CATASTROPHIC FLOODS OF BASALT: LESSONS FROM MARS

The environmental impact of volcanic eruptions is closely tied to eruption rate. However rapid global impact is only expected if an eruption is vigorous enough to inject volcanic gases and aerosols into the stratosphere or significantly change ocean chemistry. Studies in the past 20 years have shown that many flood basalt eruptions had sustained eruption rates of order 10³ -10⁴ m³/s. The predicted volcanic plumes approach, but do not cross, the tropopause at the latitudes of these eruptions. The emplacement of an entire flood basalt province, polluting the troposphere with hundreds of large eruptions in less than a million years, may still be able to have a major negative impact on the environment. However, recent studies of flows on Mars emplaced with peak eruption rates of 10⁶ – 10⁷ m³/s discovered an unfamiliar set of lava morphologies. The best-exposed example on Mars is the Athabasca Valles flood lava. Its estimated age is between 2-50 Ma, making it the youngest major volcanic event on Mars. Its volume is about 5000 km³ and lava extends to 1400 km from the fissure vents. These characteristics are not unusual for terrestrial flood lavas. What is strikingly different is that the lava filled the ~30-km-wide Athabasca valley system to a depth of ~100 m and then drained away, leaving only a few meter thick carapace. This type of deflated lava is common, albeit at a much smaller scale, in the near-vent area of basaltic eruptions on Earth. In the case of Athabasca Valles, major deflation extends for >100 km from the vent and lesser deflation for much further. Evidence of surges in eruption rate can be found from the vent area through to the distal parts of the flow. Despite an eruption duration of order weeks, the much of the margins and some interior sections are composed of slowly emplaced pahoehoe breakouts from the ponded flow. There are also suggestions that massive volumes of pyroclastic materials were produced by these eruptions, but their identification is uncertain. An only slightly smaller flood lava is found in Kasei Valles and modestly eroded the substrate and similar erosion is plausible in Athabasca Valles. These observations from Mars provide observable characteristics to search for in terrestrial flood basalt provinces to determining if similarly high eruption rates were, or were not, part of flood basalt eruptions on Earth.