The Geology of the Ka'u Desert, Hawaii as a Mars Analog

The Ka'u Desert is located on the western flank of Kilauea volcano on the Big Island of Hawaii. It is a desert because it receives little annual rainfall (about 150 mm/yr) but also because it is subjected to constant outgassing from Kilauea, which creates a harsh, acidic environment. Near the summit of Kilauea the Ka'u Desert is characterized by the Keanakako'i tephra deposit, which is several meters deep thinning out to a discontinuous deposit 1.5 km (1 mile) towards the center of the desert. The deposit itself has been incised by a number of gullies that are flat-floored and terminate in a series of amphitheater-shaped plunge pools. Most of the interior desert contains undulating weathered lava flows, extensive deposits of sand, and several more recent lava flows and volcanic edifices. The southern portion of the desert is bounded by the Hilina Pali fault scarp, which is 500 m (1,500 ft) above the nearby Pacific Ocean and contains a complex series of outwash plains, alluvial fans, and debris flows. We will present a summary of the geology of the Ka'u Desert. Contrary to published interpretations, we will present evidence that the Keanakako'i was not emplaced by two separate catastrophic eruption events but rather by two distinct eruption episodes that included multiple eruption events often interrupted by long hiatuses. Despite the morphology of the gullies contained on the Keanakako'i we will present evidence that the gullies were formed exclusively by surface runoff and not groundwater sapping, including quantitative estimates about the large amounts of discharge that occur during extreme storms. We will also present analyses of the sand deposits and determine the likely provenance of these materials. For the first time, we will also describe alluvial fans and mass wasting features on Hilina Pali and show evidence that they are part of poorly integrated channel system that originates in the Keanakako'i tephra. The Ka'u Desert represents a good Mars analog because (1) similar to valley networks, the development of channels and gullies in the Ka'u Desert has been interrupted by resurfacing events, (2) associated fluvial deposits have been heavily modified by eolian processes, and (3) the alluvial fans on the Hilina Pali have unusually large source areas.