This interesting little tidbit came through my press release monitor this morning...
ANU Media Release
WAS ANCIENT EARTH A LAVA TIME BOMB?
Ancient Earth suffered repeated episodes of catastrophic volcanism that buried much of its surface under up to 15 kilometres of lava, according to a researcher from The Australian National University.
The study by Dr Geoff Davies of the ANU Research School of Earth Sciences (RSES) used computer models of the Earth’s deep interior. The modelling showed that the volcanic episodes lasted around a million years, but haven’t happened for about three billion years and are highly unlikely to happen again. The findings were published this week in Earth and Planetary Science Letters.
Dr Davies said the modelling suggested the deep interior of the Earth bottled up its internal heat for long periods until breakthroughs triggered the volcanic outbursts.
“The effect is caused by basaltic oceanic crust that is carried deep into the Earth as tectonic plates sink into the Earth’s mantle,” he said. “The basaltic crust is a little denser than the mantle at most depths, but is lighter within a small depth range about one third of the way down. The foundered crust tends to accumulate at this depth and, if enough of it collects, can form a ‘basalt barrier’ that prevents deeper mantle material from rising to the surface where it can cool.
“The deeper mantle is then slowly heated by radioactivity. Eventually it gets hot enough to break through the barrier and rises in a rush. It melts as it reaches the lower pressures near the top of the mantle and the melted rock erupts as lava flow. The lava flows could pile up to a thickness of ten to fifteen kilometres over much of the Earth".
He added that the episodes would have changed the chemistry of the oceans and atmosphere,
leaving only the hardiest bacteria alive.
“The episodes happened about every hundred million years so the bacteria would have had plenty of time in between to recover. Eventually, as the Earth’s interior slowly cooled, the tectonic plates became thick and heavy enough to break through the basalt barrier and prevent it from reforming, and no further episodes occurred,” said Dr Davies.
The research adds an additional theory to how the Earth’s continents were formed. Geologists know that this occurred in several major bursts two to three billion years ago and different reasons have been proposed as to why it occurred. “We can now add the basalt barrier mechanism to the debate as another possible cause,” said Dr Davies.
A copy of the paper is available here: http://dx.doi.org/10.1016/j.epsl.2008.08.036