Mount Erebus is classified as a polygenetic stratovolcano. The bottom half of the volcano is a shield and the top half is a stratocone. The composition of the current eruptive products of Erebus are anorthoclase-porphyritic tephritic phonolite and phonolite, which are the bulk of exposed lava flow on the volcano. The oldest eruptive products consist of relatively undifferentiated and nonviscous basanite lavas that form the low broad platform shield of Erebus. Slightly younger basanite and phonotephrite lavas crop out on Fang Ridge—an eroded remnant of an early Erebus volcano—and at other isolated locations on the flanks of Erebus. Erebus is the world's only presently erupting phonolite volcano.