Solar system's longest lava flows found
26 November 2008
The longest lava flows in the solar system formed mountain ranges across three continents, according to new geological research.
Called the Ferrar basalts, the rocks formed by the cooling lava are now found in mountain ranges across Antarctica, South Africa, Australia and New Zealand.
'It's amazing,' says Dr Philip Leat, a geologist at the British Antarctic Survey and author of the paper. 'To find exactly the same rocks in Tasmania as in the Antarctic is very unusual - the Ferrar basalts are the only example of this kind that we know of.'
The lava came to the surface in volcanic eruptions around 183 million years ago, when Gondwana, the ancient super-continent, was still a single land-mass - although a rift was already opening across it that would eventually form the split between modern Africa and Antarctica.
In the months after the eruptions, the lava flows moved around 4100 kilometres, although along most of this length they were only around 160km wide. Researchers think the lava travelled by forcing surface rocks up and flowing laterally in the space below to form what geologists call 'sills' - horizontal incursions of lava into older rocks.
The Ferrar basalts in Antarctica with the BAS Twin Otter aircraft.
This subterranean movement helped keep the lava warm, delaying its cooling into rock and hence allowing it to spread further.
The alternative hypothesis, that the basalt outcrops were formed by 'dikes' - lava flows aligned vertically rather than horizontally relative to the ground - looks less likely. If dikes were responsible, the evidence of this is now remarkably well-hidden.
'The distance travelled by these lavas was greater than any previously known from the Earth, Mars or Venus - equivalent to the breadth of Europe from Britain to the Urals,' says Leat.
The longest lava flows detected on Mars and Venus are both around 3000km long.
'It has long been known that it is theoretically possible for lavas to travel thousands of kilometres below the surface, but this is the longest that has been observed,' Leat adds.
The Ferrar basalts' most obvious outcrops are in Antarctica; scientists estimate its total volume is around 200,000km3. When Gondwana broke up, Leat believes that the lava flows were split between several modern-day continents.
'We have known about the Ferrar basalts for decades, but until recently there has been a bit of a mystery as to what they represent and how they got there,' explains Leat.
Geologists knew the basalts were found in Antarctica, Australia and New Zealand. Their best-known outcrop was in the former's Victoria Land region. But geologists only recently discovered outcrops of Ferrar basalt in South Africa too.
The Ferrar basalts in Antarctica.
Leat travelled to Antarctica's Theron Mountains, Whichaway Nunataks and Pensacola Mountains, as well as to basalt outcrops in the other areas covered by the study. He also drew on previous work on the Antarctic Shackleton Range.
Rock samples from all these places revealed a common origin. There were two distinct kinds of lava produced in the original lava flows, with both appearing in all the mountain ranges in question.
If the distinctive signature of only one of these lava types was found in the basalts, the similarity could be coincidental. But Leat says that finding both types of lava in all these places is too unlikely to be an accident.
The Ferrar eruptions were part of a larger pattern of volcanic activity around the same time. This caused major environmental change as erupting lava gave off CO2 and sulfur, destabilising the climate and temporarily leading to anoxic, or oxygen-free, conditions in many parts of the oceans. Scientists think this sudden environmental change could have triggered the mass extinctions of the early Jurassic period.
The research is published by the Geological Society of London.
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