Geology Reference
In-Depth Information
netic data reveal the divergence of Ur from other cratons as Kenorland began its protracted
fragmentation.ThosecratonicpuzzlepiecesscatteredfromtheEquatortothepoles.Newly
opened shallow seas between the diverging pieces led to thick deposits of shallow marine
sediments. The supercontinent cycle had begun.
Hail Columbia
The addition of the supercontinent cycle to the annals of geology has made the boring bil-
lion a lot less boring. The next supercontinent episode, one more sharply in focus than
Kenorland, thanks to younger and much better preserved suites of rocks, began about two
billion years ago, at a time when Earth boasted at least five separate continent-size land-
masses.LargestamongthesewastheLaurentiansupercraton,aconglomerationofatleasta
half-dozen cratons thousands of miles across that encompasses much of what is now cent-
ral and eastern North America. (Specialists in ancient landmasses sometimes refer to this
clustering of cratons as the United Plates of America.) The original continent Ur soldiered
on as the second-largest landmass, separated from Laurentia by a substantial ocean. The
much smaller Baltica and Ukrainian cratons, which form the core of what is now eastern
Europe, and cratons representing parts of what are now South America, China, and Africa
were also large islands approaching continent size. By the time Earth was 1.9 billion years
old, these varied lands had collided at convergent plate boundaries, raising new mountain
belts and forming a supercontinent variously named Columbia, Nena, Nuna, or Hudson-
land. (The name Columbia, based on persuasive geological evidence from the vicinity of
the Columbia River, along the Washington-Oregon border, seems to be used most often.)
Thisvastbarrenland,roughlyestimatedtohavebeeneightthousandmileslongfromnorth
tosouthandthreethousandmileswide,incorporatedalmostallofEarth'scontinentalcrust.
The complexities of retroactively arranging thirty-plus cratonic fragments into one ex-
tinct supercontinent are daunting. Not surprisingly, more than one model usually vies for
acceptance. In the case of Columbia, two rather different stories emerged almost simul-
taneously in 2002. On the one hand, geochemist John Rogers of the University of North
Carolina and his colleague, Indian geologist Santosh Madhava Warrier (based at Kochi
University in Japan), proposed that Laurentia, what is nowmost ofNorth America, formed
the core of Columbia. According to Rogers and Santosh, the continent of Ur was sutured
to the west coast of Laurentia; portions of Siberia, Greenland, and Baltica were positioned
to the north; and parts of what are now Brazil and West Africa lay to the southeast. In the
same year, Guochun Zhao of the University of Hong Kong and several colleagues devised
a somewhat different configuration, in which Baltica is sutured to the east coast of Lauren-
tia, while eastern Antarctica and China are attached on the west. Given the great age of
Columbia and the preliminary nature of these reconstructions, the agreement between the
