Geology Reference
In-Depth Information
were accumulating in shallow waters around the globe—deposits now preserved as a tell-
tale sedimentary record. No ice also means no glaciers. The interval from 1.6 to 1.4 billion
years ago holds none of the characteristic glacial remains—piles of ice-rounded cobbles
and boulders, sand and gravel—that are found in most other geological ages. So the dull
Mesoproterozoic saw a great deal of change, even if those changes were geological “busi-
ness as usual.”
Supercontinent Reprise: The Assembly of Rodinia
The boring billion saw the formation of not one but two supercontinents. The scattered
fragments of Columbia drifted apart for perhaps 200 million years, but a continent can di-
verge on a globe only for so long before it starts converging once again. About 1.2 billion
years ago Ur, Laurentia, and other Mesoproterozoic continents began to reassemble into a
new landmass called Rodinia (after the Russian word for “motherland” or “birthplace”).
The rock record of far-flung localities in Europe, Asia, and North America preserves an
associated worldwide pulse of mountain-building events between 1.2 and 1.0 billion years
ago; each new mountain range elevated as converging cratons collided and crumpled.
The exact geography of Rodinia is still a matter of debate, but geological and paleo-
magnetic data, coupled with the arrangement of cratons on today's globe, place significant
constraints. Most models situate the entire supercontinent near the Equator, with Lauren-
tia—what is now most of North America—at the center, and large pieces of all the other
continentsstuckontothenorth,south,east,andwest.Accordingtoseveralreconstructions,
Baltica and chunks of what are now Brazil and West Africa lay to the southeast, with oth-
er pieces of South America to the south and fragments of Africa to the southwest, though
details of the relative positions of Australia, Antarctica, Siberia, and China are as yet un-
settled.
The distinguishing characteristic of Rodinia is its absence of certain kinds of rocks. Un-
likeanyotherintervalofthepast3billionyears,fewifanysedimentarydepositshavebeen
preserved from the period between about 1.1 billion and 850 million years ago. This hiatus
means there were probably no shallow seas between continents of the type that hosted the
1.6-billion-year-old Belt-Purcell supergroup. The conclusion: all the continents must have
fit neatly together. Nor does it appear that there were any large inland seas, of the kind
thatoncefloodedallofcentral NorthAmerica andlaidthesedimentary foundationsforthe
Great Plains some 100 million years ago. By this model, equatorial Rodinia had a hot, dry,
desertlike interior, much like Australia today. For almost 250 million years, the sediment-
ary rock cycle seems to have all but shut down.
Linda Kah makes her points methodically, but it's clear that she is passionate about her
chosen geological interval. In spite of the meager rock record near its end, the great time
