An aphyric, extrusive igneous rock of basaltic composition containing a high proportion of modal plagioclase, and an alumina content of more than 17 wt. %. Tholeiitic and calc-alkali basalts (see tholeiite; and calcalkaline) can have alumina contents of more than 17 wt. %, so the term does not refer to a specific basalt type. Rather, the variety of basalt should be specified, e.g. ‘high-alumina basalt of calc-alkali type’.
Subjects: Earth Sciences and Geography.