Glacier-clad, 6310-m-high 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 Chimborazo volcano 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 eruptions have been dominantly andesitic and constructed three edifices along an east-west line, the youngest and westernmost of which forms the current summit of Chimborazo. Although activity was at one time thought to have ceased during the very latest Pleistocene, recent work indicates that Chimborazo erupted more than a half dozen times during the Holocene, producing pyroclastic surges that reached down to 3800 m elevation.