Divergent boundary
These sediments may be interleaved with volcanic rocks, typically alkaline (having abundant sodium, Na, and other alkali elements) and bimodal in silica content (i.e., basalts and rhyolites).
Most active divergent plate boundaries occur between.
other types of deposits in rifts include lake sediments in rift centers, and conglomerates (cemented gravels) derived from rocks exposed along the rift shoulders. Divergent boundaries within continents initially produce rifts, which eventually become rift valleys. This process leaves distinctive deposits of salts and other minerals that form by being left behind during evaporation of seawater (evaporites), one of the hallmark deposits of continental rift settings. Mantle Mid-ocean ridge G Infobase PublishingĮnvironment, such as much of East Africa, the drainage may have no outlet and the water will evaporate before it can reach the sea. Mature ocean stage is like the modern Atlantic Ocean, with thick passive margin sequences developed on continental edges around a wide ocean basin. The young ocean stage, similar to the modern Red Sea, has seafloor spreading and steep rift shoulders. The young rift valley stage like that in the East African rift system has steep rift shoulders and basaltic and rhyolitic volcanoes. Faulting patterns are also asymmetric, with different styles on either side of the rift.įigure showing simplified three-stage evolution of divergent margins. (B) Shows simple shear or asymmetric rifting, where a shallow-dipping detachment fault penetrates the thickness of the lithosphere, and asthenosphere rises asymmetrically on the side of the rift where the fault enters the asthenosphere. (A) Shows pure shear model, in which the lithosphere extends symmetrically and asthenosphere rises to fill the space vacated by the extending lithosphere. Lithosphere-asthenosphere boundary G Infobase Publishing Drainage systems tend to be short, internal systems, with streams forming on the steep sides of the rift, flowing along the rift axis, and draining into deep, narrow lakes within the rift. Other articles where divergent plate boundary is discussed: earthquake: Tectonic associations: oceanic ridges, which correspond to divergent plate boundaries. Rift valleys have steep, fault-bounded sides, with rift shoulders that typically tilt slightly away from the rift valley floor. The general geomorphic feature that initially forms is known as a rift valley.
In these places the continents are beginning to break apart as immature divergent boundaries, and if successful, may form new ocean basins. Rifts are elongate depressions formed where the entire thickness of the lithosphere has ruptured in extension.