A comparative study of magma ascent and storage below impact craters on terrestrial planets

On terrestrial bodies other than Earth, volcanism and magmatism are often related to impact craters. On Venus, RADAR observations of the surface have revealed two categories of craters: bright-floored and dark-floored craters, the latter being interpreted as partial filling of the crater by lava. On the Moon, volcanic deposits and evidence of pyroclastic activities are also frequently located within impact craters, especially within floor-fractured craters. These craters are characterized by uplifted, fractured floors resulting from underlying shallow magmatic intrusions. 

The elastic stress induced within the crust by a crater excavation 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 surface unloadings 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.

Our results show that magma ascent to the crater interior requires relatively small density contrasts between the crust and magma and rather small crustal thicknesses as opposed to dyke horizontalization that results from larger crust-magma density contrasts and crustal thicknesses. Furthermore, on the Moon, craters are considerably deeper than on Venus, leading to a larger dimensionless deviatoric stress below a crater of a given radius, favoring dyke horizontalization and storage. This well explains why the magma tends to store as horizontal intrusions below floor-fractured craters on the Moon while it tends to erupt on the floor of dark-floored craters on Venus.

ACKNOWLEDGMENT: This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement No. 101001689).