Continental rifting

From GeoClasses

The East African Rift

As we saw before, the Great Lakes in the East African rift valley mark the divergent boundary where Africa is splitting apart.

Locations of East African volcanoes. Map of East Africa showing some of the historically active volcanoes(red triangles) and the Afar Triangle (shaded, center) -- a so-called triple junction (or triple point), where three plates are pulling away from one another: the Arabian Plate, and the two parts of the African Plate (the Nubian and the Somalian) splitting along the East African Rift Zone. Kios and Tilling, 1996, Dynamic Earth, USGS

Thinning and fracturing of the crust allows molten rock to emerge at the surface, creating volcanos.

This map shows the locations of volcanoes in East Africa.

Erta Ale volcano in Ethiopia Kios and Tilling, 1996, Dynamic Earth, USGS

Helicopter view (in February 1994) of the active lava lake within the summit crater of 'Erta 'Ale (Ethiopia), one of the active volcanoes in the East African Rift Zone. Two helmeted, red-suited volcanologists -- observing the activity from the crater rim -- provide scale. Red color within the crater shows where molten lava is breaking through the lava lake's solidified, black crust. Kios and Tilling, 1996, Dynamic Earth, USGS

Diagram showing continental rifting

1. Start with continental crust.
2. As forces start to pull the crust apart, the crust streches and thins.
3. This causes the crust to fracture and create a set of normal faults on either side of a central basin. The center of the rift drops down (this is called a graben).
4. Volcanism occurs along the flanks of the rift where the faulting weakens the crust.

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