Rodinia

:"For the genus of metalmark butterflies, see Rodinia (butterfly).

In geology, Rodinia (from the Russian "родина", or "motherland") refers to one of the oldest known supercontinents, which contained most or all of Earth's then-current landmass. Paleomagnetic evidence provides clues to the paleolatitude of individual formations, but not to their longitude, which geologists have pieced together by comparing similar strata, often now widely dispersed.

Lifetime

Geologic evidence suggests that Rodinia formed and broke apart in the Neoproterozoic, probably existing as a single continent from 1 billion years ago until it began to rift into eight smaller continents about 800 million years ago.cite journal | last = Torsvik | first = Trond H | year = 2003 | month = May | title = The Rodinia jigsaw puzzle | journal = Science | volume = 300 | issue = 5624 | pages = 1379 | doi = 10.1126/science.1083469 | pmid = 12775828] It is thought to have been largely responsible for the cold climate of the Neoproterozoic era.

Rodinia is thought to have been preceeded by the Vaalbara supercontinent and succeeded by the Pangaea supercontinent, with intervening periods when several continents existed.

Formation

Rodinia began forming about 1.3 billion years ago from three or four pre-existing continents, an event known as the Grenville orogeny.cite web | url = http://www.peripatus.gen.nz/Paleontology/Rodinia.html | title = Rodinia | accessdate = 2006-03-10] The absence of fossils of hard-shelled organisms and reliable paleomagnetic data make the movements of continents earlier in the Precambrian, prior to this event, uncertain. (See Columbia (supercontinent) for one possible reconstruction of an earlier supercontinent.)

The arrangement of Rodinia has been hypothesized using paleomagnetic data from the Seychelles islands and India and the Grenville mountain belts, which were formed by the Grenville orogeny and span multiple modern continents, as references.

Although the details are disputed by paleogeographers, the continental cratons that formed Rodinia appear to have clustered around Laurentia (proto-North America), which constituted Rodinia's core.

It appears that the east coast of Laurentia lay adjacent to the west coast of South America, while a conjoined Australia and Antarctica seem to have lain against the proto-North American west coast. A third craton, what would become north-central Africa, was caught in between these two colliding masses.cite web | url = http://www.scotese.com/info.htm | last = Scotese | first = Christopher R. | title = More Information About the Late Precambrian | work = Paleomap Project | accessdate = 2006-03-10] .

Other cratons such as the Kalahari (southern Africa), the Congo (west-central Africa), and the San Francisco (southeastern South America), appear to have been separate from the rest of Rodinia.

Paleogeography

Rodinia's landmass was probably centered south of the equator. [cite web | url = http://www.palaeos.com/Earth/Geography/Rodinia.htm | title = Rodinia | publisher = Palaeos | accessdate = 2006-03-10] Because Earth was at that time experiencing the Cryogenian period of glaciation, and temperatures were at least as cool as today, substantial areas of Rodinia may have been covered by glaciers or the southern polar ice cap. The interior of the continent, being so distant from the temperature-moderating effects of the ocean, was probably seasonally extremely cold (see continental climate). It was surrounded by the superocean geologists are calling Mirovia (from "mir", the Russian word for "globe").

Cold temperatures may have been exaggerated during the early stages of continental rifting. Geothermal heating peaks in crust about to be rifted; and since warmer rocks are less dense, the crustal rocks rise up relative to their surroundings. This rising creates areas of higher altitude, where the air is cooler and ice is less likely to melt with changes in season, and it may explain the evidence of abundant glaciation in the Ediacaran period.cite book | last = McMenamin | first = Mark A. | coauthors = Dianna L. McMenamin | title = The Emergence of Animals | origdate = 1990-01-15 | id = ISBN 0-231-06647-3 | chapter = The Rifting of Rodina]

The eventual rifting of the continents created new oceans, and seafloor spreading, which produces warmer less-dense rock, probably increased sea level by displacing ocean water. The result was a greater number of shallower oceans.

The evaporation from these oceans may have increased rainfall, which, in turn, increased the weathering of exposed rock. By inputting data on into computer models, it has been shown that in conjunction with quick-weathering of volcanic rock, this increased rainfall may have reduced greenhouse gas levels to below the threshold required to trigger the period of extreme glaciation known as Snowball Earth. [cite journal | last = Donnadieu | first = Yannick | coauthors = Yves Goddéris, Gilles Ramstein, Anne Nédélec, Joseph Meert | year = 2004 | month = March | title = A 'snowball Earth' climate triggered by continental break-up through changes in runoff | journal = Nature | volume = 428 | pages = 303–306 | doi = 10.1038/nature02408]

All of this tectonic activity also introduced into the marine environment biologically important nutrients, which may have played an important role in the development of the earliest animals.

eparation

In contrast to Rodinia's formation, the movements of continental masses during and since its breakup are fairly well understood. Evidence of extensive lava flows and volcanic eruptions around the Precambrian-Cambrian boundary, especially in North America, suggest that Rodinia began to rift apart no later than 750 million years ago. Other continents, including Baltica and Amazonia, rifted off Laurentia 600 to 550 million years ago, opening the Iapetus Ocean between them. The separation also led to the birth of Panthalassic Ocean (or Paleo-Pacific).

The eight continents that made up Rodinia later re-assembled into another global supercontinent called Pannotia and, after that, once more as Pangaea.

ee also

* Supercontinent cycle

External links

* [http://www.washington.edu/burkemuseum/geo_history_wa/Dance%20of%20the%20Giant%20Continents.htm "Dance of the Giant Continents: Washington's Earliest History"]
* [http://www.tsrc.uwa.edu.au/440project IGCP Special Project 440:] mapping Proterozoic supercontinents, including Rodinia
* [http://www.scotese.com/newpage13.htm PALEOMAP Project:] Plate Tectonic Animations (java)

References