PureInsight | April 27, 2008
In a paper published in this month's Geophysical Journal International,
Dr Graeme Eagles from the Earth Sciences Department at Royal Holloway,
University of London, reveals how one of the largest continents ever to
exist met its demise.
Gondwana was a "supercontinent" that existed between 500 and 180
million years ago. For the past four decades, geologists have debated
how Gondwana eventually broke up, developing a multitude of scenarios
which can be loosely grouped into two schools of thought - one theory
claiming the continent separated into many small plates, and a second
theory claiming it broke into just a few large pieces. Dr Eagles,
working with Dr Matthais Konig from the Alfred Wegener Institute for
Polar and Marine Research in Bremerhaven, Germany, has devised a new
computer model showing that the supercontinent cracked into two pieces,
too heavy to hold itself together.
Gondwana comprised of most of the landmasses in today's Southern
Hemisphere, including Antarctica, South America, Africa, Madagascar,
Australia-New Guinea, and New Zealand, as well as Arabia and the Indian
subcontinent of the Northern Hemisphere. Between around 250 and 180
million years ago, it formed part of the single supercontinent "Pangea."
Evidence suggests that Gondwana began to break up at around 183 million
years ago. Analysing magnetic and gravity anomaly data from some of
Gondwana's first cracking points – fracture zones in the Mozambique
Basin and the Riiser-Larsen Sea off Antarctica – Dr Eagles and Dr Konig
reconstructed the paths that each part of Gondwana took as it broke
apart. The computer model reveals that the supercontinent divided
into just two large, eastern and western plates. Approximately 30
million years later, these two plates started to split to form the
familiar continents of today's Southern Hemisphere.
"You could say that the process is ongoing as Africa is currently
splitting in two along the East African Rift," says Dr Eagles. "The
previously held view of Gondwana initially breaking up into many
different pieces was unnecessarily complicated. It gave fuel to the
theory that a plume of hot mantle, about 2,000 to 3,000 kilometres
wide, began the splitting process. A straight forward split takes the
spotlight off plumes as active agents in the supercontinent's breakup,
because the small number of plates involved resembles the pattern of
plate tectonics in the rest of Earth's history during which plumes have
played bit parts."
According to Dr Eagles and Dr Konig's study, because supercontinents
like Gondwana are gravitationally unstable to begin with, and have very
thick crusts in comparison to oceans, they eventually start to collapse
under their own weight.
Says Dr Eagles, "These findings are a starting point from which more
accurate and careful research can be made on the supercontinent. The
new model challenges the positions of India and Sri Lanka in Gondwana
which have been widely used for the past 40 years, assigning them very
different positions in the supercontinent. These differences have major
consequences for our understanding of Earth."
