Ur (continent)

Ur is a proposed supercontinent that formed in the Archean 3,100 million years ago (3.1 billion).

In Roger's reconstruction Ur is half a billion years older than Arctica and, in the early period of its existence, it was probably the only continent on Earth, and as such can be considered a supercontinent, though it was probably 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.

About 1,300–1,071 Mya Ur joined the continents Nena and Atlantica to form the supercontinent Rodinia. In the reconstruction of Rogers 1996 Ur remained the nucleus of East Gondwana until the break-up of Gondwana. In other reconstruction, however, India and East Antarctica did not collide until Rodinia formed 1,071 Mya. 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 Gya supercontinent.

Reconstructions of the supercontinent Vaalbara places 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.

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