With a diameter of 30 km at 2000 m elevation, 900 cu km Mount Wrangell is one of the world's largest continental-margin volcanoes. The massive andesitic shield volcano has produced fluid lava flows as long as 58 km and contains an ice-filled 4-6 km diameter caldera located within a 15-km-wide ancestral caldera. Most of he massive shield volcano was constructed during eruptions between about 600,000 and 200,000 years ago. Formation of the summit caldera followed sometime between about 200,000 and 50,000 years ago. Three post-caldera craters are located at the broad 4317-m-high summit of the volcano, along the northern and western rims of the 1-km-deep, ice-filled caldera. A steep-sided flank cinder cone, Mount Zanetti, is located 6 km NW of the summit. The westernmost cone has been the source of infrequent historical eruptions beginning in the 18th century. Increased heat flux in recent years has melted large volumes of ice in the northern crater.