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

T Ina, a distinctive 2x3km D-shaped depression, is composed of unusual bulbous-shaped mounds surrounded by optically immature hummocky/blocky floor units. The crisp appearance, optical immaturity, and low number of superposed impact craters combine to strongly suggest a geologically recent formation for Ina, but the specific formation mechanism remains controversial. We reconfirm that Ina is a summit pit crater/vent on a small shield volcano 3.5billion years old. Following detailed characterization, we interpret the range of Ina characteristics to be consistent with a two-component model of origin during the waning stages of summit pit eruption activities. The Ina pit crater floor is interpreted to be dominated by the products of late-stage, low-rise rate magmatic dike emplacement. Magma in the dike underwent significant shallow degassing and vesicle formation, followed by continued degassing below the solidified and highly microvesicular and macrovesicular lava lake crust, resulting in cracking of the crust and extrusion of gas-rich magmatic foams onto the lava lake crust to form the mounds. These unique substrate characteristics (highly porous aerogel-like foam mounds and floor terrains with large vesicles and void space) exert important effects on subsequent impact crater characteristics and populations, influencing (1) optical maturation processes, (2) regolith development, and (3) landscape evolution by modifying the nature and evolution of superposed impact craters and thus producing anomalously young crater retention ages. Accounting for these effects results in a shift of crater size-frequency distribution model ages from <100 million years to 3.5 billion years, contemporaneous with the underlying ancient shield volcano. Plain Language Summary Among the most outstanding questions in lunar evolution is the end of extrusive volcanic activity, commonly thought to be at least a billion years old. A recent study found evidence for volcanic activity within the last 100million years, in the form of irregular mare patches dated by size-frequency distributions of superposed craters. The most prominent irregular mare patch is Ina, an 2x3-km depression composed of bulbous-shaped mounds surrounded by fresh, hummocky and blocky floor units, both supporting the geologically very recent age. We undertook a detailed characterization of the setting of Ina and its interior and found that its location on the summit of a 3.5 billion years old small shield volcano, together with Hawaiian field analogs and theoretical analyses of the ascent and eruption of magma, provided new clues to its origin and age. Late-stage magma extrusion in the summit pit characterized by gas-rich Strombolian activity produced a very vesicular crust and concentration of underlying magmatic foams; cracking of the crust caused magmatic foam extrusions to produce the mounds. The very porous nature of Ina deposits decreases the superposed crater diameters and shifts the crater size-frequency distribution to 3.5 billion years, coincident with the ancient age of the volcano.