BUILDING ASCRAEUS MONS: SUMMIT CALDERAS AND THEIR BIG ERUPTIONS

Ascraeus Mons (18 km in height and ~350 km in diameter) is the northeastern-most of the large shield volcanoes located in the Tharsis province on Mars. We mapped the large summit caldera complex at a 1:100,000 scale using THEMIS, CTX, HiRISE, and HRSC imagery, and MOLA topography, and made estimates of caldera, magma chamber, and erupted volumes for each of the 9 calderas. The order of caldera formation from oldest to youngest is: the western-most crater (I), consistent with the crater-counts of Neukum et al., (2004, Nature Vol 432 971-979) and Werner (2009, Icarus 201 44–68); the northwestern caldera (II); the eastern-most caldera (III), which was modified by the collapse events of calderas IV, V, and VI, where V and VI have lower floor elevations; the two southeastern calderas (VII and VIII), such that the larger diameter crater predates the smaller crater; and finally the large central caldera (IX) with highly disrupted crater walls, that suggest multiple collapse events during caldera formation.

We then calculated the magma chamber volume (V_T) and the minimum volume erupted (V_emin) required to generate caldera collapse using the model of Geshi et al., (2014, Earth and Planetary Science Letters 396, 107–115). Estimates of V_T range from 10² - 10⁴ km³ and corresponding values of V_emin are 10 - 10² km³, consistent with volumes of individual lava flows in Tharsis. Input parameters include magma density, assumed to be 2800 kg/m³; bulk modulus of the magma assumed to be 2.11x10¹¹ Pa; caldera radius and wall slope, measured from the MOLA DEM; and depth to magma chamber, varied from 10 to 20 km. Displaced caldera volumes estimated using the Polygon Volume Tool in ESRI ArcGIS^TM range from 10² – 10³ km³, and place an upper-bound on the erupted volumes. Further sensitivity analysis will be conducted to better constrain depth to chamber and erupted volumes and later compared with other Martian volcanoes.