Glacier-clad Chimborazo, Ecuador's highest volcano, anchors the southern end of the country's "Avenue of Volcanoes" 30 km NW of the city of Riobamba. The dominantly andesitic-to-dacitic structure is mostly of Pliocene-to-Pleistocene age. The volcano collapsed about 35,000 years ago, producing a major debris avalanche, whose deposits underlie Riobamba and temporarily dammed the Río Chambo, producing an ephemeral lake. Subsequent mainly been andesitic activity constructed three edifices along an E-W line, the youngest and westernmost of which forms the current summit. Although activity was at one time thought to have ceased during the very latest Pleistocene, recent work indicates that it erupted more than a half dozen times during the Holocene, producing pyroclastic surges that reached down to 3,800 m elevation.