Post contributed by Peter Fawdon, Dept. of Earth and Planetary Sciences, Birkbeck, University of London, UK.
Volcanism is an important process that can be observed on the surface of many planetary bodies. Not all magma bodies erupt extrusively onto the planet’s surface, many simply stall within the crust, cooling slowly over millions of years to form igneous intrusions. On Earth erosion and uplift expose the frozen core of ancient volcanoes relatively frequently, however, it is considerably more difficult to investigate this intrusive magmatism on other planets.
In Nili Patera, a volcanic caldera on Mars, it is possible to see the influence of intrusive magmatism on the surface geomorphology. Nili Patera is one of two calderas in the centre of Syrtis Major Planum (8.4˚N 69.5˚W). This volcanic plain comprised of long basaltic lava flows formed in the Hesperian (~3.0 – 3.7 Ga) and predates the formation of the central calderas which themselves contain several phases of volcanic activity. Importantly this activity includes the eruption of a lava flow with a high-silica composition [Christensen et al., 2005] and the development of a resurgent dome in the western caldera floor [Fawdon et al., 2015].
Figure 1 shows a perspective view from the southeast across the floor of Nili Patera showing the context for these high-silica lava flows. In the centre of the caldera is the bright high-silica lava flow which has lobate margins and is associated with a bright cone to the north east. This volcanic cone is called Nili Tholus. High-silica lavas are not seen anywhere else on Mars and, on Earth, they are commonly associated with melt ascending form depth through the crust and fractional crystallisation whilst the magma resides within it [Cashman and Sparks, 2013].
This bright lava flow was emplaced toward the centre of the caldera from Nili Tholus in the East. However, today the lava flow can be seen sloping uphill onto an uplifted section the western caldera where a so called “resurgent dome” has formed. Figures 1 and 2 show the graben system, which runs along the apex of the resurgent dome. On Earth resurgent domes are common in the floor of calderas. They are the surface expression of magma which has risen up though the plumbing system of the volcano and has pushed up the surface above.
The bright lava flow must predate the uplift of the resurgent dome because lava cannot flow 400 m uphill over a distance of 12 km (Figure 2). However, the data also shows (Figure 3) that the lava flow overlies some small faults on a trend associated the resurgent dome uplift. This means that the two events are probably closely related in time.One explanation illustrated in figure 4 is that there was an initial body of magma stored under Nili Patera but unable to erupt. This magma was evolving towards a silica rich composition. Subsequently a second batch of magma ascended through the plumbing system under the caldera causing sufficient uplift to create the small normal fault. This could have disrupted the first batch of magma causing it to erupt. Finally, continued growth of this second intrusive magma body uplifted the western caldera floor creating the resurgent dome and tilting the lava flow seen today.
The fact we can see evidence of these processes may be a result of the unique structural setting of the caldera. During its initial formation the floor of Nili Patera collapsed over 2 km and may be as much as 1.5 km below the mean thickness of the surrounding lava plain [Hiesinger and Head, 2004].
In other martian volcanic settings these processes, if they occur, are not expressed at the surface. At Nili Patera the subsidence into the underlying Noachian crust means the intrusive magmatism which would normally never reach the surface can be seen and therefore the evolved bright lava unit, the resurgent dome, and the relationship between provide a window into the deep crustal process of Mars.
Cashman, K. V., and R. S. J. Sparks (2013), How volcanoes work: A 25 year perspective, Geol. Soc. Am. Bull., 125(5–6), 664–690, doi:10.1130/B30720.1.
Christensen, P. R. et al. (2005), Evidence for magmatic evolution and diversity on Mars from infrared observations, Nature, 436(7050), 504–509, doi:10.1038/nature03639.
Fawdon, P., J. R. Skok, M. R. Balme, C. L. Vye-Brown, D. A. Rothery, and C. J. Jordan (2015), The geological history of Nili Patera, Mars, J. Geophys. Res. Planets, 120(5), 951–977, doi:10.1002/2015JE004795.
Hiesinger, H., and J. W. Head III (2004), The Syrtis Major volcanic province, Mars: Synthesis from Mars Global Surveyor data, J Geophys Res, 109(E1), E01004, doi:10.1029/2003je002143.
Skok, J. R., J. F. Mustard, B. L. Ehlmann, R. E. Milliken, and S. L. Murchie (2010), Silica deposits in the Nili Patera caldera on the Syrtis Major volcanic complex on Mars, Nat. Geosci, 3(12), 838–841, doi:10.1038/ngeo990.