The northern basalt lava field of the Alid volcano (No. 3) is limited at its northern margin by the products of the silicic volcano of Jalua. This is a strato-volcano with a large central volcanotectonic depression open to the sea. The silicic rocks of the volcano are apparently peralkaline.
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Having only comparatively recently gained independence most accounts of Eritrean geology will be found under 'Ethiopia'. Thus, Mohr's "The geology of Ethiopia" (1962) covers Eritrea and the most useful geology map is probably the 'Geological map of Ethiopia and Somalia' on a scale of 1:2,000,000 with its accompanying general description (Merla et al., 1979). Most of the country is underlain by Precambrian metamorphic rocks but in the southeast it extends into the Afar Depression so that the volcanoes of Jalua and Alid, lying at the northern end of the Danakil Depression, comprise a small part of the huge Afar igneous province which is mainly in Ethiopia and Djibuti.
The Alid volcano lies at the northern end of the Danakil Depression. Three units are distinguished by Barberi et al. (1973): a central silicic complex and northern and southern basaltic lava fields.
This is an extensive range extending from the northern flanks of Bidu (No. 5) to the Red Sea coast. It consists predominantly of basaltic flows, eruptions being recorded for 1961 and 1963, which emanated from fissures.
The three large volcanoes of Nabro, Mallahle (Mallali) and Asavyo which constitute the Bidu massif are aligned along a north-northeasterly direction on the Danakil horst and extend southwards from Eritrea into Ethiopia. Details of this occurrence will be found under Ethiopia (No. 5).
A petrographic description of a tinguaite dyke is given by Manasse (1909), but few other details are available. It is a very fine-grained rock with a fluidal texture and comprises alkali feldspar, green pyroxene, probable lavenite, nepheline, magnetite, apatite and possible sodalite.