Museum

TUKUL 2: Geology and Volcanology

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Balchit

Balchit locality

In the vicinity of Melka Kunture, obsidian is a major component of lithic series since the Oldowan. The nearby Balchit volcanic massif constitutes a major obsidian source area.

Since 1999, new investigations have been undertaken on the prehistory and volcano-sedimentary environments of Melka Kunture and a special attention was paid to obsidian artefacts and its primary and secondary sources. Analyses were performed on several obsidian samples from various locations, both from a lava of Balchit and reworked debris or pebbles from different alluvial formations of the Awash River and its tributaries.


Location of Balchit volcanic complex

Obsidian occurrences

The only recognized source of obsidian is located near Balchit on what seems to be a flat flow dome which offers a few kilometres square outcrop with a wide variety of eruptive facies (vertical and convolute fluidal structures, finely banded perlitic lava, spherulitic facies, massive obsidian, etc). Remarkuable amygdals up to metric lenses of pure and massive obsidian are scattered among the various lava facies. The obsidian colour is dominantly black but locally blue, green, red and beige colours have been observed. It corresponds  to an obsidian sensu strictu, that is a lava which was not able to crystallize during its eruption. It is different from other obsidian rocks derived from a quick cooling, such as the base of the nearby ignimbrites. In fact there are no arguments to thinks that we are dealing with an obsidian flow dome because it appears in situ only in peripheral banded and deformed facies found in some excavations to a deepness of 1 to 2 m in the central part of the Jimjima plateau. The entire structure could have been transformed by a pumice formation process and devitrification into a bright and fluidal lava where subsisted amygdals veins and blocks of this obsidian. As products of erosion, blocks, cobbles, pebbles and gravels are found in quaternary alluviums and in minor river beds and form secondary sources which were available for prehistoric groups.

Geochemestry


View from the top towards
Balchit village with Wechecha
 

The trace element contents of two Balchit samples from the main obsidian bearing lava flow and of nine obsidian pebbles from alluvial deposits were analysed.

Six obsidians pebbles were found to present the same elemental composition as the two Balchit obsidians and therefore might come from this mother-rock.

But the results do not allow to exclude that obsidians from other sources could have supplied the archaeological sites of the Melka Kunture area.

Considerations


Obsidian outcrop at Balchit
 

Preliminary analysis show that at least four geochemically different sources could have been exploited by Ancient Man. One of these sources could be the Balchit main lava flow or one of its proximal concentrations of erosion products.

The field observations indicate that the large size of Simbiro Acheulean obsidian artefacts excludes a raw material procurement from the local alluvial deposits. It must be remember that Simbiro is more than seven Km far from Balchit, which gives an idea of the wide diffusion of this raw material.

 


Unweathered massive block of obsidian

Convolute structure

Vertical facies


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