The massive Haleakala shield volcano forms the eastern portion of the island of Maui. The summit is at the NW end of a volcanic ridge and at the head of a large scarp that opens to the ENE, interrupting the ridge which continues 7 km across the valley, formed as a result of the coalescence of headward erosion of the Koolau and Kaupo valleys. Subsequently the valley has been partially filled by a chain of young cinder cones and lava flows erupted along the rift zone that extends across the basaltic volcano from the SW to the E flanks. The most recent eruption was thought to have occurred between the exploring voyages of La Perouse in 1786 and Vancouver in 1793, but uncertainty surrounds the date of this event, which could have occurred in about 1750 CE (anthropological evidence) or several centuries earlier (radiocarbon dates).