2 March 2009
The MOC "Book": Volcanic Landforms
Posted by Ryan Anderson
More about the MOC paper! This is part six of a series of posts looking at the huge 2001 paper summarizing the results from the Mars Orbital Camera (MOC): the first high-resolution camera in orbit around Mars. Check out the previous posts if you want to get caught up: 1,2,3,4,5
Today’s topic is volcanism, something I’ve written a lot about before on the blog. Mars is essentially a volcanic planet, so this is an important part in understanding the surface that we see today. The paper first looks at lava flows on the large shield volcanoes of Tharsis, and notes that near the summit, flows are difficult to distinguish, but they become more distinct farther down:
The upper flanks of Olympus Mons are a confusing mess of interfingered lava flows and leveed lava channels that are difficult to tell apart.
Farther down on Olympus Mons, flows are more distinct. This is partly due to the fact that there are fewer of them, but they are also thicker because the lava has flowed farther and therefore was cooler and more viscous.
The authors also note that collapse features are generally rare on the large volcanoes. Ceraunius Tholus is an exception: its caldera is pock-marked with many circular collapse pits.
Collapse pits in the caldera of Ceraunius Tholus. These pits are distinct from impact craters because they have no rim, are of a more uniform size, and vary in concentration across the caldera.
There is ample evidence for volcanism other than the giant volcanoes. Vast expanses of Mars are a “platy” terrain formed by the rafting of large plates of basalt on the surface of lava lakes and seas. These have been mistakenly interpreted as evidence of sea ice on Mars because it looks very similar to ice on terrestrial oceans. This makes sense, because the process is essentially the same: you have a large body of liquid exposed to an atmosphere cold enough to freeze the surface, resulting in floating plates. One of my favorite examples from the paper was in Amazonis, where the platy lava surface is being exhumed from beneath wind-eroded yardangs. Nearby, the lava surrounds the ejecta from a crater, so that you immediately get a multiple step timeline: first the crater formed and emplaced its ejecta, then the flood of lava surrounded but didn’t quite bury the ejecta. The lava froze to form a hard, flat surface, which was then covered with a soft sedimentary rock (perhaps ash from the same eruption that formed the flood of lava). Now the soft rock is eroding away to expose the much harder plates and ejecta underneath.
This is a CTX view of a platy lava flow being exhumed from beneath yardangs in Amazonis. It is clear that the yardang material is much softer than the underlying rock, which is emerging unscathed even as the yardangs are being eroded by the wind.
MOC also confirmed that there are much smaller volcanoes, such as this one:
A small shield volcano inside the caldera of Arsia Mons. You can just barely make out a texture of lava flows radiating from the central pit.
That sums of the volcanism section! Obviously there are plenty more MOC pictures of volcanoes and volcanic features on Mars, but those were the highlights of the section for me. We also spent a while discussing the very interesting Olympica Fossae, but that’s complicated enough that it will need its own post. Stay tuned: tomorrow’s topic is valleys of all shapes and sizes!
