Up there with the worst hurricanes and earthquakes, they rank among the most powerful and frightening forces on Earth. Super-volcanoes, which produce more than 1000 cubic kilometres of lava, ash and gases, have shaped much of our planet's history and have almost certainly led to mass extinctions of life in the dim, distant past.
Now, it seems, similar super-volcanic beasts might once have shaped the geological fortunes of the fourth rock from the sun.
A quartet of Mars-orbiting spacecraft has taken surface images of an ancient region of northern Mars, dubbed Arabia Terra. Analyses of the high-resolution pictures reveal that gigantic circular pits, once assumed to have been meteorite impact craters, are most likely the calderas of immense volcanoes, now extinct.
The volcanoes would have been many hundreds of times more powerful than the Mount St Helens eruption in 1980 in the US state of Washington, where hundreds of square kilometres of land were laid waste and 11 states blanketed in ash.
The Mars discovery is based on the careful work of a joint research team. "We found a handful of features on Mars that we believe to be calderas from ancient explosive eruptions," says co-researcher Jacob Bleacher, of NASA's Goddard Space Flight Centre in Greenbelt, Maryland, in the US.
The best example, he says, is Eden Patera, a depression that formed as a result of volcanic activity. "We demonstrated that the feature lacks telltale signs of a history related to an impact event, and then shows a number of lines of evidence for volcanic activity at the site," says Dr Bleacher, who co-authored a paper in the British journal Nature with Joseph Michalski of the Planetary Science Institute and Natural History Museum in London.
"We further extrapolated a relationship with the Fretted Terrain nearby, thought to be an ash blanket of unknown origin," Dr Bleacher says.
Along with showing signs of volcanism, Eden is ideally placed to have deposited the Fretted Terrain and lacks signs of material near the vent to produce a volcanic cone. This led the research team to conclude that the area probably experienced a super-volcanic eruption.
Question of style
The significance of this discovery is the style of eruption, says planetary geologist and Mars Society Australia president Jonathan Clarke. "Unlike previous Martian volcanoes, these are highly explosive eruptions that probably formed quite quickly."
Terrestrial counterparts are the eruptions that formed places such as Lake Taal in the Philippines, Lake Toba in Indonesia, Lake Taupo in New Zealand and Yellowstone National Park in the US.
"The Martian examples are much larger than any terrestrial ones, although still only medium-sized by Martian standards," Dr Clarke says.
Super-volcanoes form by the sudden release of gases from magma from deep inside a planet. Resultant flows of incandescent ash are so fluid, they form sheets rather than a volcanic mound. "So much magma is erupted that a massive collapse structure forms, a caldera," Dr Clarke says. "Because of their low relief and central caldera, these are sometimes called inverse volcanoes."
The Martian super-volcanoes are different in composition to those on Earth, which consist of silica, potassium and sodium-rich lavas. "When magmas of this composition cool deep inside the Earth they form granite," Dr Clarke says. "The Martian examples all seem to be basalt."
The significance of these eruptions is twofold. First, they need gas-rich magmas to form; and second, they are associated with some of the most ancient exposed crust on Mars, perhaps up to 4.5 billion years old.
"This means that at least some of the early eruptions on Mars were richer in gas than later ones," Dr Clarke says. "This may have helped support the dense atmosphere on Mars necessary to allow the widespread activity of liquid water."
Volcanoes and life
Where there is water, it is said, there is often life. Indeed, although Mars is an unwelcoming place to Earthlings – its tenuous atmosphere is perishingly cold and contains no oxygen – it might once have harboured life of some sort.
Does the discovery of ancient super-volcanoes on Mars raise the chance of life having once existed there? "It does provide us with a source for the water and a means of sustaining a dense atmosphere," Dr Clarke says.
The earliest signs of life on Earth, such as the stromatolites of the Pilbara, are also found in ancient calderas, he reminds. "Hydrothermal systems in calderas, such as those of Rotorua in New Zealand and Yellowstone in the US, also appear to have been particularly hospitable for life."
Volcanoes might help create an environment that could be favourable for life to form, says Dr Beacher. "But highly explosive eruptions like super-volcanoes can be catastrophic to existing life. These eruptions can send large quantities of ash and gas into a planet's upper atmosphere, which can rapidly alter the global temperature and environment."
Mars Society Australia director Guy Murphy agrees. "The eruption of super-volcanoes would have triggered major climatic changes over a relatively short interval," he says.
In its early history, Mr Murphy says, Mars was a radically different place from the frozen world of today. "It had a thicker, warmer atmosphere, which supported liquid surface water. We knew volcanic activity was much more prevalent then, but evidence suggesting the presence of super-volcanoes is something new."
Volcanic activity, Mr Murphy adds, has the potential to create hospitable niches of warmth, liquid water and the right mix of chemicals in which life could be sparked or sustained.
Swinburne University scientist Francesco Pignatale takes a similar view. "Volcanism played a major role in creating on our planet the right environment for life," he says. "The presence of liquid water probably did the rest."
On Mars, he believes, volcanism might have helped set the right conditions for life to develop.
But doubts persist. "It's still unclear what role super-volcanoes played in the red planet's evolution," says space engineer Jason Held, director of Saber Astronautics in Sydney.
"We are only beginning to understand the relationship between life and the environment here on Earth, let alone Mars. Super-volcanoes could have caused mass extinctions. Or it may have, for a short time, created more favourable conditions."
The long view
University of Tasmania scientist Jo Whittaker is fascinated to think that other planets might have had internal geological processes similar to those on Earth. "Understanding what happened to other planets might help us understand the processes occurring on Earth today," she says.
How so? "The formation of large craters, and the linking of these to large-scale basaltic volcanism, seems quite different to processes that occur presently on Earth," Dr Whittaker says.
Large basaltic regions on Earth form in two ways: either at mid-ocean ridges, where seafloor is formed, or from eruptions of mantle plumes, such as the Siberian Traps and the Ontong Java Plateau.
Neither process is related to large volcanic craters, Dr Whittaker points out. Most of these on Earth are associated with subduction processes, whereby one crustal plate is forced beneath another. "On our planet, we tend to need continental crust plus a heat source to produce super-volcanoes," she says.
Another option proposed for the Martian super-volcanoes is that explosive eruptions from basaltic material were more prevalent on early Mars than today on Earth, owing to lower gravity and atmospheric pressures there.
"If this is the case, there are likely to be more such features on Mars," Dr Whittaker suggests. "And there must also have been a change to allow the formation of shield volcanoes, which are prevalent across Mars."
Mars has the solar system's biggest volcano, Olympus Mons, which is about 26 kilometres high and 600 kilometres wide. Being a shield volcano that was built up by repeated eruptions, it seems, from a distance at least, more like a big and relatively flat hill than a towering peak.
Why do Martian volcanoes get so big? Unlike the continually shifting tectonic plates on Earth, Mars is relatively stable. So lava flowing from the same spot can build up over time into huge volcanoes.
This much is straightforward. But not everything Martian is that clear cut. "There is an unsolved problem with Mars: its surface is covered in fine-grained layered deposits and it is not clear how they got there," says Swinburne University astrophysicist Sarah Maddison.
"One idea is that fine-grained ash was globally deposited by volcanoes," she explains. "Support for this includes the fact that sulfates are present in many of these deposits."
Also, about 70 per cent of Mars has been resurfaced by basaltic volcanism. Finally, Associate Professor Maddison notes, some areas of Mars have been affected by water and acidic conditions that might have resulted from volcanic outgassing.
The problem, she believes, is that known volcanoes on Mars, including the massive Olympus Mons, are not likely to have formed the widespread layered deposits, and in particular not the thick, lowland layered deposits at Arabia Terra, where evidence for the ancient super-volcano was found.
Hunt goes on
Although the jury is still out on whether life once existed on the red planet, every sample NASA's latest Mars rover, Curiosity, takes brings scientists a step closer to deciding whether the planet – today freezing cold, bone dry and bombarded by radiation – might once have been habitable.
Using its sophisticated on-board laboratory, the rover has found, among other things, clay minerals – including sulphur, nitrogen, hydrogen, oxygen, phosphorus and carbon – in powder drilled from sedimentary rocks. Such elements represent a sprinkling of key ingredients for life. But they fall far short of evidence for life itself, past or present.
All the same, the picture now emerging of conditions on Mars are encouraging. Current findings show that habitable environments existed there, scientists say.
The environment would have been conducive to primitive life-forms, although there is no evidence for these so far. The Earth boasts similar environments; dried lakebeds, for example, where ancient bacteria reside beneath the surface.
Today the surface of Mars is red and dead, with most surface rocks gaining their reddish colour from iron oxide. But by drilling below the rusty surface, Curiosity has exposed "grey clay". There was also a range of chemicals available, which microbial life could potentially use for sustenance. These included substances with varying degrees of oxidation, which organisms on Earth use as energy sources.
Curiosity's mission is to reach nearby Mount Sharp, or Aeolis Mons, a mountain 5.5 kilometres high, with interesting geologies billions of years old, including a welter of clays and sulphates that might once have been associated with water – and, hence, life.
Read the latest Nature research article at: www.nature.com/nature/journal/v502/n7469/full/nature12482.html
Learn about super-volcanoes at: www.coolgeography.co.uk/GCSE/AQA/Restless%20Earth/Supervolcanoes/Supervolcanoes.htm
Find out more about Saber Astronautics at: www.saberastro.com/home/company/about.html
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