Generation, ascent and eruption of magma on the Moon: New insights into source depths, magma supply, intrusions and effusive/explosive eruptions (Part 2: Predicted emplacement processes and observations)

Abstract

1 Highlights  Mare basalt eruption process predictions are compared to observed features/deposits.  Single dike event magma volume predicted to be 10 2 -10 3 km 3 ; dikes 40-100 m by 60-100 km. Shallower-source dikes continuous from source to surface; deeper dikes detach from source. Dikes reaching surface form wide range of predicted/observed effusive/explosive eruption types. Intrusion is favored in thicker farside crust; extrusion is favored on thin nearside crust. AbstractWe utilize a theoretical analysis of the generation, ascent, intrusion and eruption of basaltic magma on the Moon to develop new insights into magma source depths, supply processes, transport and emplacement mechanisms via dike intrusions, and effusive and explosive eruptions. We make predictions about the intrusion and eruption processes and compare these with the range of observed styles of mare volcanism, and related features and deposits. Density contrasts between the bulk mantle and regions with a greater abundance of heat sources will cause larger heated regions to rise as buoyant melt-rich diapirs that generate partial melts that can undergo collection into magma source regions; diapirs rise to the base of the anorthositic crustal density trap (when the crust is thicker than the elastic lithosphere) or, later in history, to the base of the lithospheric rheological trap (when the thickening lithosphere exceeds the thickness of the crust). Residual diapiric buoyancy, and continued production and arrival of diapiric material, enhances melt volume and overpressurizes the source regions, producing sufficient stress to cause brittle deformation of the elastic part of the overlying lithosphere; a magma-filled crack initiates and propagates toward the surface as a convex upward, blade-shaped dike. The volume of magma released in a single event is likely to lie in the range 10 2 km 3 to 10 3 km 3 , corresponding to dikes with widths of 40-100 m and both vertical and horizontal extents of 60-100 km, favoring eruption on the lunar nearside. Shallower magma sources produce dikes that are continuous from the source region to the surface, but deeper sources will propagate dikes that detach from the source region and ascend as discrete penny-shaped structures. As the Moon cools with time, the lithosphere thickens, source regions become less abundant, and rheological traps become increasingly deep; the state of stress in the lithosphere becomes increasingly contractional, inhibiting dike emplacement and surface eruptions.In contrast to small dike volumes and low propagation velocities in terrestrial environments, lunar dike propagation velocities are typically sufficiently high that shallow sill formation is not favored; local low-density breccia zones beneath impact crater floors, however, may cause lateral magma migration to form laccoliths (e.g., Vitello Crater) and sills (e.g., Humboldt Crater) in floor-fractured craters. Dikes emplaced into the shallow crust may stall and produce crater chains due to active andpassive gas venting (e.g., Men...

Geological Characterization of the Ina Shield Volcano Summit Pit Crater on the Moon: Evidence for Extrusion of Waning‐Stage Lava Lake Magmatic Foams and Anomalously Young Crater Retention Ages

“…On the basis of (1) our latest physical volcanology analysis of lunar dike evolution processes and final‐stage shield‐building eruptions (Head & Wilson, ; Wilson & Head, ); (2) analog studies of the morphology, topography, and magmatic‐volcanic processes of terrestrial small shield volcanoes in Hawai'i (Qiao et al, , Figure ); and (3) following our comprehensive geological characterization of the context and interior of Ina pit crater presented above (sections and ) and prior investigations (e.g., Braden et al, ; Garry et al, ; Qiao et al, ; Strain & El‐Baz, ), we examine the Ina feature in the context of lunar shield‐building eruptions. We begin with the crucial observation and interpretation that the Ina feature is a summit pit crater/vent atop an ~22‐km‐diameter, ~3.5‐Ga‐old shield volcano (Qiao et al, ; Strain & El‐Baz, ) and then examine the successive phases of a lunar shield‐building eruptions (Wilson & Head, ), with a special focus on final‐stage summit pit crater activities.…”

“…The diameter of the Ina pit crater also lies on the summit crater diameter/base diameter trend line of lunar small shields (Figure b). Lunar small shield volcanoes are generally interpreted to develop when dikes propagate to the surface and shields build up through multiple phases of flows erupted from a common pit crater source, dominated by accumulating low‐effusion rate, cooling‐limited flows (Head & Wilson, , ). The Ina shield volcano is well developed in the southern portion, while the growth of the northern part is affected by the preexisting ejecta deposits.…”

“…For comparison, we also transfer the map of the Ina interior units to the upper flank of the shield volcano and count the superposed impact craters there (Figure b). During the crater counting investigations, special care has been taken to eliminate contamination by secondary impact craters and endogenous pits according to their morphologic characteristics (e.g., Head & Wilson, ; Oberbeck & Morrison, ; Shoemaker, ). The crater counting results are mapped in Figures a and b and reported in the standard cumulative SFD plots (Figure c) and tabular form (Table ).…”

“…Recent theoretical treatments of final‐stage shield‐building magmatic activity and volatile exsolution physics (Head & Wilson, ; Wilson & Head, ) provided a framework to interpret Ina as a drained summit pit crater lava lake atop an ancient shield volcano ~3.5 billion years (Ga) old, contemporaneous with the major phase of lunar mare volcanism (Qiao et al, ; Wilson & Head, ). In this hypothesis, the floor hummocky and blocky units are the solidified lava lake crust, which is several meters thick and very vesicular, both at the microvesicular and macroporosity scales due to the presence of large void spaces generated by crust deformation, bubble coalescence, and disruption during the very late stage volatile‐release‐driven Strombolian eruptions.…”

“…(7) what are the outstanding questions that can be addressed to resolve the origin of Ina (and other IMPs)? Then, on the basis of terrestrial analog observations of terrestrial small shield volcanoes in Hawai'i, and lunar mare basalt ascent and eruption theory and observations (Head & Wilson, ; Wilson & Head, ), we address alternative formation mechanisms of the ranges of characteristics associated with Ina and their postemplacement geologic modification, especially the observed anomalously young crater retention ages. We also discuss the implications for the origin of other lunar IMPs, the duration of mare volcanism, and the potential of Ina as a target for future surface exploration missions.…”