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Extraterrestrial Volcanism

Introduction

An interesting subject? Oh yes. A simple one? Yeah, right.

It is important to note that my studies deal with terrestrial volcanism, and so the information contained in this article is unlikely to be as specific as that of other articles written by myself. Still, I hope to supply a wealth of information through this writing that is difficult to find elsewhere on the internet.

The big question is “why do we study volcanism on other planetary bodies?” Well, the most accurate answer is: because it’s interesting. Every volcanically active planetary body we’ve found differs from Earth in some fairly dramatic ways. This sheds insight on how physical properties of the environment affect the eruption; these changes include: gravity, atmospheric pressure, planetary composition, and surface temperature. Because each planetary body presents a unique environment to our study, we will look at each one separately.

Lunar Volcanism

The moon, as you may have guessed, is not volcanically active. It was at one time, however, and is mostly composed of basalt on the surface. During the moon’s past, most volcanic activity was expressed as a style known as Mare volcanism, comparable to Earth’s flood basalts. Mare is the Latin word for sea; mares were so named because when viewed through telescopes, they appeared to be large smooth areas. These were initially thought to be oceans, and though it was later found that there are no oceans on the moon, the name stuck. Mares are actually large regions of basalt; these areas appear smooth because there is a very notable lack of impact craters on them. While this lack is far from 100%, it is important because it helps us with two important questions: how old are these volcanics and when did most of the moon’s meteorite impacts occur? Well, we know that most of the moon’s volcanoes occurred 3-4 billion years ago, although there are one or two that may be as young as one billion years old. There are no fresh volcanoes on the moon because it has cooled significantly since its formation.

The interior of the Earth is kept hot mostly by the decay of radioactive uranium within the mantle and core. While the moon also has this uranium, there is much less…due to the smaller size of the moon. Hence, due to the smaller size (which means the interior can cool more quickly) and the less uranium, the moon’s interior is completely or almost completely solid. Unless there is a devastating asteroid impact to remelt the moon’s interior, it will probably never see an eruption again. The mares of the moon are extremely well preserved due to other important factors- like the lack of wind and rain, which prevents erosion. Moreover, on earth the oceanic crust is recycled every hundred-thousand years or so, but there are no plate tectonics on the moon which means that those mares are permanent features.

These mare flood basalts aren’t the only style of volcanism present on the moon, however. There are also fields of small shield volcanoes, which are typically several hundred meters tall and several thousand wide. A bizarre type of structure, known as “sinuous rilles,” frequently wind their way through mare basalt fields. These rilles are dramatic valleys, which can be wider than a kilometer, stretch over a hundred kilometers, and reach depths of hundreds of meters. It seems that these are lava channels, but they differ from terrestrial (earth) lava channels in their formation. On earth, lava flows build walls on either side of themselves and will continue to flow between these two lava levees. Here the wall has been built up; on the moon, we have no built wall; just a steep valley through which the lava once traveled. Current theory maintains, however, that due to the extremely high temperature of magma and the massive volumes, the lava thermally eroded a channel for itself; that is, it melted through the pre-existing rock. Hence, we have the formation of these dramatic valleys: sinuous rilles.

Of course, it’s not all docile flows, though; there are pyroclastics found. In some locations, lunar exploration missions have discovered green glass beads. In other places, there are black and orange glass beads. These beads are thought to be the product of lava fountaining on the moon! As olvine-rich magma is ejected through a narrow vent, the rapid decompression of the lava and it’s dissolved gasses cause it to jet into a sort of lava mist. As the mist cools, it forms gas-free droplets that fall back to the surface. Most of the gas has exsolved due to the lack of atmospheric pressure. On earth, drops deform into tear shapes as they encounter wind resistance. On the moon, however, winds are scarce…there being no atmosphere and all. Hence, the droplets form spherical beads as they fall back to the surface. It is unknown how high these drops may reach since we have never witnessed an eruption. They may reach great distances, however, due to a lack of wind resistance and a lower gravitational field.

Venutian Volcanism

Venus was named after the Roman goddess for its serenity and beauty. After all, the pastel-colored planet seemed perfectly calm to early astronomers. In recent times, we’ve found that Venus is anything but. With howling winds, an atmosphere composed of acid gasses, surface temperatures hot enough to melt lead, an atmospheric pressure equal to 1000 meters beneath our oceans, and the entire surface being continually remolded by lava, the peaceful planet turned out to be an explorer’s worst nightmare. Few surface probes have been sent to Venus as the conditions are simply too rough to endure. The intense heat, pressure, and atmospheric acids would wreak havoc on the probe if not properly designed. The thick clouds of CO2 that shroud the planet also tend to muffle any signal sent back to Earth. What we know of our neighbor has been mostly gleaned from orbiting satellites that have done a pretty efficient job of mapping the surface using radar. What we’ve discovered is that there are only two groups of people that would probably enjoy inhabiting Venus: meteorologists and volcanologists.

Venus has a large number of volcanoes, comparable to Earth, many of which are unnamed. The size of the volcanoes can be enormous with some exceeding one thousand kilometers in diameter. They are certainly not all huge, though, with most less than 20 kilometers in diameter. Many of the smallest volcanoes may yet be unidentified and, because of their number, most go unstudied. While Venus’ volcanoes tend to have huge diameters, their altitudes rarely exceed 1.5 km. This basically means that they are shields to the extreme. This flatness of the Venutian volcanoes is likely due to very fluid lava (due to chemical composition) and the high surface temperature, which does not cool the lava quickly. Volcanism often occurs along rifts, though there are no plate tectonics on Venus. Most Venutian volcanism is best described by hotspot models.

Understanding the characteristics of these volcanoes can be extremely challenging because the vast majority of our data comes from radar reflectivity; if it’s more reflective, it shows as white. If it’s less, it shows as black. We see that at large volcanoes, the central region is dark while the outside border appears very bright. This means one of several things: that the center of the volcano’s surface is composed of ash or smooth lava, and that the outside border is very rough (possibly indicating an a’a style lava flow). Not all of Venus’ volcanoes are flat, however. Some smaller ones (~20 km in diameter) show very steep sides. While we don’t why this is exactly, we know this is in part due to higher viscosity magma reaching the surface. The higher viscosity may be due to more silicic magma, although it could be caused by a higher volatile content. On Earth, volatiles tend to exsolve violently, but with the incredibly high pressure of Venus, exsolution may be limited and result in a higher viscosity (the bubble-rich foam is resistant to flow). Also present on Venus are hundreds of volcanic fields, each containing hundreds of smaller volcanoes. At larger volcanoes, we find calderas present (much as we do on Earth). The word “Patera” is usually assigned to them, which describes a large irregular depression on foreign planets.

Lava takes on a number of characteristics on Venus. In many places there are Plains lavas, which are relatively flat…much as the name suggests. These flows are almost entirely emplaced at the same time, much as a flood basalt behaves. Fluctii are a different kind of flow field, which is composed of many individual flows. These differ from the plains lavas because the former were deposited as a single event while fluctii are the products of multiple flows and eruptions. Large lava channels transect the landscape, with widths as much as 10 km (for comparison, most lava channels at Kilauea are a few meters). These channels can extend hundreds of kilometers, with one setting the solar system record at 6800 km (the Nile river is only about 6500 km). Some lava flows are thought to be komatiites (which are rare on Earth) that are more fluid than basalt. Still other flows may be composed of carbonatite or sulfur-based lava! This is all theoretical of course; there’s no way to know for sure at present.

Unusual volcanic features occur, which include radial fracture centers. Here a network of fractures spread from a single point outward. That single point is typically a large uplifted region. This suggests two mechanisms for the formation of these fractures: they formed from the stresses that occurred during uplift, or they outline subsurface dikes that have been emplaced. In my opinion, the two are probably related. Perhaps the uplift is associated with the emplacement of a large subsurface magma body. This body may then have fed dikes that propagated along pre-existing cracks that formed during the uplift. It’s all highly theoretical and so I hope that I’ve not lost you. If the dike idea is accurate, then these radial dike swarms may be a precursor to other unusual features like:

Coronae. Coronae are large circular features that are traced by ridges and fractures, which help to reveal their shape. A Venutian corona may be present as a dome, a plateau, or a depression. It is thought that they probably begin as radial dike swarms. After a time, uplift ceases and the underlying magma body flattens out. This results in surface collapse. This process may or may not repeat itself, but almost undoubtedly is the reason for these structures’ existence.

Arachnoids. These structures are named for their spider-like appearance. The best way to describe these features without a picture (though a web search should find some for you) is to say it appears as though a corona was placed on top of a radial fracture center. The corona would be the spider’s body, and the fractures its legs. These are thought to form in a manner similar to that of coronae.

And here we come to the end of our section on the volcanoes of Venus. As is always the case in these articles, there is much more to learn; I simply present a basic overview of the science.

Martian Volcanics

Compared to volcanism on Venus, that of Mars is relatively simple. In addition, due to the amount of effort that has been put into studying Mars, we have a better understanding of its volcanism than any other planet. Regardless, Mars does not currently display active volcanic activity, although it certainly has occurred during its past.

The distribution of Mars’ volcanoes is not uniform, but displays a sort of “bunching.” The largest batch of volcanoes occurs atop an area known as the Tharsis region. This region consists of a 4000 km diameter bulge that rises 10 km from the surface. On this bulge reside the four tallest volcanoes on Mars, including Olympus Mons which sets the size record for all mountains in the solar system. Olympus Mons is 500 kilometers in diameter and reaches a height of roughly 25 km…that is about 81250 feet! Mars’ largest volcanoes are all shields that appear much as our earthly shield volcanoes do, but on a much larger scale. It is thought that, as Mars does not display plate tectonics, Olympus Mons and the rest of the large shields were formed via hotspot volcanism; this may explain the massive Tharsis bulge. Much like our terrestrial volcanoes, Mars’ shields contain numerous intersecting lava flows while fan deposits at the slopes were probably caused by landslides.

All four of Mars’ large shield volcanoes contain summit calderas. Also containing a summit caldera is Elysium Mons, a large volcano outside of the Tharsis Region. Elysium Mons is significantly steeper than the shields of Tharsis, and is thought to have been created by more viscous magmas. The shape of the volcano is comparable to composite cones in the Andes and Cascades, but it has been decided (with little opposition) that this volcano represents a hotspot comparable to Africa’s Tibesti volcano.

Alba Patera is an unusual feature, with a slope of less than one degree. It’s diameter is very close to that of Olympus Mons, but barely rises from the surface. Current theory maintains that this feature may be akin to the coronae seen on Venus and discussed earlier.

Paterae; a patera is an irregular crater with outward facing slopes along the edges. Most Paterae craters are around half the size of the entire feature, and so it is thought that many of them may be enormous calderas atop ancient shield volcanoes. Since many appear in the Tharsis Region, the idea is well supported. Highland Paterae differ from typical paterae on Mars, however, because they display large pyroclastic deposits. As the name suggests, these paterae occur atop the highlands of Mars. They typically display extremely gradual slopes that are composed of volcanic ash (though a physical sample has never been taken). Cut into the slopes are numerous channels whose origins are still being debated, but may be volcanic.

Tholi are smaller dome mountains, which indicate effusion of more viscous lavas. It is likely that Elysium Mons would have been named Elysium Tholus if it were a good deal smaller. For domes too small to be called tholi, we have an unnamed type usually referred to as a small construct. These are usually interpreted to be cinder cones, or at least structures analogous to cinder cones. Some people suggest a maar style origin for these features and still others claim a periglacial origin for them. In my opinion, the most realistic proposal is the cinder cone style origin. These occur in clusters, which are spread across several locations on the red planet.

The final type of volcanic feature on Mars is the volcanic plain. These plains may have a number of different origins. They can stretch over hundreds of kilometers or simply fill an impact crater. Many are relatively featureless in even the best photographs, while others show an immense number of structures. Many show enormous lava flows that snake along the plains for hundreds of kilometers. It is thought that while some may be composed of lava flows, still others may be composed of volcanic ash. In some places, this ash is thought to be over 2 km thick! The source for many volcanic plains remain hidden, however, and it is difficult to postulate much further on their features or origins without actually sampling or running ground-based studies at the sites.

Volcanism of Io

Io is a planetary body you may or may not be familiar with. It is, in fact, one of Jupiter’s moons and by far the most volcanically active body in the solar system. There are a number of reasons we might not expect this. The biggest reason is that this moon is small and far from the sun, meaning that its hot interior should cool extremely quickly. This is not the case, however, because of Jupiter’s intense gravitational field.

Of all Jupiter’s discovered satellites, Io is the closest orbiter. Intense tidal forces, caused by the enormous gravity of our largest planet, cause the interior to remain fluid. The action is similar to that of the moon on our oceans, but in a very different and much more dramatic fashion. The model of how these volcanoes operate is very similar to our terrestrial hotspot models. The lack of surface impact craters, which virtually all other moons contain, indicates that the surface is completely buried to an average depth of 1 km every million years. Large volcanic centers dot the surface, as well as calderas that are as much as 200 km wide and 2 km deep. What we don’t see on Io are very tall volcanoes, such as occur on Earth and Mars. Instead, we usually see volcanic plains which indicate very low viscosity lavas. Mountains do dot the Ionian surface, but are rare and do not occur in groupings of any kind. These were probably once volcanoes in the past.

Dramatic coloring of Io’s volcanic plains include yellows and bright greens. These colors are likely the result of sulfur-rich lava flows and SO2 frost that was erupted from nearby volcanic vents. While reds and oranges are also abundant, much of Io’s coloring remains unexplained…especially since it appears to be short-lived. Many regions across Io seem to change color on a fairly regular basis. Most of the coloring on Io, however, appears to be the result of sulfur occurring in various forms and minerals. Despite the amount of sulfur present however, most of the activity seems to be silica-based. This is because sulfur vaporizes on Io’s surface (partially due to the lack of atmosphere) at around 500 kelvin. Hotspot temperature measurements from satellites indicate temperatures as high as 2000 K. It is impossible to know, with any certainty, just what these lavas are composed of until we sample these flows, however.

Discovery of Io’s volcanic activity was fairly recent. Earth based observations during the 1970’s indicated that the surface was dominated by sulfur and far hotter than we thought it should be. This indicated that the surface may be active. Activity was finally confirmed by images from the Voyager spacecraft which recorded a pair of umbrella-shaped plumes rising high above the surface. In fact, plumes from Io have been observed to reach heights of 300 km and have probably exceeded this dramatically in the past (consider how poorly studied Io has been). Also, compare this to terrestrial plumes which rarely exceed heights of 15 km…and this facilitated by the work of atmospheric convection which is totally absent on Io. Eruption velocities from the vent for these plumes have been calculated to be as high as 1 km/sec. That is over 2200 mph! Because there is no wind on this moon, volcanic plumes tend to form beautiful umbrella shapes above the surface.

Conclusion

There is one more style of volcanism that has not been discussed here, commonly called cryovolcanism. Because this style of activity is highly theoretical and currently in doubt, it will be discussed in a future section that will be named “volcanic analogies” or something along those lines. For this article, we have discussed the volcanic behavior of our Moon, Venus, Mars, and Io…the only known active bodies in the solar system other than Earth. We’ve covered the different eruption styles, the manner in which they occur, and the features they leave at the surface. It’s difficult to know much more about them until we begin putting geologists on these bodies…something unlikely to happen during any of our lifetimes. Ongoing studies will continue to bring new data to light, helping to reveal the very processes that may be occurring on our own planet, albeit in a different form. If you’re interested in learning more on the subject, I would encourage you to research science journals on the subject.