Shape of scoria cones on Mars: Insights from numerical modeling of ballistic pathways

Abstract

Morphological observations of scoria cones on Mars show that their cross-sectional shapes are different from those on Earth. Due to lower gravity and atmospheric pressure on Mars, particles are spread over a larger area than on Earth. Hence, erupted volumes are typically not large enough for the flank slopes to attain the angle of repose, in contrast to Earth where this is common. The distribution of ejected material forming scoria cones on Mars, therefore, is ruled mainly by ballistic distribution and not by redistribution of flank material by avalanching after the static angle of repose is reached. As a consequence, the flank slopes of the Martian scoria cones do not reach the critical angle of repose in spite of a large volume of ejected material. Therefore, the topography of scoria cones on Mars is governed mainly by ballistic distribution of ejected particles and is not influenced by redistribution of flank material by avalanching. The growth of a scoria cone can be studied numerically by tracking the ballistic trajectories and tracing the cumulative deposition of repeatedly ejected particles. We apply this approach to a specific volcanic field, Ulysses Colles on Mars, and compare our numerical results with observations. The scoria cones in this region are not significantly affected by erosion and their morphological shape still preserves a record of physical conditions at the time of eruption. We demonstrate that the topography of these scoria cones can be rather well (with accuracy of ∼10 m) reproduced provided that the ejection velocities are a factor of ∼2 larger and the ejected particles are about ten times finer than typical on Earth, corresponding to a mean particle velocity of ∼92 m/s and a real particle size of about 4 mm. This finding is in agreement with previous theoretical works that argued for larger magma fragmentation and higher ejection velocities on Mars than on Earth due to lower gravity and different environmental conditions.

The Apparent Absence of Kilometer‐Sized Pyroclastic Volcanoes on Mercury: Are We Looking Right?

“…Such low-relief shapes are in contrast to pyroclastic volcanoes on Earth or Mars, where a conical edifice is generated, because atmospheric drag decreases the speed of the ejected particles and prevents widespread dispersal of the particles from the vent (e.g., Brož et al, 2014;Riedel et al, 2003). This landform shape is a result of the dispersal of the erupted material across such a large area that even an amount of material larger by 3 orders of magnitude than is typical for Earth would be insufficient to build a substantial (at least several hundreds of meters high) topographic feature composed of accumulated pyroclastic ejecta.…”

“…For Mars, Brož et al (2014Brož et al ( , 2015 attempted to reproduce the shapes of the putative scoria cones using Earth-like values of σ μ and including the effect of air resistance. The ejection angle, measured from the vertical, is characterized by a normal distribution centered at 0 with standard deviation σ α , which represents the mean angular radius of the ejection cone (see Figures S1-S4 in the supporting information for details).…”

“…The lack of identified low-volume volcanoes on Mercury, and hence the unavailability of any data about their volumes, motivates us to assume in a first pass that pyroclastic cones would be formed by the same amount of material on Mercury as the most voluminous Martian putative scoria cone (4.2 km 3 : Brož & Hauber, 2012;Brož et al, 2015) and that the parameters of the eruption would be the same on both bodies (see description of Figure 1 for details or Brož et al, 2014Brož et al, , 2015. The only difference in model setup we consider here is the lack of an atmosphere for Mercury.…”

“…The results are summarized in Figure 2. The final edifices would have gentler flank slopes (maximally 13.8°on Mercury vs. 24°on Mars in the steepest part of the cones) for the same sets of parameters for both eruptions, including an ejection speed of Until now, we have considered only solutions based on the assumption that explosive volcanism would occur on Mercury with a similar set of parameters as determined for low-volume explosive eruptions on Mars (Brož et al, 2014(Brož et al, , 2015, and references therein). If the material is ejected at higher initial speeds on Mercury than expected for Mars, the amount of material necessary to construct a landform of such height must further increase (Figure 2a).…”

Plateaus and sinuous ridges as the fingerprints of lava flow inflation in the Eastern Tharsis Plains of Mars

“…Although a fluvial erosion interpretation is difficult to explain if no depositional fans are associated with the channels, it is possible that these features might have been buried by subsequent lava emplacement, if the plains flows are younger than the rift aprons (including any fluvial activity). This is possible as the abundant volcanic vents identified on the Tharsis plains (Plescia, 2004;Bleacher et al, 2007aBleacher et al, , 2009Keszthelyi et al, 2008;Baptista et al, 2008;Baratoux et al, 2009;Hauber et al, 2009Hauber et al, , 2011Wilson et al, 2009;Brož and Hauber, 2012;Richardson et al, 2013;Brož et al, 2014) could be comparatively young (Hauber et al, 2011) and could represent volcanic plains that are genetically unique from the rift apron flows. Regardless, active lava flows can also produce depositional fans at slope breaks as lava is debouched from channels onto the lower slope terrain (Carr and Greeley, 1980).…”