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The Ashangi Basalts are the earliest
Tertiary Tertiary (from Latin, meaning 'third' or 'of the third degree/order..') may refer to: * Tertiary period, an obsolete geologic period spanning from 66 to 2.6 million years ago * Tertiary (chemistry), a term describing bonding patterns in organic ch ...
volcanic rock Volcanic rocks (often shortened to volcanics in scientific contexts) are rocks formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and me ...
s in north
Ethiopia Ethiopia, officially the Federal Democratic Republic of Ethiopia, is a landlocked country located in the Horn of Africa region of East Africa. It shares borders with Eritrea to the north, Djibouti to the northeast, Somalia to the east, Ken ...
, hence they are in the lowest position. These dark
porphyritic Porphyritic is an adjective used in geology to describe igneous rocks with a distinct difference in the size of mineral crystals, with the larger crystals known as phenocrysts. Both extrusive and intrusive rocks can be porphyritic, meaning ...
basalts are separated from the Mesozoic formations below it by basal conglomerates. The basalts hold
phenocrysts 300px, Swiss side of the Mont Blanc massif, has large white phenocrysts of plagioclase (that have trapezoid shapes when cut through). 1 euro coin (diameter 2.3 cm) for scale. A phenocryst is an early forming, relatively large and usually conspic ...
that developed before the magma reached the earth surface. These basalts are weathered, partially eroded and have a sub-horizontal stratification, particularly at the lower part. This series was created during the first period of the
flood basalt 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 (geolo ...
eruptions in north Ethiopia, in the
Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...
.


Name and definition

The name was coined by geologist
William Thomas Blanford William Thomas Blanford (7 October 183223 June 1905) was an England, English geologist and natural history, naturalist. He is best remembered as the editor of a major series on ''The Fauna of British India, Including Ceylon and Burma''. Biogra ...
, 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, ...
in 1868,{{cite book , last1=Blanford , first1=W.T. , title=Observations on the geology and zoology of Abyssinia, made during the progress of the British expedition to that country in 1867-68 , url=https://archive.org/details/cu31924024736666 , date=1870 , publisher=Macmillan and Co. , location=London after Lake Hashenge in Tigray, which is surrounded by large outcrops of the formation. So far the nomenclature has not been proposed for recognition to the
International Commission on Stratigraphy The International Commission on Stratigraphy (ICS), sometimes unofficially referred to as the International Stratigraphic Commission, is a daughter or major subcommittee grade scientific organization that concerns itself with stratigraphy, strati ...
.


Stratigraphic context

The formation covers
Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
sedimentary rocks, particularly Amba Aradam Formation and Adigrat Sandstone. In many places, on its upper side, deposits of intra-volcanic sedimentary rock occur.


Environment

These volcanic rocks formed through melting of
Earth's mantle Earth's mantle is a layer of silicate mineral, silicate rock between the Earth's crust, crust and the Earth's outer core, outer core. It has a mass of and makes up 67% of the mass of Earth. It has a thickness of making up about 46% of Earth's ...
. After
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...
generation, lava flowed out over the surface, in successive flows. Some part of it disappeared within the next 1 to 3 million years due to weathering and erosion, while other parts remained present. Within the basalt, columnar joints occur as a result of cooling; they are visible along the flanks of the ridges. The columnar joints are perpendicular to the surface of the lava flows; they are mostly vertical, but may also be strongly inclined. The lower succession, the one that overlies the Amba Aradam Formation and the Adigrat Sandstone holds vertical and closely spaced columnar joints. A common characteristic of the columnar joints observed in the Ashangi Basalts is their
pentagonal In geometry, a pentagon () is any five-sided polygon or 5-gon. The sum of the internal angles in a simple pentagon is 540°. A pentagon may be simple or self-intersecting. A self-intersecting ''regular pentagon'' (or ''star pentagon'') is cal ...
or
hexagonal In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A regular hexagon is d ...
shape (in plan view).


Lithology

The Ashangi Basalts are made of coarse-grained
plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...
minerals, surrounded by a matrix of fine-grained plagioclases and
pyroxene The pyroxenes (commonly abbreviated 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 ions of calcium (Ca), sodium (Na), iron ( ...
s; some iron oxides are also present.


Geographical extent

The Ashangi formation occurs widely in
Tigray Region The Tigray Region (or simply Tigray; officially the Tigray National Regional State) is the northernmost Regions of Ethiopia, regional state in Ethiopia. The Tigray Region is the homeland of the Tigrayan, Irob people, Irob and Kunama people. I ...
, including the May Ch'ew area and Dogu’a Tembien.


References

Geologic formations of Ethiopia Basalt formations Oligocene Africa Oligocene volcanism