Overview
The shield volcano Olympus Mons (Latin for Mount Olympus) is one of the broadest volcanoes and certainly the tallest in the Solar System. Olympus Mons has a relief of about 22 km and a basal extent in excess of 600 km. The style of volcanic activity at Olympus Mons is predominantly effusive, and the volcano is thought to have been active since 4 Ga (Werner, 2009).
Olympus Mons is located at 18° N, 133° W, on the northwestern edge of a large volcanic region named the Tharsis province (fig. 1). Shield volcanoes are the most striking features in the Tharsis province. In morphology these volcanoes share many similarities to shield volcanoes on Earth, but there are systematic differences in size, as the Martian examples are typically 2-3 times larger in terms of height and width (Carr et al., 1977). Even more extreme are the differences in the dimensions of the calderas which commonly occur at the summits of these volcanoes: the calderas on the Tharsis shields vary in diameter between ~20 and ~100 km, as compared with Hawaiian shield diameters that do not greatly exceed 5 km.
Figure 1. Mars Orbiter Laser Altimeter background image of the Tharsis Province, Mars. Cold (blue) color indicates low elevation, and warmer (red) color indicates high elevation. The words 'Mons', 'Planum', and 'Planitia' are Latin for 'mountain', 'plateau', and 'plains', respectively. Olympus Mons is located on the northwestern edge of the Tharsis Rise. To the north and west of Olympus are low lying plains, and to the south and east are smooth volcanic lava flow deposits.
The primary reason for the difference in size is due to the lack of plate tectonics on Mars (Greeley and Spudis, 1981). As a result, the shield volcanoes in Tharsis (and elsewhere on Mars) were constructed from the continuous deposition of lava from the same point source, or hot spot (Greeley and Spudis, 1981). Additional factors that contribute to the difference in volcano shape include lower gravitational acceleration, which is about one third of Earth's, and higher effusion rates and lower viscosity lavas (Greeley and Spudis, 1981).
The current understanding of the volcanic activity on Mars is that volcanism was very active early in the Martian history, and decreased in intensity in the past 3.6 Gyr (Wilson et al., 2001; Werner, 2009; Carr and Head, 2010). Early martian history (before about 3.6 Ga ago) is characterized by high effusion rates (Bleacher et al., 2007), and long lived volcanic eruptions (Werner et al., 2001). As a result, all volcanic constructs were formed and built up to their present size prior to ~3.6 Ga (Werner et al., 2007). Volcanic activity in Tharsis since ~3.6 Ga is believed to have become more episodic, occurring once every few hundred million years (Wilson et al., 2001), with the most recent activity 2 Ma ago on the flanks of Olympus Mons (Neukum et al., 2004).
Crater Retention Ages: A Planetary Scientists Best Friend
Age dates ascribed to volcanic units on Mars can be derived qualitatively, using cross cutting relationships, or numerically, through the use of crater statistical techniques. The most widely used age dating method is crater statistics because the technique provides reliable dates for nearly all surfaces on Mars. The theory behind crater retention age dating is that more densely cratered surfaces are older than sparsely cratered surfaces because of differences in exposure to bolide impacts (Hartmann, 2005). Numerical ages from craters have been determined from cratering rate frequencies and dated Lunar rock samples (Hartman, 2005). From this lunar data, isochrones for various bins of crater diameters over a given areal extent have been established for Mars (e.g. Hartmann, 2005). This method has allowed planetary scientists to determine the approximate ages for surface units on Mars, and is important to be familiar with to understand the volcanic history of Olympus Mons.
Figure 2. (a) Example of a young, rough-textured lava flow with extremely few impact craters, overlapping older sparsely cratered lava plains. (b) Crater count data suggests that older lavas date back to a few hundred Ma, and younger lavas appear to date from a few tens of Ma. This data is taken from Figure 7 in Hartmann, (2005).
Regional Tectonics
Loading of the crust by the addition of magma pressure in Tharsis, and the addition of weight has produced radial dikes and has controlled the regional tectonic regime. Take note that Olympus Mons is located off to the northwest of the major zone of fracturing.
Figure 3. Distribution of radial grabens around the Tharsis Rise. Taken from Mege and Masson, 1996.
In addition to the mapping of surface units and inferences drawn therein to learn about the stress orientations, numerical modeling techniques have been used to gain some insight into the distribution of stress on Mars (Sleep and Phillips, 1982).
Figure 4. The horizontal principal stress with the largest magnitude is indicated by the longer arrows and the other principal horizontal stress by a short arrow. The arrowheads indicate extension and arrows without heads indicate compression. The distribution of stress determined favors the formation of dikes radial to the Tharsis province, which is observed by Mege and Masson, (1996). The stress model shown here was produced by Sleep and Phillips, (1982).
Figure 5. Mars Orbiter Laser Altimeter topography of Mars is provided for comparison with with the above figure.