Geography of Ethiopia - Geology

Geology

The East African tableland is continued into Ethiopia. A pioneering study of the geology of Ethiopia was W. T. Blanford's work in 1870. More recent work has focussed on the Afar Depression, due to its importance as one of two places on Earth where a mid-ocean ridge can be studied on land (the other is Iceland).

The following formations are represented:

Sedimentary and metamorphic

• Recent: Coral, alluvium, sand
• Tertiary: Limestones of Harrar
• Jurassic: Antalo Limestones
• Triassic: Adigrat Sandstones
• Archaean: Gneisses, schists, slaty rocks

Igneous

• Recent: Aden Volcanic Series
• Tertiary, Cretaceous: Magdala group
• Jurassic: Ashangi group

Archaean.--The metamorphic rocks compose the main mass of the tableland, and are exposed in every deep valley in Tigre and along the valley of the Blue Nile. Mica schists form the prevalent rocks. Hornblende schist also occur and a compact felspathic rock in the Suris defile. The foliae of the schists strike north and south.

Triassic.--In the region of Adigrat the metamorphic rocks are invariably overlain by white and brown sandstones, unfossiliferous, and attaining a maximum thickness of 300 m. They are overlain by the fossiliferous limestones of the Antalo group. Around Chelga and Adigrat coal-bearing beds occur, which Blanford suggests may be of the same age as the coal-bearing strata of India. The Adigrat Sandstone possibly represents some portion of the Karoo System of South Africa.

Jurassic.--The fossiliferous limestones of Antalo are generally horizontal, but are in places much disturbed when interstratified with trap rocks. The fossils are all characteristic Oolite forms and include species of Hemicidaris, Pholadomya, Ceromya, Trigonia and Alaria.

Igneous Rocks.--Above a height of 2,400 m (8,000 ft) the country consists of bedded traps belonging to two distinct and unconformable groups. The lower (Ashangi group) consists of basalts and dolerites often amygdaloidal. Their relation to the Antalo limestones is uncertain, but Blanford considers them to be not later in age than the Oolite. The upper (Magdala group) contains much trachytic rock of considerable thickness, lying perfectly horizontally, and giving rise to a series of terraced ridges characteristic of central Ethiopia. They are interbedded with unfossiliferous sandstones and shales. Of more recent date (probably Tertiary) are some igneous rocks, rich in alkalis, occurring in certain localities in southern Ethiopia. Of still more recent date are the basalts and ashes west of Massawa and around Annesley Bay and known as the Aden Volcanic Series. With regard to the older igneous rocks, the enormous amount they have suffered from denudation is a prominent feature. They have been worn into deep and narrow ravines, sometimes to a depth of 1,000 to 1,200 m (3,000 to 4,000 ft).