Three phases of volcanic activity, older, middle, and younger, are represented on Moheli. The island was originally a large central-type shield volcano, made up of ancient lava flows, but it presently has an elongate form reflecting fissure eruptions of intermediate phase lavas.
Silica-undersaturated lava, alkalic basalt, basanite and melanephelinite, with lesser amounts of nephelinite, trachyte, and phonolite are the main rock types found on Moheli. Intrusive equivalents occur in the form of alkali gabbro, theralite, and ijolite. Inclusions of ultramafic rocks are common, and rare sandstone inclusions are also present.
Two separate petrochemical variation trends are proposed, namely a low-pressure trend from melanephelinite through nephelinite and trachyte to phonolite, caused by fractionation of olivine, clinopyroxene, and (later) feldspar. A high-pressure trend from alkali basalt through basanite to melanephelinite probably results from variable (low) degrees of partial melting of upper mantle material. A striking low potassium content and consequent low K:Rb ratios in the basanite are tentatively explained as resulting from fractionation of amphibole.