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On the Moon, the low-density crust exerts a strong filter to magma ascent. Many lunar craters are filled with mare, and evidence of pyroclastic activity and shallow magmatism is often located within craters called floor-fractured craters (FFCs). Interpreting quantitative observations on mare-filled craters and FFCs based on mechanical models, we show that a surface unloading caused by an impact crater provides a driving overpressure to the magma stalling at depth. This overpressure counterbalances the melt negative buoyancy, favoring its ascent through the crust. Providing a large unloading and a thin crust, magma can ascend up to the crater floor. FFC characteristics are consistent with a magma denser than the crust by 200-300 kg/m(3) and an elastic lithosphere thickness larger than 70 km. Our study suggests that small impact cratering likely induced magmatism and thereby crustal evolution in the early times of terrestrial planets.

Plain Language Summary On the terrestrial planets showing particularly old surfaces, such as on the Moon, volcanic deposits and evidence of magma intrusions are often found on the floors of relatively small impact craters, too small for the melt to have formed because of the impact itself. Why then is the process of magma ascent and eruption linked to that of cratering? Here we show that the unloading associated to an impact crater decompresses the elastic medium situated beneath. If magma is stalling at depth, it becomes pressurized relatively to its encasing rocks, which allows for its ascent in the crust. Magma might erupt at the surface if the unloading is sufficiently large and the crust relatively thin. Our model well explains observations on evidence of magma eruption versus intrusion at different craters varying by their sizes and by the crustal thickness at their locations on the Moon. We suggest that, in the early times of planets where magmatism and cratering were dominant, magma ascent caused by crater unloading might have modified the initial crustal composition and structure.