The Alaji (upper) Basalts are the youngest series of the

Ethiopian Ethiopians are the native inhabitants of Ethiopia, as well as the global diaspora of Ethiopia. Ethiopians constitute several component ethnic groups, many of which are closely related to ethnic groups in neighboring Eritrea and other parts of ...

flood basalts A flood basalt (or plateau basalt) is the result of a giant volcanic eruption or series of eruptions that covers large stretches of land or the ocean floor with basalt lava. Many flood basalts have been attributed to the onset of a hotspot reach ...

. The most recent flows are only 15 million years old.

Name and definition

The name was coined by geologist

William Thomas Blanford William Thomas Blanford (7 October 183223 June 1905) was an English geologist and naturalist. He is best remembered as the editor of a major series on ''The Fauna of British India, Including Ceylon and Burma''. Biography Blanford was born ...

, who accompanied the

British Expedition to Abyssinia The British Expedition to Abyssinia was a rescue mission and punitive expedition carried out in 1868 by the armed forces of the British Empire against the Ethiopian Empire (also known at the time as Abyssinia). Emperor Tewodros II of Ethiopia, t ...

in 1868, after the Imba Alaje mountain. So far the nomenclature has not been proposed for recognition to the

International Commission on Stratigraphy The International Commission on Stratigraphy (ICS), sometimes referred to unofficially as the "International Stratigraphic Commission", is a daughter or major subcommittee grade scientific daughter organization that concerns itself with stratigra ...

Stratigraphic context

Uppermost

Tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...

flood basalts in Ethiopia. Locally they are covered by

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58shield volcano A shield volcano is a type of volcano named for its low profile, resembling a warrior's shield lying on the ground. It is formed by the eruption of highly fluid (low viscosity) lava, which travels farther and forms thinner flows than the more v ...

es, such as the

Simien Mountains The Simien Mountains (Amharic: ስሜን ተራራ or Səmen; also spelled Simen and Semien), in northern Ethiopia, north east of Gondar in Amhara region, are part of the Ethiopian Highlands. They are a World Heritage Site and include the Simien M ...

, or

Mount Guna Mount Guna (Amharic: ጉና ተራራ, ''Guna Terara'') is a mountain and shield volcano located near the cities of Nefas Mewcha and Debre Tabor, in the northern Amhara region of Ethiopia. It is the highest point in the South Gondar Zone, with a ...

. These flows have been deposited on the lower

Ashangi Basalts The Ashangi Basalts are the earliest Tertiary volcanic rocks in north Ethiopia, hence they are in the lowest position. These dark porphyritic basalts are separated from the Mesozoic formations below it by basal Conglomerate (geology), conglomerates ...

and locally on intra-volcanic sedimentary rock.

Environment

Like all

volcanic rocks Volcanic rock (often shortened to volcanics in scientific contexts) is a Rock (geology), rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of Volcano, volcanic origin. Like all rock types, ...

, the Alaji Basalts originate from initial melting of the Earth's mantle. After

extrusion Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex c ...

, 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 Columnar jointing is a geological structure where sets of intersecting closely spaced fractures, referred to as joints, result in the formation of a regular array of polygonal prisms, or columns. Columnar jointing occurs in many types of igneou ...

) are omnipresent. The basalts comprise successive flows. During cooling, newly developed crystals within the lava solidify and develop

congealing stress Congelation (from Latin: , ) was a term used in medieval and early modern alchemy for the process known today as crystallization. In the ('The Secret of Alchemy') attributed to Khalid ibn Yazid (), it is one of "the four principal operations", a ...

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.

Lithology

The Alaji Basalts hold alternating layers of

rhyolitic Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

and

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

ic rocks. Its thickness varies between 80 and 220 metres. The upper layer is capped by

trachytic Trachyte () is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava enriched with silica and ...

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 Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...

minerals and a small amount of altered

pyroxenes The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...

Rock sample 4 Aphanitic basalt Addi Amyuq pass

Geographical extent

The formation outcrops widely in the

Ethiopian highlands The Ethiopian Highlands is a rugged mass of mountains in Ethiopia in Northeast Africa. It forms the largest continuous area of its elevation in the continent, with little of its surface falling below , while the summits reach heights of up to . ...

, particularly in the upper landscape positions.{{cite journal , last1=Kieffer , first1=B., and colleagues , title=Flood and shield basalts from Ethiopia: magmas from the African superswell , journal=J. Petrol. , date=2004 , volume=45 , issue=4 , pages=793–834 , url=https://academic.oup.com/petrology/article-abstract/45/4/793/1529659, doi=10.1093/petrology/egg112 , doi-access=free

References

Geologic formations of Ethiopia