Suitable analogs to evaluate the origins of volcanic terrains on the Moon and Mars?
Introduction
Linear or curvilinear eruptive fissures, the surface expressions of steeply-dipping pressurized feeder dikes, are the dominant volcanic vents through which relatively low viscosity basaltic magma reaches the surface of a planet. Fissures, usually transient features in the growth of lava terrains, are the sources for pyroclastic ejecta, low-volume proximal lava flows, and high-volume lava flows that stretch for many kilometers. They occur in many geologic settings: rift zones with lava-filled depressions and grabens, large mafic shield volcanoes covered by numerous thin lava flows, lava plains comprising coalesced low shields, and vast regions covered by thick sequences of flood basalt. These volcanic terrains and their variants portray crustal extension due to magmatic or tectonic forces, or both, and some of them ultimately develop landscapes characterized by sub-parallel fissures. While recent studies have focused on interpreting equivalent processes on currently inaccessible planetary terrains, there is a need to demonstrate the similarities of Earth analogs to planetary fissure systems, although surficial features or geologic settings often are somewhat different (e.g., Carr, 1973; Greeley and King, 1977; Greeley and Schultz, 1977; Moore and Hodges, 1980; Wilson and Head, 1994; Hodges and Moore, 1994; Spudis, 1999; Plescia, 2004; Hauber et al., 2009; Head and Wilson, 2017).
In order to classify volcanic fissures on Earth as planetary terrains, we evaluate relatively young features of the Great Rift and surroundings on the eastern Snake River Plain (ESRP) of Idaho, USA, and selected features in Hawai’i, Iceland, and northern Africa. Physiography examined in the field and in aerial imagery is used to categorize fissures associated with volcanism on Earth, which can then be compared as analogs to volcanic features on other planetary bodies. The intention of this study is to elucidate the association of fissure-related structures in two volcanic terrains on the Moon: lunar floor-fractured craters (FFCs), and lunar mare and cryptomare basalts. We extend this comparative study to the volcanic plains in the greater Tharsis region of Mars. The leading question here is whether or not the categorized fissure types on Earth are suitable analogs for broadly similar features in these terrains.
Fissure eruptions were likely dominant features during the growth of all three volcanic terrains discussed here; yet, this hypothesis may be difficult to test with currently available data. The classification of similar types using analogs also depends on recognizing how landforms develop in geologic settings that differ from those on Earth. Differences in gravity, atmospheres (or lack of), and planetary interiors (crust-mantle compositions and thicknesses, tectonism, thermal structures, etc.), can account for overall differences in morphology and morphometry of volcanic features. These factors are treated in comprehensive models of magma ascent, emplacement, and eruption on both the Moon (Head and Wilson, 2017) and Mars (Wilson and Head, 1994), with detailed consideration of known or theoretical conditions related to the development of volcanic features, deposits, and structures. For example, compared to Earth, the lower gravity and atmospheric pressure of either Mars or the Moon will cause gas-driven eruptions (typical of basaltic systems) to initiate at greater depths of gas exsolution and magma disruption. Moreover, compared to Earth, both the Moon and Mars will have broader dispersal of pyroclastic ejecta and lava fields, as well as lower relief volcanic constructs.
We hypothesize that while fissure systems on any planetary body may be similar at a fundamental level, differences will become apparent at complex levels due to the presence (or lack) of some distinctive associated features. A secondary issue, therefore, is how broadly each type of fissure system must be defined in order to be considered a suitable planetary analog. Volcanism on the Moon and on Mars, most of which occurred early in their geologic histories, is manifested on both bodies in generally dissimilar patterns (e.g., Carr, 1973; Neal and Taylor, 1992; Hodges and Moore, 1994; Wilson and Head, 1994; Spudis, 1999; Carr and Head, 2010; Head and Wilson, 2017, and many others). One of the primary morphological differences is that most vents related to the prodigious lunar mare basalts and martian lava plains are likely buried beneath lava flows, yet on Mars many vents, cones and flows remain visible.
Although recent high-resolution orbital platforms have enhanced our views of the Moon, no attempt is made here to reclassify or revise the interpretations derived from surface features or analyzed samples in many previous studies (e.g., Schultz, 1976; Wilson and Head, 1981, 2018a; Head and Wilson, 1992, 2017; Jolliff et al., 2000; Plescia, 2004; Shearer et al., 2006; Wilson et al., 2011; Hurwitz et al., 2013; Spudis et al., 2013; Jozwiak et al., 2012, 2015, 2017). Instead, this survey of fissure vents and volcanic features on the Moon focuses on determining the similarities and differences among them and how they are related to analogs on Earth. Morphometric assessment of lunar vents in both FFC and mare settings is used to quantify such associations with the intent to improve the understanding of magmatic processes, whether observed or implied.
A slightly different approach is used to evaluate volcanic fissures on Mars, mainly because of the difference in available imagery and the current knowledge of specific, albeit significant, volcanic landforms. While a variety of volcanic features on Mars have been extensively studied on the main volcanic edifices and lava plains (e.g., Hodges, 1979; Hodges and Moore, 1994; Wilson and Head, 1994, 2002; Moore and Hodges, 1980; Scott et al., 2002; Mège et al., 2003; Wyrick et al., 2004; Bleacher et al., 2009; Hauber et al., 2009, 2011; Wilson et al., 2009; Brož and Hauber, 2013; Bamberg et al., 2014; Henderson et al., 2015; Richardson et al., 2017; Peters and Christiansen, 2017), we focus on potential fissure vents around Alba Mons and low shields on the flanks of major Tharsis shields. This comparison follows the premise that basaltic fissures may have manifested in various forms on other planetary bodies even if the visible evidence has been degraded over time.
Thus, a selected subset of key visible landforms on Mars is presented, rather than attempting to include all of the many volcanic landforms, with the intent to broadly outline their distinction from volcanic systems on the Moon. One important consideration is eruptive environment, especially gravity, atmospheric and subsurface hydrologic influences compared to the Moon (Wilson and Head, 1994; Head and Wilson, 2017). We purposely avoided landforms such as tuff rings that represent basaltic eruptions influenced by water reservoirs or permafrost although they represent significant volcanic processes on Mars (e.g., Brož and Hauber, 2013). Moreover, we briefly address FFCs on Mars, another significant landform (Bamberg et al., 2014), but without detailed assessment mainly because they have not been analyzed and modeled in as much detail as the FFCs on the Moon (e.g., Jozwiak et al., 2012, 2015; 2017; Wilson and Head, 2018), nor does available imagery reveal fissure vents with sufficient resolution. Both of these volcanic landforms on Mars deserve a more comprehensive assessment, including comparative analyses, in future studies.
