Update, May 21, 2015: They may have found a “supervolcano” caldera on Mars, per this story today. This Siloe Patera is about 40 x 30 km. For comparison, the nested caldera complex on the Martian volcano Olympus Mons is 60 x 80 km, and here on Earth, Yellowstone Caldera is 45 x 85 km.
Original post: It has been a while, but last time we looked at some of Earth’s calderas and the difference between volcanic craters and calderas. A terrestrial caldera is a rather circular collapse feature bigger than 1 km (a little over half a mile). It collapses into an emptied or partly emptied magma chamber. Most calderas are complex, showing multiple collapse features.
On Earth, volcanoes like Kilauea show these multiple collapses, too.
Calderas on other planets are big, usually more than 10 km (over 6 miles) wide. On Venus, the average-sized caldera is twice as big as Earth’s.
The Moon has impact craters and a history of flood volcanism (the dark patches on its near face that we call “The Man in the Moon” are flood basalts). It has some surprisingly young volcanoes, too, but it doesn’t have calderas. Volcanoes have been spotted on Mercury’s cratered surface, too, but again – no calderas.
The three stony planets (Earth, Venus, and Mars) have calderas that probably have formed by similar types of volcanism, though the details vary. (There are calderas on Io, one of Jupiter’s moons, too, but the volcanism comes from a different type of process, so let’s save that for another day.)
What are volcanoes on Venus and Mars like? Even though we can’t go there yet, spacecraft have collected lots of data. Astrogeologists study relationships between the magma depth, tectonic features, and topography of the caldera to find out more about volcanic processes like heat flow, crust density and thickness, and the presence of volatiles. In some cases, where volcanic features on Earth have changed because of weathering, they can learn more about terrestrial volcanoes because similar ones up there are more intact.
Most venusian and martian volcanoes are made out of “runny” basalt lava. That’s why most extraterrestrial calderas are on shield volcanoes (Kilauea and Mauna Loa are also shields). There are two types of extraterrestrial shields, based on their relief above the nearby terrain. A mons (“mountain,” pleural montes) has a large central caldera and gentle slope sat less than 5 degrees. An example, of course, is Olympus Mons.
A patera (“saucer,” pleural paterae) usually has an irregular scalloped caldera and flanks that slope at less than 1 degree. Earth doesn’t have paterae.
Venus, the second planet from the Sun, formed near Earth when the Solar System first took shape. Experts therefore think that Venus may be very similar to our home planet in terms of composition. It certainly resembles Earth in size and density, and it probably also has about the same amount of internal heat rising up to the surface from an inner core.
When it comes to volcanoes, though, there’s a big difference. Venus doesn’t appear to have plate tectonics, although its surface is quite young geologically. Many experts think that internal heat builds up underneath the crust of Venus until there is some sort major resurfacing event or events. No rock samples are available for dating, but according to crater counts, the last resurfacing event on Venus happened some 300-500 million years ago, which is pretty recent on the geological scale.
Spacecraft don’t last long under the hellish conditions on the surface of Venus, but orbiter studies have shown that the planet has almost 170 big volcanoes (each, on average, about the size of the island of Hawaii). Some of these may have been active recently. Almost a hundred of them have calderas, each one typically about 70 km (over 40 miles) wide. These shield calderas are located in the Atla-Beta-Themis triangle (I don’t know where this is, but here’s more information).
Two of the major factors for Earth’s explosive caldera aren’t present on Venus, as far as we know. No water has been detected there, and it doesn’t appear to have plate tectonics. However, the calderas of Venus appear to be dynamic, like those on our own shield volcanoes where magma often comes up through interconnected spaces underneath the volcano, with multiple collapses followed by infilling lava eruptions over time.
On Venus, there is little evidence for collapse. Instead, topography and tectonic features suggest calderas may form through downsag subsidence (basically, the rock bends instead of breaking).
Not all of the volcanoes on Mars are caldera shields, but the most spectacular ones are. These do collapse.
Whoa! Did that monstrous form all at once, dropping like a piston? Scientists think not, that instead it happened slowly, like the 1924 collapse of Kilauea’s Halema’uma’u crater, and perhaps as dramatically, if there was any water in the vicinity.
The calderas of Mars are so much bigger that a single collapse event would have been catastrophic. Many probably collapsed slowly during many relatively smaller eruptions rather than from one big one.
However, some apparently did have a piston-like collapse, like the one at Elysium Mons.
Mars is being studied intensively now, and each day there are more discoveries. Let’s just close with one of the most beautiful pictures I’ve seen anywhere, and not just related to science, either. It’s a closeup of the complex Olympus Mons caldera.
Calderas and caldera structures: a review. Cole, J. W., et al. (2005)
Calderas on Venus and Earth (II): Comparison and Models of Formation. Krassilnikov, A. S., and Head, J. (2003)
The geomorphology of planetary calderas. Mouginis-Mark, F. J. and Rowland, S. K. (2001)
Calderas: A planetary perspective, J. Geophys. Res., 89(B10), 8391–8406, doi:10.1029/JB089iB10p08391. Wood, C. A. (1984)
Extra-terrestrial Volcanoes. Oregon State.
Categories: Sunday morning volcano