Alaji Basalts

The Alaji (upper) Basalts are the youngest series of the Ethiopian flood basalts. The most recent flows are only 15 million years old.[1][2]

Alaji Basalts Stratigraphic range: Miocene PreꞒ Ꞓ O S D C P T J K Pg N
Mt. Gumawta, Tembien highlands
Type	Geological formation
Underlies	Pliocene shield volcanoes, locally
Overlies	Ashangi Basalts, Intra-volcanic sedimentary rock
Thickness	220 m (720 ft)
Lithology
Primary	Basalt, Rhyolite
Other	Trachyte
Location
Coordinates	12.9817°N 39.5198°E
Region	Tigray
Country	Ethiopia
Extent	Ethiopian Highlands
Type section
Named for	Imba Alaje Mountain
Named by	William Thomas Blanford
Alaji Basalts (Ethiopia)

Name and definition

The name was coined by geologist William Thomas Blanford, who accompanied the British Expedition to Abyssinia in 1868,[3] after the Imba Alaje mountain. So far the nomenclature has not been proposed for recognition to the International Commission on Stratigraphy.

Stratigraphic context

Uppermost Tertiary flood basalts in Ethiopia. Locally they are covered by Pliocene shield volcanoes, such as the Simien Mountains, or Mount Guna. These flows have been deposited on the lower Ashangi Basalts and locally on intra-volcanic sedimentary rock.

Environment

Like all volcanic rocks, the Alaji Basalts originate from initial melting of the Earth's mantle. After extrusion, the magmatic structures form at the surface. Common volcanic structures such as lava tubes or ropy lavas are absent in the Alaji Basalts, but (columnar joints) are omnipresent. The basalts comprise successive flows. During cooling, newly developed crystals within the lava solidify and develop congealing stress that favours the formation of columnar joints (intersecting fractures). They are perpendicular to the surface of the lava flow: mostly vertical, but sometimes also inclined or almost horizontal.[4][5]

Lithology

The Bi’en ridge near Hagere Selam in the Tigray Region is composed of Alaji Basalts. Weathering of the rock has led to the occurrence of fertile Luvisols

The Alaji Basalts hold alternating layers of rhyolitic and basaltic rocks. Its thickness varies between 80 and 220 metres. The upper layer is capped by trachytic materials. The lower part of the Alaji Basalts is composed of medium- to fine-grained basalts. The middle succession holds elongated plagioclase-dominated trachytes. The mineralogy of the basaltic rocks is marked by well-aligned elongated pyroxenes and micro-plagioclases. In the mineralogy of the trachytes one notices especially the by twinning of feldspar minerals and a small amount of altered pyroxenes.[4]

Rock sample of aphanitic basalt, collected at Addi Amyuq pass

Geographical extent

The formation outcrops widely in the Ethiopian highlands, particularly in the upper landscape positions.[6]

References

1. Mohr, P.; Zanettin, B. (1988). The Ethiopian flood basalt province. In: Continental flood basalts. Dordrecht: Springer. pp. 63–110. doi:10.1007/978-94-015-7805-9_3.
2. Hofmann, C., and colleagues (1997). "Timing of the Ethiopian flood basalt event and implications for plume birth and environmental change". Nature. 389: 838–841. doi:10.1038/39853.
3. Blanford, W.T. (1870). Observations on the geology and zoology of Abyssinia, made during the progress of the British expedition to that country in 1867-68. London: Macmillan and Co.
4. Miruts Hagos and colleagues (2019). The volcanic rock cover of the Dogu'a Tembien massif. In: Geo-Trekking in Ethiopia's Tropical Mountains, the Dogu'a Tembien District. SpringerNature. ISBN 978-3-030-04954-6.
5. Pik, R., and colleagues (1998). "The northwestern Ethiopian Plateau flood basalts: classification and spatial distribution of magma types". J Volcanol Geotherm Res. 81 (1–2): 91–111. doi:10.1016/S0377-0273(97)00073-5.
6. Kieffer, B., and colleagues (2004). "Flood and shield basalts from Ethiopia: magmas from the African superswell". J. Petrol. 45 (4): 793–834. doi:10.1093/petrology/egg112.