Post contributed by Dr. Petr Brož, Institute of Geophysics of the Czech Academy of Science
Volcanism is an important process which shapes the surfaces of all terrestrial planets, and is still active on Earth, Jupiter’s moon Io, and perhaps on Venus. On Earth, volcanoes with wide variety of shapes and sizes exist; however, the size of volcanoes is anti-correlated with their frequency, i.e. small volcanoes are much more numerous than large ones. The most common terrestrial volcanoes are represented by kilometre-sized scoria cones (Figure 1a). These are conical edifices of pyroclastic material originating from explosive volcanic activity. Degassing of ascending magma causes magma fragmentation on eruption piling up the pyroclasts around the vent as a cone. Interestingly, scoria cones as known from Earth, have not been observed commonly on any other terrestrial body in the solar system despite the fact that magma degassing, and hence magma fragmentation, has to occur on these bodies as well.
On Mars, the existence of scoria cones has been suggested in several regions, either as parasitic cones on the flanks of larger volcanoes (Bleacher et al., 2007), or as large clusters of edifices forming small volcanic fields (Meresse et al., 2008; Lanz et al., 2010; Brož and Hauber, 2012). The interpretation of these edifices as martian scoria cones was mainly based on their apparent morphological similarity with terrestrial scoria cones (Figure 1). It has been recognized that putative martian scoria cones differ in size and shape from terrestrial analogues (Meresse et al., 2008; Brož and Hauber, 2012). Martian scoria cones are usually larger in basal diameter, taller, more voluminous by 1 to 2 orders of magnitude than their terrestrial counterparts, and the flanks do not exhibit slopes over 30° (e.g., Brož and Hauber, 2012). The large basal diameter of the martian cones can be explained by lower values of gravitational acceleration and atmospheric density on Mars compared to on Earth, which allow the scoria particles to be ejected farther from the vent and deposited across a wider area than in terrestrial conditions (McGetchin et al., 1974; Dehn and Sheridan, 1990; Wilson and Head, 1994; Brož et al., 2014).
When we investigated morphometry of martian putative scoria cones in detail, we found that even though martian cones are taller and have larger volumes than on Earth, the amount of scoria material is typically not sufficient for the critical angle of repose to be attained over the main part of their flanks (Brož et al., 2014) as it is common on Earth (Riedel et al., 2003). The principal mechanism of scoria cones formation on Mars is thus the ballistic emplacement of ejected particles which accumulate around the vent over time, rather than a redistribution of particles by avalanching processes typical of terrestrial scoria cones (Riedel et al., 2003). This suggests that avalanche redistribution during growth of martian scoria cones plays only a minor role and allows us to reconstruct their growth by modelling the ballistic trajectories and recording the cumulative deposition of repeatedly ejected particles. By measuring the shapes of 28 martian scoria cones within three different areas, we inferred that they had been formed by Strombolian volcanic eruptions with an ejection velocity about 2 times larger and a particle size about 20 times smaller than typical for Earth (Brož et al., 2015).
These observations and inferences enabled us to reconstruct the series events required to build a martian scoria cone (Brož et al., 2015) summarized on Figure 2. Initially, cones on Mars and Earth develop in a comparable manner by gradually increasing their height and slope angle. Because of the differences in the ballistic range, ejected particles are deposited over a much smaller area on Earth than on Mars and, for the same volume of ejecta, the terrestrial cone is steeper than the martian one. Once the angle of repose on Earth (~30°) has been reached, the slope inclination remains constant. Continuing growth is accommodated by an increase of the cone width, hence terrestrial cones show a correlation between the height and the basal diameter. In contrast, the martian scoria cones are built by ballistic deposition only and, despite of substantial volumes of ejected material, they never reach the angle of repose, because the ejecta is distributed over much greater area. Each scoria cone on Mars thus contains a record of specific physical conditions at the time of eruption which can be, at least partly, reconstructed from its shape. Our research highlights the significance of environmental setting on physical processes driving the volcanic activity on Mars, and more broadly on other bodies of the Solar system.
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