he gradual stretching of the lithosphere causes generally causes an extension of two to four times the length of the original lithosphere before separation occurs. Thus to accommodate the stretching the lithosphere must reduce in thickness by one half to a quarter of the original thickness, resulting in the formation of a basin.
As the lithosphere is not ductile, stretching takes place by the formation of a series of asymmetric normal faults. As with any movement within a fault system, strain build up can occur that when released can result in devastating earthquakes. Thus, earthquakes are common in rift zones.
The thinning of the lithosphere causes by rifting reduces the lithostatic pressure on the asthenosphere. As the asthenosphere is very hot the reduction is pressure causes partial melting of the ultramafic rocks to create magma with a mafic composition. The magma created is less dense than the overlying lithosphere and rises due to buoyancy. This magma easily finds its way to the surface through the weakened lithosphere, causing basaltic volcanism to commonly occur in continental rifts. In addition to basaltic volcanism, violent andesitic and rhyolitic volcanism can also occur in rifts. This occurs when the mafic magma becomes trapped in the lithosphere and causes secondary melting of the broadly granitic rocks in the lithosphere.
If the rifting is termed "successful", extension of the lithosphere occurs until such a time as lithosphere separates. By this time, the basin that forms is sufficiently deep to be in-filled by the ocean and form part of the continental shelf. When the lithosphere separates a rift-drift transition takes place, where the stretching ceases and the sea floor spreading begins to take place, as magma moves up through the ridge. If sea floor spreading continues, the result will be the formation of a new ocean basin.
Long before the formation of an ocean basin, depression created by rifting is partially in-filled by erosion of the surrounding high-lands. As subsidence continues lakes form along the axis of the basin and fine grained mud is deposited onto the clastic debris from the initial infilling. As subsidence continues, the basin sinks below sea level, and is infilled by a shallow sea. These shallow water have high evapouration rates and thus evapourite deposits of halite and gypsum occur. These deposits eventually end up as part of final ocean basin.
The sequence of coarse sediments, mudstone layers and a salt cap, combined with the large amount of organic material deposited in the shallow aquatic environments, meat rift zones contain valuable petroleum reservoirs.