The massive Haleakala shield volcano forms the eastern portion of the dumbbell-shaped island of Maui. The summit contains a dramatic 3.5 x 9.5 km summit crater that is widely breached on the N and SE sides. The "crater" is not of volcanic origin, but formed as a result of the coalescence of headward erosion of the Koolau and Kaupo valleys. Subsequently the crater has been partially filled by a chain of young cinder cones and lava flows erupted along a major rift zone that extends across the basaltic volcano from the SW to the E flanks. Another less prominent rift zone trends north from the summit. 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).