Evidence of volcanism and magmatism are often associated with impact craters on terrestrial planets. On Venus, radar observations of the surface have revealed two categories of craters in the ancient crustal plateaus: bright-floored and dark-floored craters, the latter being interpreted as partial filling of the crater by lava. On the other hand, on the Moon, ascending dykes in the low-density highland crust tend to horizontalize into shallow sill-like intrusions, generating floor-fractured craters that are characterized by uplifted and fractured floors resulting from the underlying magmatic intrusions.

The stress field induced within the crust by a crater unloading indeed has two competitive effects. It induces a depressurization of the encasing elastic medium, which provides a driving pressure to the magma. This allows its ascent through the crust despite the magma's negative buoyancy and explains why the magma tends to erupt preferentially within impact craters (Michaut and Pinel, 2018). However, the state of stress below the unloading is such that the minimum compressive stress is vertical at the unloading axis, which tends to horizontalize the dyke intrusion, therefore favoring magma storage below a crater at the expense of eruption.

We calculated the stress fields generated by impact craters of different radius on top of a semi-infinite half-space and use them in numerical mechanical models of magma ascent (Maccaferri et al, 2011) to evaluate the path followed by a dyke below a crater. We identify several types of behavior (ascent to the crater floor, horizontalization of the intrusion, storage at depth, ascent to the planet surface) depending on the physical properties of the magma and crust, as well as on the dyke and crater unloading characteristics. We draw a regime diagram for magma ascent below craters as a function of two characteristic dimensionless numbers depending on these different physical parameters.

We show that dyke deviation and hence magma storage is favored by larger crater depths on smaller gravity bodies, explaining the different observations between the Moon and Venus.