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Sunday, January 07, 2007

A brief history of supercontinents

Nearly everyone who has taken an introductory geology course has heard of the 'supercontinent' of Pangea (meaning 'all lands). The supercontinent was composed of two parts, Laurasia (North America and Eurasia) and Gondwana (the southern continents). This supercontinent was fairly short-lived, but gave geologists a glimpse into the past and future of continental motion.
Gondwana can rightly be considered a supercontinent of its own. The continent was composed of Africa, Australia, Antartica, India, Madagascar, South America, Sri Lanka and had numerous smaller blocks around its periphery. The formation of Gondwana took place at about the same time as the Cambrian explosion of life and the shallow equatorial seas along some of the margins served as a warm, nutrient-rich ecosystem.
The configurations of continents before Pangea becomes more difficult. We must rely on paleomagnetism (fossil magnetism in rocks that tells us 'where' the rocks formed), geochronology (radioactive decay tells us when the rock formed), geology (some features may 'line up' when continents are placed together and any other information we can use. For example, in Pangea, the Appalachian mountains line up nicely with the Caledonian mountains in Europe when the Atlantic is closed.
The existence of supercontinents besides Pangea and Gondwana has been a difficult chase, but it now appears that at least one, and perhaps more, supercontinents existed prior to Pangea and Gondwana. The first of these has been nicknamed Rodinia (from the Russian prefix 'rodit' meaning mother). Rodinia was so named because it was thought to be both the mother of all supercontinents and also because the margins along which the continent disaggregated became the birthplace of modern life.
Did Rodinia exist? It sure appears that there was a large supercontinent near the end of the Precambrian (~1100 million years ago until ~750 million years ago). The evidence comes from paleomagnetism, geochronology and geology. The problem is trying to decide what pieces belong where! Perhaps the most compelling evidence for this supercontinent comes from a study of the sedimentary history of the margins of North America. It seems that no matter where we look along the margins we see evidence of rifting (or breakup). Since breakup requires at least two pieces, these rifted margins along North America suggest that a number of other pieces resided as neighbors to North America. There is still great debate as to what belongs where, but very few doubt the existence of a Precambrian supercontinent.
What about before Rodinia? Evidence has been interpreted to support an earlier supercontinent known as Columbia or Nuna. Evidence supporting the existence of this supercontinent is derived primarily from the study of paleomagnetism and geochronology of collisional belts (where the continents smashed into each other). Since these mountains have long since been eroded, we are looking at the cores of the belts and trying to figure out what the supercontinent may have looked like. This supercontinent is thought to have formed about 2100-1800 million years ago and broken up at around 1500 million years ago.
Supercontinents prior to Columbia are even harder to discover. In part this is because the rock record becomes increasingly rare the further back in time we go.
What about the future? If supercontinents do form, breakup and form again, then what does the future hold? One could argue that Africa+Eurasia constitutes a supercontinent today. We do know that continents continue to move and as long as there is heat within the earth to drive this motion, other supercontinents may form. Two such 'future' supercontinents have been proposed. The first "Pangea Ultima" considers what would happen if the Atlantic ocean closed. The second "Amasia" offers a glimpse of the world without a Pacific Ocean.


Joe Meert


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