Ur (continent) facts for kids
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Ur, in the Eoarchean Era
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| Historical continent | |
|---|---|
| Formed | 3.1 Ga |
| Type | Supercontinent |
| Today part of |
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Ur is a hypothetical supercontinent that formed in the Archean eon around 3.1 billion years ago (Ga). In a reconstruction by Rogers, Ur is half a billion years older than Arctica and, in the early period of its existence, probably the only continent on Earth, making it a supercontinent despite probably being smaller than present-day Australia. In more recent works geologists often refer to both Ur and other proposed Archaean continental assemblages as supercratons. Ur can, nevertheless, be half a billion years younger than Vaalbara, but the concepts of these two early cratonic assemblages are incompatible.
Incompatible reconstructions
About 1.3–1.1 Ga, Ur joined the continents Nena and Atlantica to form the supercontinent Rodinia. In the reconstruction of Rogers 1996, Ur remained the nucleus of eastern Gondwana until the break-up of Gondwana. In other reconstructions, however, India and East Antarctica did not collide until Rodinia formed 1.1 Ga. Furthermore, in the early Archaean Earth's mantle was 200 °C hotter than today, and many characteristics of modern tectonics, such as ophiolites, blueschists, lawsonite-bearing eclogites, and ultra-high-pressure rocks, did not exist or were rare. This makes most proposed Archaean supercontinents controversial, including Rogers's 3 Ga supercontinent.
Reconstructions of Vaalbara place two cratons—Kaapvaal in southern Africa and Pilbara in western Australia—next to each other based on stratigraphic similarities. In Roger's configuration of Ur, these cratons are placed far apart in their Gondwana configuration. This configuration is contradicted by widespread Precambrian collisional events between Australia and Africa.
Another possible supercraton, Zimgarn, proposed by Smirnov and others 2013 and named after the Zimbabwe and Yilgarn cratons, is distinct from both Vaalbara and Ur. Vaalbara and Zimgarn, according to this proposal, both disintegrated about 2.1–2.0 Ga to reassemble as the Kalahari and West Australian cratons 2.5–1.5 Ga. Smirnov et al. based this reconstruction on: (1) Zimgarn was still undergoing cratonisation when an extensive carbonate platform developed over Vaalbara; (2) the magmatic signatures are different for the two supercratons during the period 2.6–2.0 Ga; and (3) paleomagnetic latitudes for 2.7 Ga are slightly different.
See also
In Spanish: Ur (supercontinente) para niños
