaiian and strombolian explosive activity, and to the production of unusual eruption products neither predicted nor observed on Earth in the terminal stages of eruptions. These include the unusual mounds and rough (hummocky, blocky) floors of some small-shield summit pit crater floors, elongate depressions andmare flows (similar to those named “irregular mare patches”, IMPs, by Braden et al., 2014). We examine the ascent and eruption of magma in the waning stages of the eruptive process in small-shield summit pit craterfloors and show thatmany IMP characteristics can be plausibly explained by basaltic magma behavior as the rise rate of the ascending magma slows to zero, volatiles exsolve in the dike and lava lake to form a very vesicular foam, and the dike begins to close. Stresses in the very vesicular andporous lava lake crust produce fractures throughwhich the foamextrudes at a rate determined by its non-Newtonian rheology.Waning-stage extrusion of viscousmagmatic foams to the surface produces convex mounds whose physical properties inhibit typical impact crater formation and regolith development, creating an artificially young crater retention age. This mechanism for the production and extrusion of very vesicular magmatic foams is also applicable towaning-stage dike closure associatedwith pit craters atop dikes, and fissure eruptions in the lunar maria, providing an explanation for many irregular mare patches. Thismechanism implies that IMPs and associated mare structures (small shields, pit craters and fissure flows) formed synchronously billions of years ago, in contrast to very young ages (less than ~100 million years) proposed for IMPs by some workers. © 2017 Elsevier B.V. All rights reserved.