Rifts

Active rift systems may exhibit very steep escarpments that drop from the rift shoulders to the base of the rift valley floor, typically forming an elongate depression that may extend for hundreds or even thousands of miles. The world's best known example of a continental rift is the East African rift, extending from the Ethiopian Afar to Mozambique. other spectacular examples include Lake Baikal in siberia, the Rio Grande in the desert southwest of Arizona and New Mexico, and the Alaotra rift in Madagascar. Most of these rifts have coarse-grained sediments deposited along their margins, and fine-grained and even lake sediments in their centers. Volcanic centers are sporadically developed.

Many rifts in continents are associated with incipient breaking apart of the continent to form an oceanic basin. These types of rift system typically form three arms that develop over domed areas above upwelling mantle material, such as is observed in east Africa. Two of the three arms may link with other three-pronged rift systems developed over adjacent domes, forming a linked elongate rift system that then spreads to form an ocean basin. This type of development leaves behind some failed rift arms that will come to reside on the margins of young oceans when the successful rift arms begin to spread. These failed rift arms then become sites of increased sedimentation and subsidence, and also tend to be low-lying areas, and form the tectonic setting where many of the world's major rivers flow (for example, the Nile, Amazon, and mississippi). other rifts form at high angles to collisional mountain belts, and still others form in regions of widespread continental extension such as the basin and range province of the southwestern united states.

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