TECTONIC SETTINGS FAVORING HIGH-FLUX EFFUSIVE ERUPTIONS

Historic effusive eruptions on Earth have not exceeded 10,000 m³/s but active eruptions exceeding 100,000 m³/s have been observed on Jupiter’s moon, Io. Furthermore, there is strong evidence of a geologically recent eruption on Mars exceeding 1,000,000 m³/s. Such extreme fluxes are calculated to have resulted in turbulent flow with significant potential for erosion by lava. Similar high-flux eruptions are invoked to explain some features on the Moon and Venus, especially channels that appear incised. These observations beg the question of what conditions favor high-flux eruptions.

For a given dike geometry, a higher driving pressure will increase flux. At a given driving pressure, a wider and/or longer dike can lead to a higher eruptive flux, but creating a large dike requires both high volume and high driving pressure. Therefore, conditions are needed that produce high driving pressure as well as a magma source with sufficient volume to feed the eruption. The driving pressure is primarily controlled by the hydrostatic buoyancy of the magma (P=ΔρgH, where P is the driving pressure, Δρ is the density difference between the magma and the surrounding rocks, g is gravitational acceleration, and H is the height of the magma column). Exsolving volatiles greatly reduce the density of the magma, especially as it nears the surface. However, both Ionian and lunar lavas are relatively volatile-poor when compared to terrestrial magmas so volatile content alone is unlikely to explain why the Earth has a dearth of high-flux eruptions. The fact that the Earth has the highest g is also unhelpful in explaining the lack of high-flux eruptions. Therefore, the most likely explanation is related to the height of the magma column.

Tectonic settings that favor the production of large deep magma chambers under a thick dense lithosphere could be the key. Counter-intuitively, situations that hinder magma ascent (e.g., compressive stresses) may also be helpful because they will make it difficult for the magma to ascend to produce shallower magma chambers. Io and Mars have thick cold lithospheres, and Io also has many regions under compression. If this hypothesis is correct, high-flux eruptions are not expected on Venus or locations on the Moon with a thin crust. Upcoming missions to Venus and the Moon should provide the data needed to test this hypothesis.