Mars is smaller than Earth, but it hosts some giants.
Olympus Mons is the Solar System’s tallest known volcano. The Tharsis Rise, an outward bulge in the planet’s surface, is crowned by the three enormous shields: Ascraeus, Pavonis, and Arias. Last but not least, monopolizing northern Tharsis, is Alba Mons – a volcano with no earthly equivalent.
Some or all of these volcanoes may still be active.
Known as Nix Olympica (the Snows of Olympus) since the 19th century, this high-albedo feature near the Martian equator was renamed Olympus Mons in the second half of the 20th century when Mars-orbiting satellite imagery showed it to be a shield volcano.
And what a volcano it is!
That’s not an exaggeration – according to NASA, all the Hawaiian Islands could indeed fit into Olympus Mons!
Those cliffs around the volcano’s base stand as much as 5 miles above the surrounding plains. Close up, they show interbedded areas of hard rock (probably lava) and soft (dust or volcanic ash) material. Scientists believe they formed as landslides and perpendicular faulting from extension after the weight of all that piled-up lava exceeded its strength.
A 1.2-mile-deep depression surrounds Olympus where the volcano has depressed the planet’s crust. Beyond this is an area of rugged terrain, called an aureole, that extends up to 470 miles from the volcano. It probably formed in giant landslides off of the flanks of Olympus Mons, possibly when permafrost melted.
About 750 miles southeast of Olympus Mons, as much as 25% of the surface of Mars starts to bulge outward as the Tharsis Rise, reaching an elevation of a little over 4 miles above the datum (what sea level would be on Earth). On top of that four-mile bulge are three giant volcanoes. (Montes is the pleural of mons, Latin for mountain.)
Ascraeus Mons is the northernmost and tallest of the three at 59,793 feet above the Mars datum. That’s only about 10,000 feet lower than neighboring Olympus Mons. It is some 300 miles in diameter and, because it is a shield volcano, Ascraeus has very gentle slopes. At its summit sits a 15-mile-wide central caldera made out of four smaller calderas.
Pavonis Mons, the smallest, sits in the middle. It may be modest by Tharsis Rise standards, but Pavonis is still almost twice as high as Mount Everest. It has two nested calderas at the summit, the smaller one over 3 miles deep.
Arsia Mons is over 52,000 feet high and has a massive a 72-mile-wide summit caldera. With a diameter of 295 miles, Arsia is the biggest of the Tharsis giants in terms of volume.
Thin carbon dioxide/water clouds form over all the high volcanoes, but Arsia has some unique weather:
A repeated weather phenomenon occurs each year near the start of southern winter over Arsia Mons. Just before southern winter begins, sunlight warms the air on the slopes of the volcano. This air rises, bringing small amounts of dust with it. Eventually the rising air converges over the volcano’s caldera and the fine sediment blown up from the volcano’s slopes coalesces into a spiraling cloud of dust that is thick enough to observe from orbit. The spiral dust cloud over Arsia Mons repeats each year, but observations and computer calculations indicate it can only form during a short period of time each year. Similar spiral clouds have not been seen over the other large Tharsis volcanoes, but other types of clouds have been seen. The spiral dust cloud over Arsia Mons can tower 15 to 30 kilometres (9.3 to 18.6 mi) above the volcano.
All of the great volcanoes are covered in dust that has accumulated during the great dust storms. This makes it difficult to identify some surface features on the volcanic edifices, but all of them definitely show lava flows, collapse features, landslides and evidence of tectonic movement.
Scientists are especially fascinated by the way the three Tharsis Montes line up southwest to northeast on the planet’s bulge and how evenly spaced they are. Each peak is about 430 miles from its neighbor. No definitive explanation for this has yet been given.
Just over 1,000 miles northeast of Olympus Mons and dominating the northern part of the Tharsis Rise is Alba Mons, one of the biggest known volcanoes in the Solar System.
It’s less than a third the height of Olympus Mons, but its flow fields extend almost 840 miles from the summit.
The Tharsis shield volcanoes have very gentle slopes (down to 4 degrees on Pavonis). The steepest slope on Alba Mons is only 0.5 degrees!
Olympus Mons covers an area comparable to the US state of Arizona or the countries of France or Germany. Alba Mons covers an area the size of Libya.
But comparisons are inexact. There is no volcano on Earth that resembles Alba Mons, which is basically just a very slightly raised but massive planetary bleb.
No one has seen hot lava flowing on Mars, but that doesn’t mean its volcanoes are all extinct.
No accurate dating is yet possible for these five giant volcanoes. The Mars rovers were sent to investigate possible sites for life and for human settlements – they haven’t visited the big volcanoes to pick up and test rock samples.
However, because the entire Solar System underwent the Late Heavy Bombardment about 4 billion years ago, general ages on Mars can be determined by counting impact craters and then comparing results to studies of craters on the Moon.
The most heavily cratered (oldest) surfaces are found at Alba Mons and in parts of Ascraeus caldera, some of which could be as much as 3.6 billion years old. The least cratered (youngest) areas are found on the western flank of Olympus Mons and might have formed 2.4 million years ago – in geology terms, that’s just yesterday.
Other parts of the Ascraeus caldera may have been erupted about 100 million years ago. The whole Arsia Mons summit caldera is about 130 million years old. Astrogeologists say that Pavonis shares the same history as its two Tharsis companions.
As for Olympus Mons, its nested calderas formed some 100-200 million years ago.
On Earth, plate tectonics is responsible for most volcanism, which occurs around the edges of the huge plates of crust that cover our planet’s surface. Intraplate volcanoes like the Hawaiian Islands may be caused by upwelling plumes of molten rock from deep in the mantle, though the existence of mantle plumes is still controversial.
No one has yet found proof that Mars has ongoing plate tectonics. The Tharsis region therefore presents a problem. The Martian crust there must have cooled and hardened very early in the planet’s history, since it still bulges despite carrying the combined weight of some of the biggest volcanoes in the Solar System.
How, then, did volcanism happen afterwards?
A University of Colorado researcher has proposed that, instead of many individual mantle plumes, like those of Earth, its mantle might contain a single rising plume of molten material. The linear arrangement of the three Tharsis Montes suggests that there may be enough melt down there to allow the hard outer crustal plate to rotate sideways.
This “shell tectonics” theory has impressed experts because it explains not only the Tharsis bulge and volcanism there but also the unusual arrangement of the planet’s surface (highlands in the south and lowlands in the north).
This process might have been more active in the distant past, but there is no reason why it isn’t still happening.
Olympus Mons, Alba Mons and the Tharsis Montes may not be extinct, just sleeping.
There aren’t yet enough instruments on Mars to study the planet’s interior as well as we have studied the interiors of Earth and the Moon. For now, all we can do is marvel at Olympus Mars, Alba Patera and the volcanoes of the Tharsis Rise.
Catherine L. Johnson and Roger J. Phillips. Evolution of the Tharsis region of Mars: insights from magnetic field observations. Earth and Planetary Science Letters, 230 (2005) 241–25.
Linda T. Elkins-Tanton, The Solar System: Mars. New York. Chelsea House, 2006
NASA. Olympus Mons.
Wikipedia. Volcanology of Mars
Shijie Zhong. Migration of Tharsis volcanism on Mars caused by differential rotation of the lithosphere. Nature Geoscience 2, 19 – 23 (2009)
Categories: Sunday morning volcano