Afar Triple Junction - Geology

Geology

The geology of the Afar Triple Junction incorporates multiple rock types. Most of this area is composed of basalt, a volcanic rock formed by rapid cooling of magma at the Earth’s surface. There are also sedimentary and metamorphic rocks, but not as much as the igneous (basalt). The vast majority of the basalt was created between 1 and 24 million years ago, with production peaking towards the end of this period. Volcanic activity continues today, although on a smaller scale, as active volcanoes continue to erupt; Erta Ale is perhaps the most famous active volcano in the Afar. Volcanism today is mainly in northern Ethiopia and farther southward along the East African Rift. Thicknesses of basalt can reach upwards of 4 kilometers within the Afar Triple Junction.

Before the creation of the rifts, which now define the Afar Triple Junction, the area was dominated by igneous and metamorphic rocks. These were formed between 500 and 600 million years ago during a mountain building event caused by the collision of East and West Gondwana. These rocks are referred to as basement rocks, which are buried by future successions of rocks and represent the earliest portion of geologic history.

Clues as to the history of the Afar Triple Junction lies in the presence of limestones, evaporites, and ancient coral reefs found in and around the Afar Depression, centralized in western Djibouti. The presence of these types of rocks is strong evidence that the area known as the Afar Depression was once covered by shallow seawater, possibly an extension of the Red Sea. The rifts of the Afar Triple Junction and the valleys they form are mostly filled with sedimentary rocks (see yellow bands on geologic map) which are representative of the rocks from which they were formed. These include sediments eroded from both basement rocks and volcanic rocks, good candidates for erosion based on their topography, as they sit at higher elevations on the borders of the rifts.