Ma’Alalta (Pierre Pruvost)

The earliest phase in the Ma’Alalta volcano consists of basaltic lava flows that emanated from fissures. These were followed by emplacement of three ignimbrite units the first two of which, both 4 m thick, are overlain by pumice horizons. Before eruption of the third ignimbrite basaltic activity resumed with the building of scoriaceous cones. The third phase was the building of the main strato-volcano which involved eruption of obsidian and trachytic flows, pumice and other pyroclastic layers and domes, all of which are exposed in the walls of the large summit caldera. Final activity involved the building of rhyolitic lava domes and outpouring of obsidian flows along fissures south of the strato-volcano; there are some active fumaroles amongst these rocks. Many of the rhyolitic flows are crystal free but the ignimbrites exposed in the caldera walls are rich in crystals of sanidine, perthite, rare oligoclase-andesine, often rimmed by sanidine with quartz in some rocks. A slightly sodic pyroxene and fayalite are usual while sodic amphibole, aenigmatite and biotite are less plentiful. An ignimbrite on the southeastern margin of the volcano contains aegirine-augite, katophorite, aenigmatite and biotite. Ten whole rock analyses are available in Barberi et al. (1970), who also give Sr isotope data for a basalt, a trachyte, an ignimbrite and two pantellerites. Evolution of the rhyolitic rocks is discussed by Barberi et al. (1975a).