Assessing the Origin and Evolution of Effusive Volcanic Channels on Mars

Introduction:  Channels of alluvial, fluvial, glacial, and volcanic origin (Figure 1) are observed on Mars and may be distinguished as volcanic channels formed via lava eruption by sharp, parallel channel boundaries, source vents, associated effusive deposits, consistent or decreasing width along their lengths, and channel shallowing along length. Some lava channels have been interpreted to have formed by either thermal, mechanical, or thermomechanical (both) erosion into the substrate.  These enigmatic channels are observed on Mercury, Venus, the Moon, and Mars, with the most notable variant being sinuous rilles [1-5]. Unlike channelized lava flows which represent a particular lava flow regime and result in constructive landforms, the channels investigated in this study have been interpreted as primarily erosive features [2-4; 6-7].  The primary objectives of this work were to: [1] establish a geospatial dataset of volcanic channels on Mars interpreted to have formed via lava effusion; [2] classify observed channels based on morphology; [3] quantify the morphology of channels; and [4] understand spatial and temporal distribution of channels across Mars.  The data presented here represents an overarching effort to characterize a global dataset.

Methods:   An initial global survey of the entire surface of Mars was conducted in order to identify volcanic channels formed via the eruption of lava and build a geospatial dataset of effusive volcanic channels. We utilized the Java Mission-planning and Analysis for Remote Sensing (JMARS) GIS to access planetary datasets and perform data analysis.  To identify channels, we utilized THEMIS (100 m/px), CTX (~5 m/px), and HiRISE (~0.5 m/px).  For topographic analyses, we utilized the MOLA 128 ppd DEM (200 m/px) and HRSC DEMs where available (~50-70 m/px).  We developed a confidence ranking to assess which channels were likely created by effusive volcanism. We rated channels from 1 = very low confidence in a volcanic origin to 5 = very high confidence in volcanic origin. Confidence in a volcanic origin was determined by identification of a likely volcanic source, presence of erupted deposits, morphological comparison to other similar volcanic features, including those on Venus, the Moon, and Earth, and geological context.  Only channels ranked 4 (likely) or 5 (very likely) were considered for further study.

To quantify channel morphology, we measured and calculated the following channel dimensions:  length of main channel, width, depth (where available), mean slope, and sinuosity. Length was measured by drawing a profile along the channel centerline (thalweg) from its source - or where first observed - to its terminus. To calculate mean width and depth, we drew equally spaced transects (every ~0.5 – 2 km) perpendicular to channel length, measuring the distance between channel walls and the vertical distance between the top of the channel wall and the bottom of the channel floor. The channel slope was calculated by dividing the total relief by channel length, and then taking the arctan of the quotient.  To calculate sinuosity, we divided the channel length by the meander belt length [3].  We also measured the distance from the volcano summit by measuring the distance from the edge of the summit caldera to the ‘source’ or ‘head’ of the channel.  All of the volcanoes in the study had a summit caldera.  Surface ages were determined using an existing global geologic map [8] and supplemented with data from a global surface dating study by [9]. 

Preliminary Results:  To date, we have mapped approximately 350 volcanic channels interpreted to have formed via flowing lava, although analysis is ongoing.  The channels occurred on the rift aprons of the Tharsis Montes and 6 Martian central volcanoes (Ceraunius, Uranus, and Hecates Tholi; Elysium Mons and its rise (above -0.3 km in elevation); Syrtis Major; and Alba Mons) [8-9].  Classification of channel morphology is ongoing although four categories of channels have emerged:  sinuous, non-sinuous (fairly straight), pit crater, and bifurcated.  Preliminary channel lengths of channels on six central volcanoes range from ~7 to 319 km, with a mean of 61 km and a median of 42 km.  Channel widths were between ~0.15 and 3 km, with a mean of 0.54 km and a median of 0.47 km. On average, observed channels had a slope of 2.2 degrees (median of 0.76) and a sinuosity value of 1.11 (median of 1.09).  Given the large dataset, analysis is ongoing.

Preliminary Discussion:  The presence of lava formed volcanic channels across Martian geologic history suggests the planet was capable of producing low viscosity lavas erupted at sustained effusion rates for much of its history.  The lava channels observed on six central volcanoes are broadly consistent in morphology with lunar examples of volcanic channels [3,7], with similar mean sinuosities [1.11 vs 1.2 for the Moon], lengths [61 km vs ~68 km for the Moon], and widths [540 m vs 637 m for the Moon].  Longer channels are additionally observed with higher frequency on the lower flanks of volcanoes, which could suggest easier magma ascent in areas with thinner crust and may hint at the presence of more than one magmatic source feeding channels.

Figure 1:  HiRISE image of a sinuous rille on the lower flanks of the Elysium rise. Note the lack of levees which would indicate a constructional process.  This section of the channel is 750m wide and 40 m deep

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