Museum

TUKUL 2: Geology and Volcanology

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Different Types of Lava

The raw materials utilized by prehistoric man at Melka Kunture are essentially volcanic rocks. Stone tools have been especially obtained by rocks chosen for their good lithological quality and for their resistance during utilization. Generally speaking, these rocks must be compact and homogeneous, hard and poorly porous, vitreous or with a fine texture. Good rocks with these qualities are the obsidian from Balchit, some obsidian facies, basaltic and trachytic lavas, while porous lava, too much soft, has not been generally utilized. Many of these lavas are covered by a more or less thick peripheral alteration cortex (patina) which complicates or sometimes avoids their petrologic identification.

 

Basalts

 

Aphiric basalts do not exhibit visible crystals. They are constituted by a compact texture formed by very fine minerals (microlites), plagioclase, augite and olivine, and by glass giving them a dark grey-blue colour. They rarely have more than 50% of SiO2, which allows them to be fairly fluid at the eruption temperature. Their hardness makes them difficult to be worked, but they have a good resistance to choc and to use. 

Feebly porphyric basalts have some visible (1 to 3 mm) crystals (olivine  and augite) contained within a microlithic or vitreous groundmass. 

Porphyric basalts have up to 1 cm large crystals (phenocrystals), equally contained within a microlithic or vitreous groundmass. The facility to be cut and the hardness of these two facies derive by their compact and fine grained texture containing the crystals.

The microdoleritic basalts are formed by a fine grained and grey texture rich in plagioclase, the arrangements of which contain the augite and olivine microlites. Some basalts with a larger doleritic texture, with several interstitial wides, outcrop around Melka Kunture. These lavas have not been utilized in Prehistoric times because of their porosity and fragility. Their flows are easily visible in the country because the erosion shaped their surface as large blocks spreading, for example, in the area some kilometres North from Awash. 

Trachy-basalts have rare olivine crystals and they contain phenocrystals of feldspars (plagioclase or alkaline feldspar such as the anorthose) within a texture of basaltic composition whose compactness determines the rock hardness. 


Porphyric basalt

Aphiric basalt

Porphyric doleritic basalt

 

Differentiated Rocks

 

Trachy-andesites are similar to trachy-basalts but they are generally more bright and less hard. They have phenocrystals of alkaline feldspars (sanidine or anorthose). Due to their chemical composition, with a more important quantity of SiO2, they are close to trachyte. 

Aphyric fluidal lava, associated to the limit-fault (South/South East) of Melka Kunture basin, has been distinguished for its abundance. Its grey to blue facies resembles the benmoreite (similar to alkaline trachytes with for example 62/65% of SiO2). Notwithstanding its peculiar fluidal texture it can be very compact but it often appears with vesicles oriented in the sense of fluidality. 

Aphyric and subaphiric differentiated lavas are trachytes or rhyolites (60/70% of SiO2) composed by a compact grey, green or yellow fine grained texture, more or less bright, with variable porosity sometimes with some small crystals. At the time of eruption they were fairly viscous. 

Porphyric differenciated lavas are generally brighter and less compact than the preceding ones. They usually have some phenocrystals of alkaline fel


Porphyric differenciated lava

Aphiric fluidal trachytic lava

 

Ignimbrite

 

The welded ignimbrite regionally extended (possibly several hundreds of square kilometres) is the more remarkable formation of this type in the area of Melka Kunture. Issued in the course of an enormous eruption, it moved like an aerosol before to leave its lava elements which have been welded when gas energy diminished. This porous and fragile rock has been utilized as pebbles, fragments or blocks, certainly because of its large diffusion around Melka.  


Welded ignimbrite

Other types of welded ignimbrite with various facies and lithological characters are present in the Melka Kunture area, some of them could have been utilized for making artefacts because of their good compactness and hardness.

Obsidian

 

Balchit obsidian, largely utilized during the entire history of Melka Kunture, comes from a complex volcanic system located 7 km North from Melka. It is a real obsidian sensu strictu, that means a rhyolitic rock not crystallized while setting. This rock has very often a black colour  with a very fine fluidity. However some facies, affected by pneumatolithic actions or by oxidations, have a green or red colour. This natural glass, easy to break,  is very compact and without porosity and it was used for different kinds of tools.


Obsidian

Obsidianic lavas may be distinguished from the real obsidian such as the one from Balchit. They have been produced by a quick cooling; this is the case, for example, of the base of some welded ignimbrites. But, depending on the features of the considered area, especially the thickness reached by the cooling and the presence or absence of water, the vitrification phenomenon is more or less important and the rock more or less compact and useful for lithic tools production.  

Occasional Rocks

 

The syenitic inclusions are the only grained rocks found amongst the lithic tools of Melka Kunture sites. They belong to some levels of a magma chamber where already crystallized minerals have been accumulated before being brought up, dislocated and thrown out the volcano in occasion of a great eruption which expelled in the same time the magma encumbering the chamber.  They are especially constituted by alkaline feldspars and they document the trachytic, phonolithic or rhyolitic composition of the magma.  Their coarse grained texture avoided their utilization for an accurate  lithic technology. 

The flint and opale, usually classed as sedimentary rocks, are certainly results of volcanism, because, in the context of the Melka Kunture Basin surrounded by volcanoes, they must correspond to precipitations of amorphic silica from hydrothermal circulations directly linked to volcanism. Their compact texture could allow their utilization for lithic production, but their generally small dimensions could only be utilized for small tools.

 

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