The phonolite or

clinkstone Phonolite is an uncommon extrusive rock, of intermediate chemical composition between felsic and mafic, with texture ranging from aphanitic (fine-grained) to porphyritic (mixed fine- and coarse-grained). Phonolite is a variation of the igneous ...

of northern

Ethiopia Ethiopia, , om, Itiyoophiyaa, so, Itoobiya, ti, ኢትዮጵያ, Ítiyop'iya, aa, Itiyoppiya officially the Federal Democratic Republic of Ethiopia, is a landlocked country in the Horn of Africa. It shares borders with Eritrea to the ...

is a shallow-seated

igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma o ...

. The phonolites intruded and punctured the

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

sedimentary rocks and the

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 some 19 to 11 million years ago.

Stratigraphic context

The formation occurs as plugs that have punctured the Tertiary

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 reac ...

s of the Ethiopian plateau, and form the highest peaks in their surrounding.

Lithology

These phonolites are composed of

alkali pyroxene In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a s ...

s and

nepheline Nepheline, also called nephelite (), is a rock-forming mineral in the feldspathoid groupa silica-undersaturated aluminosilicate, Na3 K Al4 Si4 O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatite ...

s; there are also elongated

plagioclase Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...

crystals. Pyroxenes have a skeletal texture, whereas nepheline crystals are hexagonal to rectangular. The phonolites of pink to light grey colour. The outcrops hold massive (up to 25 metres long), moderately weathered blocks, generally of elongated shape; in some locations it also forms spherical outcrops. Rock sample Phonolite Addi Amyuq

Geographical extent

These silica-poor rocks belong to the Adwa-

Aksum Axum, or Aksum (pronounced: ), is a town in the Tigray Region of Ethiopia with a population of 66,900 residents (as of 2015). It is the site of the historic capital of the Aksumite Empire, a naval and trading power that ruled the whole regio ...

trachy-phonolite volcanic field. They are also exposed in Addi Amyuq, some 3 kilometres north of Hagere Selam, on the northern slope of the ridge. The morphology of the Addi Amyuq phonolite outcrop is similar to that of Gobo Dura where the Aksum obelisks were quarried. Phonolite_dimension_stones_Khunale

Obelisks and church bells

The phonolites are among the best exploited rocks of northern Ethiopia. Notably, they were used to carve the monolithic obelisks and other monumental stones during the

Aksumite kingdom The Kingdom of Aksum ( gez, መንግሥተ አክሱም, ), also known as the Kingdom of Axum or the Aksumite Empire, was a kingdom centered in Northeast Africa and South Arabia from Classical antiquity to the Middle Ages. Based primarily in wha ...

. In Tembien they are used as church bells (in line with the name

) and dimension stones.

References

{{Reflist Geologic formations of Ethiopia