Bayuda Northern Volcanic Field

Within an area of approximately 10x16 km are concentrated more than 100 deeply eroded volcanic plugs with a number of feeder dykes. Many of the plugs stand proud as inselbergs up to 180 m high, but others are eroded down to basement level. The columnar joint patterns seem to indicate that some of the plugs did not reach the surface but ended as domes within Nubian Sandstone. The most abundant rocks are tephrites, basanites and phonotephrites in which phenocrysts of olivine, pyroxene, amphibole and occasionally nepheline are set in a matrix of plagioclase, pyroxene and abundant opaque minerals. Three small plugs to the east of Jebel Barget Amr consist of dense green latites of zoned plagioclase, abundant aegirine-augite and opaques. Jebel Barget Amr and Jebel Umm Sheriba, both of which form high inselbergs, are phonolite plugs with spectacular columnar jointing. The rock includes aegirine, K-feldspar and opaques in a fine-grained matrix; nepheline has been identified by X-ray diffraction. The volcano of Jebel Dahamil Atella is situated some 10 km northwest of the field along the intersection of a series of faults that have preserved Nubian Sandstone in a downfaulted block. The volcano is deeply eroded and consists of nephelinites which were erupted from three centres in the form of pahoehoe lavas and pyroclastics. The nephelinite consists of a matrix of nepheline, a little plagioclase, andesine, titanaugite and opaques with small phenocrysts of olivine and titanaugite.