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Evidence for Rodinia

August 19, 2011

Picture from Wikipedia - Break up of pangaea into modern continents

Most of us are aware of Pangaea, earth’s super-continent that existed about 250 million years ago before breaking up into the present day continents. Geologists have known that there have been previous super-continents before Pangaea which broke up and coalesced back again.

The first super land mass was Columbia, formed about 2 billion years ago. Columbia broke up into fragments which accreted back together again about 1 billion years ago, forming the super-continent Rodinia. It was conjectured that about 750 million years ago, Rodinia broke up into several land masses including Laurentia, the core of the present North American continent, and Gondwana, consisting of most of the present Southern Hemisphere continents. Antarctica was thought to be entirely a part of the ancient Gondwana land mass. Now, a team of geologists in the USA, UK and Australia have found evidence that a part of present day East Antarctica, the Coats Land crustal block, was once part of the ancient Laurentia, bordering present day Texas.

The scientists compared the concentrations of different lead isotopes (Pb-204, Pb-206 and Pb-207) in the igneous (lava) rocks from Coats Land to that of the Red Bluff granites in the Franklin Mountains in Texas, and found remarkable agreement between the two. Rocks from the Keweenawan province of North America, near the great lakes, also had similar isotopic concentrations. The composition of lead isotopes in igneous rocks is determined by the initial lead concentration of its magma source, and from the radioactive decay of uranium and thorium. Interestingly, rocks from the Umkondo province in the Kalahari region of Southern Africa, which was earlier a part of Gondwana, had a discernably different lead isotope composition. This is clear evidence that the Coats Land crustal block had the same origins as Laurentia, but separate from Gondwana.

Incorporating previously obtained paleomagnetic data, the exact location of the Coats Land cluster block about 1 billion years ago was pinned down to the southwestern end of Laurentia, next to present day Franklin Mountains in Texas. It seems likely that the Kalahari region of the ancient Gondwana collided with Laurentia around 1 billion years ago, forming the super-continent Rodinia. The authors suspect that Gondwana broke away from Laurentia again about 500 million years ago, resulting in the Coats Land cluster block also breaking away.

It seems modern day Antarctica is made up partly from Laurentia and partly from Gondwana, the two pedestals of ancient Rodinia.

 

Loewy, S., Dalziel, I., Pisarevsky, S., Connelly, J., Tait, J., Hanson, R., & Bullen, D. (2011). Coats Land crustal block, East Antarctica: A tectonic tracer for Laurentia? Geology, 39 (9), 859-862 DOI: 10.1130/G32029.1

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