Like all volcanic rocks, the Alaji Basalts originate from initial melting of the Earth's mantle. After extrusion, the magmatic structures form at the surface. Common volcanic structures such as lava tubes or ropy lavas are absent in the Alaji Basalts, but (columnar joints) are omnipresent. The basalts comprise successive flows. During cooling, newly developed crystals within the lava solidify and develop congealing stress that favours the formation of columnar joints (intersecting fractures). They are perpendicular to the surface of the lava flow: mostly vertical, but sometimes also inclined or almost horizontal.[4][5]
Lithology
[edit]The Bi’en ridge near Hagere Selam in the Tigray Region is composed of Alaji Basalts. Weathering of the rock has led to the occurrence of fertile Luvisols
The Alaji Basalts hold alternating layers of rhyolitic and basaltic rocks. Its thickness varies between 80 and 220 metres. The upper layer is capped by trachytic materials. The lower part of the Alaji Basalts is composed of medium- to fine-grained basalts. The middle succession holds elongated plagioclase-dominated trachytes. The mineralogy of the basaltic rocks is marked by well-aligned elongated pyroxenes and micro-plagioclases. In the mineralogy of the trachytes one notices especially the by twinning of feldspar minerals and a small amount of altered pyroxenes.[4]
^Mohr, P.; Zanettin, B. (1988). The Ethiopian flood basalt province. In: Continental flood basalts. Dordrecht: Springer. pp. 63–110. doi:10.1007/978-94-015-7805-9_3.
^Hofmann, C., and colleagues (1997). "Timing of the Ethiopian flood basalt event and implications for plume birth and environmental change". Nature. 389: 838–841. doi:10.1038/39853. S2CID205027259.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^Pik, R., and colleagues (1998). "The northwestern Ethiopian Plateau flood basalts: classification and spatial distribution of magma types". J Volcanol Geotherm Res. 81 (1–2): 91–111. Bibcode:1998JVGR...81...91P. doi:10.1016/S0377-0273(97)00073-5.{{cite journal}}: CS1 maint: multiple names: authors list (link)
Ethiopia
