Composition of Magmas and Lavas The composition of magmas and lavas is controlled by the most abundant elements in the Earth-Si, Al, Fe, Ca, Mg, Na, K, H, and o. Three distinct types of magma are more common than others: Basaltic, containing about 50 percent SiO2 Andesitic, about 60 percent SiO2 Rhyolitic, about 70 percent SioComposition of Magmas and Lavas ◼ The composition of magmas and lavas is controlled by the most abundant elements in the Earth—Si, Al, Fe, Ca, Mg, Na, K, H, and O. ◼ Three distinct types of magma are more common than others: ◼ Basaltic, containing about 50 percent SiO2. ◼ Andesitic, about 60 percent SiO2. ◼ Rhyolitic, about 70 percent SiO2