The last time I wrote an article here, it was to talk about Afar Region volcanoes, and I promised more to come. This is still a project that I have in mind, but that will have to wait a little longer until I can get an extended period of free time. Today we will go farther away than Afar, to a place whose hellish conditions make the Ethiopian deserts seem like a paradise. I’m here to answer one question: Is Venusian volcanism active, and how and where is this volcanism occurring?

The age and evolution of Venusian volcanism is something that has interested me for quite some time. Understanding extraterrestrial volcanoes might help us understand volcanism as a whole much better. The more I learn about extraterrestrial volcanism, the more I think we are the weird ones. The more I learn, the more I see similarities between Lunar, Martian, and Venusian volcanism, as opposed to Terrestrial volcanic activity, although each rocky body gives volcanoes its personal touch. It might also come as a surprise that I consider Venus to have a more normal volcanism,  when our sister planet is full of bizarre volcanic features, from pancakes to spiders, or strange concentric patterns that seem completely alien. But from what I learned about much of the weirdness of Venusian volcanoes often comes from what happens to them after they erupt rather than the volcanism itself, if Earth has erosion, then Venus and it’s highly deformable crust has a lot of ways of fracturing, folding, sinking, or wrinkling volcanoes to change their shape, which we will see more about in a moment. This is added to how Magellan radar images look weird to the untrained observer.

The issue of active volcanoes on Venus has been turning up in Volcanocafé from time to time, usually when new scientific articles come up. One of them is quite relevant to this question, and must be mentioned, this is a 2023 article called “Surface changes observed on a Venusian volcano during the Magellan mission” by Robert R. Herrick and Scott Hensley. This article found changes to a pit crater, in between two images taken during the Magellan Mission that took place 30 years ago. The pit crater seemed to have enlarged and partly filled with lava in between the images. This seems to confirm that some level of activity is ongoing on Maat Mons volcano.

How to reveal Venusian geologic history

The Magellan Spacecraft that orbited Venus between 1990 and 1994 is the best data we have on our sister planet. However, Magellan had overcome the issue of the planet’s opaque atmosphere. Apart from having an atmosphere nearly 100 times heavier than that of Earth, this atmosphere is also constantly covered in clouds of water and sulfuric acid. These clouds help Venus acquire its remarkable brightness in the sky, but at the same time obscure its surface. So to map this surface, the Magellan mission employed radar, which uses microwave radiation that pierces the atmosphere to see surface features. These images are usually viewed in black and white, and they don’t show the real colour of the surface but rather its texture and angle of incidence, among other properties. Other than being able to see obstacles like cliffs or steep hills due to the incidence angle, the texture is one of the more important aspects. The bright areas in Magellan are those with a rough surface texture, for example, young lava flows like those of Maat Mons will have some of the brightest colours because lava has a very irregular surface, something you will know full well if you have ever tried to walk on a’a lava. Faults or cliffs will also show very brightly due to the irregularities of the rock face. Instead, dark areas correspond to those covered in sand or dust, making the surface smooth, while ponded lava lake surfaces can also show as relatively dark areas. There is also low-resolution stereographic topography data of Venus’ surface that I’ll also rely on, revealing the size and locations of mountains and depressions, and helps see how the volcanoes visible on the radar became deformed as time and the planet’s hot plastic crust came into play.

I view the aforementioned data with the Google Venus kmz file for Google Earth that was produced in collaboration between various scientists and Google Earth, and that I will link to at the end of the article.

So we have the data, but now an interpretation is needed. Which principles can be used to establish the age of this alien surface from satellite-based data alone? There are quite a few:

Impact Craters: The approach of using crater density to date the geologic age of the terrain has been used successfully on Mars and the Moon. Incoming meteorites gradually turn the surface of a planet into gruyere cheese, or at least before it gets resurfaced by lava. Venus, however, burns up most meteorites in its thick atmosphere, while only the biggest ones do get through and impact the surface. Craters larger than 35 kilometers follow a similar distribution to craters on other bodies, but the relative frequency of craters under 35 km is smaller than usual since most burn up (or blow up) in the atmosphere. Few craters make crater counting a limited tool that can only be used on very large surfaces, however, it has allowed to establish that the Venusian surface has an average age of about 500±200 million years. This is a lot. On Earth, the surface is constantly changing; one can barely recognize a one-million-year-old scoria cone, but on Venus, we are often looking at volcanoes that are hundreds of millions of years old. Venus is no Io, contrary to what some might expect. Something else about crater distribution in Venus is that it’s relatively homogeneous, which comes to show that most of the surface formed within a short time. Though there is some difference since it has been shown that the young parts of the planet, rifts, coronae, and young volcanoes are about half the age of the rest of the planet (on average). In other planets like Mars, the difference in crater density is much greater, given that the highlands of Mars date to just after the formation of the planet, while much of the Tharsis Rise, in contrast, is floored by lavas that are only about 100 million years old. So the relative homogeneity of Venus’ surface crater density and the very limited spatial resolution of the crater counting due to most incoming impactors burning up has led to a unique interpretation of the planet’s geologic history, that there was possibly one massive resurfacing event or period followed by quiescence. But because the technique can’t apply to small surfaces, there is always the doubt whether small areas of the planet have been resurfaced much later by volcanism, prompting much debate about how dead or active the planet is.

Wrinkles: As the surface ages, it grows wrinkles, and I’m not joking. Most of the planet seems to have contracted over time, probably to balance the areas that are rifting. Wrinkles are born out of the squeeze. They have the structure of folds or thrust faults. I’m unsure whether this process is homogeneous over the surface of the planet, but it won’t be too important for the article anyway, since I’m already selecting through other means volcanoes that are too young to be wrinkled.

Fractures: Some fractures on Venus are due to dike intrusions, which show as white filaments in Magellan images, and often can be traced for a thousand kilometers in length. Dikes are not unexpected on an active volcano, but there are other types of fractures that can also be found; some are related to spreading or uplift/subsidence, while others are tension fractures along the crest of folds. Lavas that predate those fractures are cut to pieces, while lava that postdates the fractures will pond inside the topographic pockets created, erasing faults within the areas flooded. Many volcanoes are in major rifts, and we will see some cases of volcanoes that erupt inside the grabens or other volcanoes that are older than the rifting and are cut by it (like Yolkai-Estsan).

Deformation: Venusian volcanoes also tend to deform over time, summit areas often develop into a subsidence bowl, where lava flows that formed downslope are left pointing upslope. I think this happens when dense crystals accumulate in the central magma storage, and because these are denser than the basaltic crust, their weight pulls the summit of the volcano downwards into the planet. The counterpart to Venusian magma chambers on Earth are probably the structures known as layered intrusions (named because of the distinctive alternating layers of crystals like olivine, pyroxene, or plagioclase that make them up). On Earth, layered intrusions happen in ocean island volcanoes and in flood basalts; most flood basalts likely hosted gigantic layered intrusions in their center, and a bunch of them are known, although many have likely been obliterated by continental break-up that usually happens after flood basalts. Some layered intrusions can grow gargantuan; for example, the Bushveld layered intrusion in South Africa is thought to have been a magma chamber 400 kilometers wide! On Venus, I believe these same layered intrusions are linked to the structures known as coronae, which sometimes reach hundreds of kilometers across and in one case even 2000 km (Artemis Corona, which is the largest volcano-tectonic structure in the Solar System).

Volcanic edifices also seem to subside as a whole too, due to their weight, so the oldest, most wrinkled, and dust-covered volcanoes have practically zero prominence and are usually below the mean altitude of the planet. Instead, the volcanoes with the youngest dust-free surfaces are usually among the taller ones. So elevation can also inform about the age.

The surface roughness/smoothness: This aspect is the most important to this article. Lavas at elevations lower than 2 km (above the mean planet surface) tend to collect dust, either from the lava itself breaking down or dust carried from elsewhere, which makes them darker in Magellan images. Lavas from above 2 km elevation instead seem to get brighter (coarser surface), probably as weathering and “wind” erosion strip the softer sand and leave behind a field of boulders, which results in many Venusian volcanoes having a bright halo around their peaks in Magellan images, although this could be some change in other properties of the rock too. This brightening happens much quicker than the darkening at low elevations, and part of Maat Mons’ flank already has a slight brightening visible. Other events can also redistribute the dust and alter surface brightness in the radar. Landslides leave dark smooth deposits, and some volcanoes can probably smooth the surface with pyroclastic material, which I think happens around the summit of Maat Mons. Meteorite impacts also make some changes; they knock away the surface dust (or fill the ground with rubble) from an area around the crater and drop it into a ring around it, making a brighter circle surrounded by a dark ring or bow-shaped area that is elongated in the direction of the Venusian wind. Even the meteors that burst in the atmosphere can have a similar effect, they create a bright circle where the dust is presumably blown away by the shockwave, but with a darker area just below the airburst, which I picture like the trees standing upright in the center of the Tunguska explosion. In the areas where the dust cover is appropriate to leave airburst deposits, these seem more common than actual craters.

The Ozza-age volcanoes

The surface roughness/smoothness is the main criterion that can be used to find young lava flows, since it’s the most direct way of finding the age of the lava. I called the volcanoes with the youngest looking lavas (those that don’t seem, or mostly don’t seem to be smoothed by dust/weathering) Ozza age volcanoes, given that Ozza Mons is by far the biggest system among them. I reason that the active volcanoes must be among them, assuming a similar rate of weathering across the planet.

A total of 16 volcanoes could be classified as Ozza age, give or take a few, since the distinction was not always fully clear. Do consider that Venusian volcanoes are gigantic, often far superior in size to terrestrial stratovolcanoes or shield volcanoes. All of these youngest systems turned up in the same part of the planet, being found between 180 and 300 degrees longitude and in tropical regions, below 25 degrees latitude. The area is not too surprising since the main rift systems of Venus run around most of the planet within the tropics, and also because the volcanoes are clustered around three topographic swells called Atla Regio, Beta Regio, and Phoebe Regio. Though I’m surprised the area around Artemis Corona, which hosts very important rifts and many of the largest Coronae in Venus, did not show to have any particularly young volcanism. The map below shows the location and name of these volcanoes, though not all have been named.

However, most of these volcanoes do not seem to be active, to the point that I think the list can be narrowed to only four potentially active volcanoes. How is this? Well, the lava flow roughness is not the only principle that can be used to determine the age of the volcanoes. Many of these are deformed or faulted, which shows they haven’t erupted in a long enough interval, which makes me think they shouldn’t be considered active, at least not by Earth standards. Most of the volcanoes are located along the tropical rifts of Venus which can be used as an age constraint, given that the majority of them, for example Yolkai-Estsan Mons, or two of the Devana Chasma volcanoes have massive fault scarps cutting through their edifices and summits, which are the result of rifting, but there are no lavas that seem to overlie or pond in these fault scarps, so there must have been a long interval volcanic inactivity and tectonism acting on their volcanic edifices.

Another example of a volcano that no longer seems to be active despite having some of the youngest lavas of Venus is Sapas Mons, which is the only one of the Ozza-age volcanoes that is not on top of a rift, located on the flank of the hyperactive Atla Regio. Sapas, like most Venusian volcanoes, is gigantic. Its lavas cover an area of almost 170,000 km2, which is nearly twice the size of Iceland! This is also just a bit less than its younger neighbour Maat Mons. At present Sapas is a very low volcano that rises barely 2 kilometers above the average height of its surroundings, but I think this could be an effect of subsidence that has elapsed since its construction, originally I think Sapas it may have been more like the actively growing Maat Mons volcano that presently towers nearly 7 kilometers above the average height of its surroundings, and 8 kilometers above its lowest elevation lavas. But the most clear sign that Sapas is long inactive is the shape of the summit area.

Originally, the shield volcano had two peaks, each hosting a steeply perched lava lake or piston uplift structure over 10 kilometers wide. The steep flanks of the two peaks gave way to landslides that ran downslope, similar to many lava flows that extended radially from the twin summits, following the downslope direction. However, the volcano has deformed in such a way that these lava flows and landslides now point upslope; as indicated in Magellan topography data the entire summit area has subsided into a double bowl 100 kilometers long and 3 kilometers deep, as deep as the tip of the lowest elevation lava flow of Sapas, likely due to the weight of its dense central intrusive core/ layered intrusion. If Sapas Mons was still active, it would have erupted inside the subsidence bowl, making a ponded lava plain, which is not the case, which precludes the volcano from having erupted in a really long time.

There are additional signs of Sapas’ long dormancy. First, the ridge around the subsidence bowl is fractured in places, likely due to tension along the crest of the fold, and the fractures cut through the lavas. Second, is that a bright halo of erosion/weathering encircles the volcano above 2 km above the mean Venus elevation. And lastly, although the lavas are generally of similar brightness to those of Maat, beyond the bright halo, Sapas lacks lavas as bright as the brightest/roughest low-altitude flows of volcanoes such as Maat, Ozza, or even Theia.

Another volcano that looks very young is the impressive Theia Mons volcano, a junction between three major rift valleys and two minor ones that forms the roof of Beta Regio. The volcano covers over 300,000 km2 in young Ozza-age lava, and probably has older lavas over a larger area. Its young edifice postdates the bulk of rifting of Devana Chasma (but not all), the rift being much deeper in adjacent areas than on the edifice of Theia. However, the summit of the volcano has experienced massive subsidence and forms a vast 70 km wide depression that is 4 km deep, making even the largest terrestrial calderas look small. Though this may not be a caldera collapse, but the subsidence of its central intrusive complex. Since the central depression reaches as deep as the lowest elevation lavas of the shield, then if the volcano was active, eruptions should be ponding inside of it, but the Magellan radar shows a dust-covered summit depression cut by fault scarps and no sign of recent lava filling it. This volcano is another example of why almost all Venusian volcanoes can be ruled out as being active, for one reason or another, or usually several.

Something else I’ve done is to try and estimate the age of the tropical rift systems with crater counting. Ozza-age lavas cover a very small area of the planet, making it impossible to use crater counting, but the rifts are more widespread and they are linked to the youngest Venusian volcanoes. These rifts often seem to entirely postdate the many volcanoes that cover all of Venus’ surface, showing that at some point there may have been a transition from a very volcanically active stage to a stage of weak volcanic activity, where tectonic processes instead dominated. However, some volcanoes have grown after the bulk of activity of the major rift systems, like Theia or Ozza Mons, that occupy the two largest rift junctions of the planet. And four, as we will see later, have lavas that postdate even the youngest rifting. So, having an idea of the absolute (average) age of the last rifting across the tropical rift systems was important to judge whether the planet is geologically active or not, and no doubt this has been done already, but I wanted to do it myself. I can only do so with craters of over 30 kilometers since the smaller ones don’t have the typical size distribution and do not always make it possible to see if the floor is faulted.

The result is that in an area of 12 million square kilometers that is affected by rifting there are 12 meteorite craters that predate the rifting, 2 additional craters that may postdate rifting but were too small to tell for sure but anyways also too small to be included in the counting, and two >30 km craters that respectively postdate the rifting and Ozza-age lavas inside Dali Chasma. One of these two last craters has 34 km diameter and is located north of Yolkai-Estsan Mons, it postdates fault systems belonging to Ganis Chasma that in turn cut and thus postdate Yolkai-Estsan Ozza-age lavas. The other crater is a 40 km diameter impact on Ozza Mons (Uvaysi crater) in the center of Dali Chasma rift system, the impact showered the whole SW sector of Ozza, including some of the best preserved lavas of the planet in smooth dust deposits. The bottom of the crater doesn’t seem cut by Dali Chasma faults, but the rim does have some faults that likely postdate the crater. I think a fissure of Maat Mons likely erupted inside Uvaysi and resurfaced the floor, but the crater itself, while postdating the main shield-building episode of Ozza, predates some of the last rifting of Dali Chasma that cuts through all but the last Ozza lavas.

Venus has 189 catalogued craters of over 30 km in diameter and an estimated mean age of 500 Ma, so taking into account the area of the planet, it is possible to compare the mean crater density of the planet with that of the rifting area, and the density should be proportional to the age. Considering the one crater, this gives a mean age of 100 million years for the rifting areas, which is a lot more than I was expecting to be honest, but at the same time shows the rifts are much younger than the rest of the planet, so there is clearly a protracted history of volcano-tectonic activity here. This orientative age doesn’t imply there’s no active rifting on Venus, given the crater that does postdate all rifting is away from the central axis of the rift, an area that may have been abandoned in favor of more focused activity. But combined with the crater on Ozza’s flank, it does seem to point at Venusian volcanism being very old, and it also points to rifting being far slower than on Earth’s continental rifts, if active at all.  While there is a lot of uncertainty, even the 16 Venusian volcanoes of youngest appearance are likely to have had most of their activity before 100 million years ago. This volcanic slumber gets so bad that I’m not sure which planet has erupted more lava in the last 200 million years, Mars or Venus.

The potentially active Venusian volcanoes

So, which are the volcanoes that could be presently active? The first is an area of 3,300 km2 of lava, very bright in the radar, that is ponded inside Devana Chasma in Phoebe Regio. The volcano has no name, and I simply call it the South Devana Chasma volcano. Since the topography is complicated and the lavas appear to mantle it, then perhaps the lava is hundreds of meters thick, probably over 1,000 km3 in volume, which is not much. There is also an older apron of Ozza-age lava covering 30,000 km2 that was erupted before Devana Chasma started to open; additional episodes must have followed inside the rift, associated with continuing faulting, until the final episode that entirely or almost entirely postdates rifting. If the rift valley is still active, then this unnamed volcano must be active too.

Another potentially active volcano is Kono Mons, which is located in another major rift system of Venus, known as Zverine Chasma.  The volcano grew as a shield that covered an area of 70,000 km2, though only a few of these lavas can be considered Ozza-aged I think, the rest are older, then the rift valley opened and split the volcano in two. More lava was erupted inside Zverine Chasma, maybe 1 km thick and covering 6000 km2, so 6000 km3 in volume. The surface of these lavas does not have any rift-related faults on them, so they must postdate all rifting, but there are small fractures and deformation that seem associated to subsidence or uplift of the volcano’s center. If the rift system is still active, then Kono Mons must be actively erupting too, given the lack of rift fractures on the lava, making Kono a potentially active volcano and among the few youngest eruptors in Venus.

The last two potentially active volcanoes, which also have the best chances of being active, and likely contain the vast majority of lava erupted in Venus during “Ozza times”. They are the two goddesses of Atla Regio, Maat and Ozza Mons. Because I think these volcanoes deserve their own article, I will publish a second part looking into the construction of the volcanoes, the way they erupt, and how various features like coronae, giant dikes, and lava flows may be related.

Conclusion

Since I’ve learned of Venusian volcanism, I’ve been swinging between two ideas of Venus: that of a dead planet that was resurfaced during an apocalyptic, very old volcanic episode, and that of a live planet that still has several volcanoes going off at any given time. It turns out that it is not exactly any of the two. The volcano-tectonic activity that we see on the planet seems to have spanned hundreds of millions of years, but it can’t be considered to reach a level of volcanic activity anywhere near what it once was. I thought some volcanoes, like Theia Mons or Tepev Mons, that are relatively tall, would turn out to be active, but closer inspection shows them to be long dead. As far as I can see only 4 volcanoes may still be actively erupting, and only one of them vigorously, Maat Mons, the others seem to be but a shadow of what ancient Venusian volcanoes were hundreds of millions of years ago, when in a relatively short span pf the planets history they covered all the surface of the planet in lava and cut by countless giant dikes, likely feeding layered intrusions that were hundreds of kilometers across and more, and channels of lava that run for several thousands of kilometers in lenght. It’s unclear if this has been a continuous fading trend of the planet’s volcanic activity towards oblivion, or if volcanism is something cyclic or episodic that at present is going through a low phase, but it’s clear that the Earth is a far more consistently geologically active planet than its siblings.

References

Richard C. Ghail, David Hall, Philippa J. Mason, Robert R. Herrick, Lynn M. Carter, Ed Williams. VenSAR on EnVision: Taking earth observation radar to Venus, International Journal of Applied Earth Observation and Geoinformation, Volume 64, 2018, Pages 365-376, ISSN 1569-8432, https://doi.org/10.1016/j.jag.2017.02.008.
(https://www.sciencedirect.com/science/article/pii/S030324341730034X)

David Sandwell, Ross Beyer, Ekaterina Tymofyeyeva, Catherine Johnson, Stafford Marquardt, Jenifer Austin, Kurt Schwehr. Google Venus: https://topex.ucsd.edu/venus/index.html

Robert R. Herrick, Virgil L. Sharpton, Michael C. Malin, Suzanne N. Lyons, and Kimberly Feely. (Venus Crater Database) “Morphology and Morphometry of Impact Craters”, (1997, U. of Arizona Press, eds. S. W. Bougher, D. M. Hunten, and R. J. Phillips, pp. 1015-1046). https://www.lpi.usra.edu/resources/vc/vchome.html