Armitage and Collier Split Continents

When continents split to form new oceans, their history determines whether they break apart with a bang or a whimper.

In an article published in Nature, John Armitage and Jenny Collier, from ESE, together with Tim Minshull from the National Oceanography Centre, University of Southampton, show that when continents split to form new oceans, their history determines whether they break apart with a bang or a whimper.

Previous work shows that the breakup of continents occurs either with a bang - marked by a period of excessive volcanism with the extrusion and intrusion of massive amounts of basalt rocks; or with a whimper, with hardly a volcano in sight. The consensus among Earth Scientists prior to this new work was that these very different styles of breakup were controlled by the temperature of the mantle beneath the separating continents. If hot mantle is present, for example due to a hot spot, then a volcanic margin will form. If the mantle is relatively cool, for example where the crust is thinned slowly, a non-volcanic margin will form. Armitage and Collier, however, show that the formation of new oceans is not as simple as hot or not, the geological history of the crust also is crucially important.

The new research was inspired by field experiments conducted in the Indian Ocean between 2001 and 2003. In these experiments researchers from Imperial College used seismic equipment to look for volcanic rocks within the Seychelles-Indian continental margins. As these continents tore apart shortly after the eruption of the Deccan Flood basalt province, formed by a mantle hot spot, conventional theory predicted that the break-up should also have been accompanied by massive volcanism - but they found very little. The near absence of volcanism was a surprise since in the North Atlantic the presence of the Iceland hotspot generated large amounts of volcanism. Clearly the presence of hot mantle alone does not determine whether volcanism occurs. "The break-up between India and the Seychelles was thought to have all the right conditions to generate volcanic margins. Our work demonstrates how much we still have to learn about how our planet works" says Dr Jenny Collier.

To explain the surprising lack of volcanism as the Indian Ocean was formed Armitage and Collier developed a finite-element model to describe the mantle melting that occurs as continents split. The model takes into account earlier thinning of the crust due to extension and its affect on the underlying mantle. For the North Atlantic Armitage and Collier found that earlier extension focused the hot upwelling mantle allowing it to reach shallow depths quickly - causing large amounts of melting and volcanism. For the Indian Ocean earlier extension had a different affect - it tapped the hot mantle and depleted it sufficiently before separation occurred with the result that little mantle melting and volcanism accompanied continental breakup. Dr John Armitage adds: "We have found that rift history can either suppress or enhance the production of magma and is just as important as mantle temperature in controlling what happens."

Armitage and Collier's research marks a new milestone our understanding of how volcanism and plate tectonics are related. The research underlines that detailed knowledge of the pre-existing geology and timing of previous events when combined with cutting edge models are on the verge of being able to predict how our planet behaves.

Personnel

• John Armitage
• Jenny Collier