Morphometry of eroded ignimbrite and lava flow margins: Application to Tyrrhenus Mons, Mars
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Tyrrhenus Mons (21°S, 106°E), Mars, is a low-relief (≤1800 m), central-vent, highland volcano with flank slopes ≤ 2.0°. The wide extent of its deposits, low flank slopes, the layering of its flank material, and its pervasive erosion are consistent with a pyroclastic origin. However, these observations cannot unequivocally disprove that the volcanic edifice could have been built by multiple, stacked, low-viscosity lava flows. The objective of the proposed research is to constrain the nature of the deposits surrounding Tyrrhenus Mons, by quantifying their planform shape. In general, the nature of volcanic deposits is a direct link to the internal processes that rule the volcanic activity—processes that cannot yet be directly observed. In particular, volatile content (H 2 O and CO 2 ) is a key factor for an effusive or an explosive outcome. Moreover, the development of a method that enables the identification of the type of volcanic deposits (specifically, lava flows versus pyroclastic deposits) through remote sensing would be useful for the study of inaccessible areas on Earth. In this project, data for Mars are obtained through Java Mission-planning and Analysis for Remote Sensing (JMARS) imagery. Image interpretation is based on terrestrial analogs, observed through Google Earth. The main employed instruments are: Mars Orbiter Laser Altimeter (MOLA), Thermal Emission Imaging System (THEMIS) Day IR, Mars Reconnaissance Orbiter Context Camera (CTX), High Resolution Imaging Science Experiment (HiRISE), Google Earth Pro, QGIS, and GNU Image Manipulation Program (GIMP). Three continental flood basalt lava flows and three regionally extensive ignimbrites have been identified on Earth and sections of their margins have been traced using Google Earth Pro. The outlined margins have been analysed through the software FracLac for ImageJ to obtain their fractal dimension. On Tyrrhenus Mons, sections of the lower summit shield materials and the basal shield materials units have been outlined through JMARS and GIMP and the obtained contours have been analysed through FracLac as well. Obtained average fractal dimension values ranges are 1.09 – 1.40 for lava deposits margins, 1.17 – 1.58 for pyroclastic deposits margins, and 1.20 – 1.23 for Tyrrhenus Mons deposits margins. Including the standard deviation, the martian range is contained in both the lava and the pyroclastic ranges, but the martian range fits better within the lava range of fractal dimensions. In fact, the martian range overlaps the pyroclastic one only in the lowest values. The results are not conclusive, but, based on fractal dimension measured through remote sensing, Tyrrhenus Mons’ analysed deposits are more consistent with lava flows rather than with pyroclastic deposits. On Earth, if an ignimbrite is thoroughly welded, the difference between ignimbrite and lava is not easily detectible using only the fractal dimension of the deposit margin. For this reason, I propose that Tyrrhenus Mons shield deposits are constituted by either lava flows or extensively welded ignimbrites.