Scoria cones as a rare trace of explosive volcanism on Mars
Since the first more detailed images of Mars from the 1970s, planetary geologists have known that the Red Planet is covered with enormous volcanic structures. The Mariner 9 mission revealed shield volcanoes and vast lava plains on scales Earth almost does not have: Olympus Mons rises as the tallest volcano in the Solar System, almost three times higher than Mount Everest, while Alba Mons spreads so widely that its diameter is compared with the length of the continental United States. On that “planetary” scale, Mars is dominated by calmer, effusive basaltic volcanism—outpourings of low-viscosity, “fluid” lava that spreads in sheets and layers, building huge uplands and lava plains.
But alongside that predominantly calm pattern, geologists have been trying for decades to explain one inconsistency: why there are relatively few clear traces of explosive eruptions on Mars. Theory suggests the opposite. Atmospheric pressure on Mars is on average about 160 times lower than on Earth, and gravity roughly one-third of Earth’s; precisely such conditions, as planetary geologist Petr Brož from the Czech Academy of Sciences explains, should facilitate the development of explosive eruptions because gases in magma more easily create powerful fountains and disperse material. And that is exactly why groups of cones resembling scoria cones in the Ulysses Colles area, on the edge of the vast Tharsis volcanic region, are especially interesting: they look like clear confirmation that Mars, at least intermittently, produced “moderately” explosive eruptions that on Earth we call Strombolian.
Ulysses Colles: details from Mars revealed by the CTX camera
To understand the story, remote-sensing data are key. The CTX (Context Camera) on NASA’s Mars Reconnaissance Orbiter photographed Ulysses Colles on May 7, 2014, and the scene gave planetary geologists a rare “window” into explosive volcanism on Mars. Ulysses Colles lies on the southern edge of the Ulysses Fossae system, a group of troughs and tectonic depressions within Tharsis. In the images, alongside numerous impact craters and textured surfaces of old lava flows, rounded hills with circular openings at the top stand out—a shape that on Earth we most often associate with scoria cones.
In the same frame, grabens are also visible—linear block structures of the crust that have subsided between fault systems. Such cracks and depressions are not just a picturesque detail: they are important context that helps distinguish volcanic forms from other possible processes. Ulysses Colles is not an isolated “island” of cones, but part of a broader volcanic-tectonic complex in which effusive lava flows, tectonic extension, and erosion overlap in layers billions of years old. Precisely because of that age and layering, an open question remains—how many times and in what sequence volcanism there “switched” from calm outpouring to explosive episodes.
Arizona as an analogy: the same geometric “signature” in the San Francisco Volcanic Field
To interpret Martian forms better, geologists compare them with places on Earth where processes can be studied up close. In this case, an almost textbook analogy lies in northern Arizona, in the San Francisco Volcanic Field (SFVF). Landsat 8, a satellite that has for years systematically imaged Earth’s surface, recorded on June 19, 2025 a series of cones with dark lava flows similar to those from Ulysses Colles. Here too, grabens are visible in the frame, and around the bases of the cones spread darker, “rougher” areas of lava flows.
SP Crater (also known as S P Mountain) stands out in particular, a scoria cone with a long, dark lava flow that extends to the north. According to data from the U.S. Geological Survey, that flow is about 7 kilometers long and for decades has served as a training ground for NASA astronauts in geology. In two places the flow “spills” into a graben, creating a recognizable crescent-like pattern—a geometric trace that allows scientists to reconstruct lava movement directions, relationships between fractures and later outpourings, and the relative age of individual phases.
How scoria cones form and why they should be larger on Mars
Scoria cones (in English, scoria cones, often also cinder cones) form when gas-rich magma erupts from a vent into the air as fountains, during which the molten material rapidly cools into small fragments—scoria—which then accumulates around the opening and builds steep, conical structures. Such eruptions are usually considered “mildly explosive” and are most often classified as Strombolian, with occasional, rhythmic ejection of incandescent fragments and short-lived lava fountains. Planetary geologist Ian Flynn from the University of Pittsburgh notes that this is a much gentler pattern than the rare, very violent eruptions that create high ash columns and widespread deposits of pyroclastic material.
On Mars, the same basic mechanism should work, but with different environmental “physics.” Lower gravity allows particles ejected from the vent to fly farther before they fall, and lower air pressure reduces drag and makes it easier for volcanic fountains to expand. The result would, at least in expectation, be taller and broader cones with gentler slopes—exactly what is often cited in morphometric analyses of Martian candidates as a difference relative to terrestrial analogs. In that sense, the cones in Ulysses Colles become a natural laboratory: they are clear enough to be measured and compared, and rare enough that each new example significantly changes the picture of Martian volcanism.
Why there are so few traces of explosive volcanism on Mars
If conditions on Mars facilitate explosive eruptions, why do we see relatively few of them? One answer is that explosive volcanism may never have been common. Another is that it was present, but its traces were “erased” by later processes: younger effusive flows could have covered older pyroclastic deposits, and long-term erosion by wind and meteorite impacts could have deformed or destroyed recognizable forms. Brož notes that on Mars so far only dozens to a few hundred candidates for scoria cones have been identified, while on Earth there are tens of thousands, and they also make up the majority of volcanoes on land.
In that uncertainty, the problem of chronological sequence is also important. Patrick Whelley, a NASA volcanologist involved in developing equipment and methods for exploring the Moon and Mars, emphasizes that on Mars it is often unclear whether lava flows formed before the cones or vice versa. It is possible that the flow is older, and the cone formed later “on top of it.” It is equally possible that the cone formed first, and then the vent became blocked and magma found an outlet to the side, creating a flow that looks “younger.” In geology, such relationships form the core of interpretation, but on Mars they are resolved almost exclusively on the basis of satellite images, topography, and crater statistics, without the possibility of direct sampling.
Sunset Crater and the “living” history of terrain: from an eruption about 800 years ago to Mars billions of years old
Comparisons with Arizona have an additional advantage: on Earth, the age of many eruptions can be approximately dated and linked to traces in the landscape and human history. Sunset Crater, a scoria cone southeast of SP Crater, erupted about 800 years ago and is considered the youngest among roughly 600 similar cones in the San Francisco Volcanic Field. The comparable cone in Ulysses Colles, by contrast, is estimated to be billions of years old. That temporal “abyss” is not just an interesting contrast; it also shows how long Mars can preserve landforms—but also how difficult it is to be sure what in that long history has been covered, altered, or misinterpreted.
That is precisely why field studies of terrestrial analogs remain important. Observing cones, lava flows, and grabens from close range helps scientists recognize “small” signs—flow margins, surface textures, relationships between deposits—that in satellite images appear only as shades. Such knowledge is then transferred into the interpretation of Martian imagery and into planning future missions, including training astronauts at locations such as SP Crater.
“If it looks like a duck...” : caution in planetary geology and the risk of confusion with mud volcanoes
Planetary comparisons have limits, and scientists regularly emphasize them. Brož warns that in planetary science it is often—half jokingly—said that even if something looks, behaves, and “sounds” like a duck, it still does not mean it is a duck. In other words, morphological similarity is not always proof of the same process. Scoria cones, for example, can be confused with mud volcanoes or other forms formed from subsurface fluids. In some environments, especially where there are no clear contexts of lava flows, distinguishing them can be extremely difficult.
Brož’s laboratory research further complicates the picture because it suggests that on Mars, mud flows under certain conditions can behave and look similar to some types of lava flows, and can even show unusual phenomena such as boiling and “levitation” of material. This is a reminder that Mars must not be interpreted exclusively through “Earthly eyes”: the planet has a different atmosphere, different gravity, and a different geological history. Today, the Ulysses Colles cones are, according to available analyses, considered very convincing evidence of explosive volcanism, but scientists are still seeking additional criteria that will reduce the possibility of misidentification in other regions.
From “mild” explosions to supereruptions: what Mars reveals, and what it still hides
Mars also bears traces of much stronger explosive events than the Strombolian eruptions that build scoria cones. Such very explosive episodes do not leave small cones, but a different geological signature: large hollows and irregular depressions that in planetary nomenclature are often called paterae (paterae), as well as broad and thin deposits of ash and other easily erodible material. Wind can then carve that material into yardangs—elongated ridges shaped by erosion and “sandblasting” by windborne particles. Comparing with such forms is important because it helps distinguish the spectrum of Martian volcanism: from calm lava outpourings, through “mild” explosions that build cones, to rare, highly energetic events that reshape large areas.
In the terrestrial context, the difference between “mildly explosive” eruptions and those that eject ash columns tens of kilometers high is especially visible in the example of the Hunga Tonga–Hunga Ha'apai eruption of January 2022, when satellite measurements recorded that the column reached about 58 kilometers in height. Such extremes serve as a reminder that “explosive” covers a very wide range of processes—and that on Mars, although more rarely, traces of different types of eruptions can be sought, but in different geomorphological “signatures.”
Why this comparison matters and what comes next
In the story of Ulysses Colles and the Arizona analogs, it is not just about comparing two beautiful satellite photographs. It is about a method in which the known is used as a key to the unknown: fieldwork, measurements, and understanding of processes on Earth help scientists interpret relief on Mars from a distance, determine which data need to be collected and which instruments to develop. At the same time, Mars “returns” the challenge: it forces researchers to broaden their intuition and not take for granted that the same form always means the same process.
The scoria cones in Ulysses Colles are therefore important both as a scientific clue and as a warning. They complement a picture of Martian volcanism that for decades was focused on effusive flows and enormous shield uplands, while at the same time opening new questions about how widespread explosive volcanism really was, how many of its traces are buried beneath younger lava and dust, and how future missions—robotic or human—will choose places where those traces can be verified up close.
Sources:- NASA Earth Observatory (NASA Science) – article and satellite views comparing cones on Earth and Mars, with imaging dates ( link )
- U.S. Geological Survey – Sunset Crater and the context of the San Francisco Volcanic Field ( link )
- U.S. Geological Survey – overview of the San Francisco Volcanic Field (number of cones and basic information) ( link )
- Mars Education (Arizona State University) – explanation of the term patera/paterae in Martian nomenclature ( link )
- NASA Earth Observatory – analysis of the Hunga Tonga–Hunga Ha'apai eruption and satellite measurement of plume height ( link )
- U.S. Geological Survey Astrogeology – technical description and data context of the CTX instrument on Mars Reconnaissance Orbiter ( link )
Find accommodation nearby
Creation time: 4 hours ago