flow dunes that crop out from underneath the tephra. Deposits from the 1600 eruption and previous events also crop out within the amphitheatre walls. Another southeastward-opening landslide scar lies just north of Huaynaputina. One of these funnel-shaped vents is a trough that cuts into the amphitheatre. The trough appears to be a remnant of a fissure vent. A second vent appears to have been about wide before the development of a third vent, which has mostly obscured the first two. The third vent is steep-walled, with a depth of ; it contains a pit that is wide, set within a small mound that is in part nested within the second vent. This third vent is surrounded by concentric faults. At least one of the vents has been described as an ash cone. A fourth vent lies on the southern slope of the composite volcano outside of the amphitheatre and has been described as a maar. It is about wide and deep and appears to have formed during a phreatomagmatic eruption. These vents lie at an elevation of about , making them among the highest vents of a Plinian eruption in the world. Slumps have buried parts of the amphitheatre. Dacitic dykes crop out within the amphitheatre and are aligned along a northwest–south trending lineament that the younger vents are also located on. These dykes and a dacitic lava dome of similar composition were formed before the 1600 eruption. Faults with recognizable scarps occur within the amphitheatre and have offset the younger vents; some of these faults existed before the 1600 eruption while others were activated during the event. Surroundings The terrain west of the volcano is a high plateau at an elevation of about ; north of Huaynaputina the volcano Ubinas and the depression of Laguna Salinas lie on the plateau, while the peaks Cerro El Volcán and Cerro Chen are situated south of it. The lava dome Cerro El Volcán and another small lava dome, Cerro Las Chilcas, lie south from Huaynaputina. Northeast-east of Huaynaputina, the terrain drops off steeply ( vertically and horizontally) into the Río Tambo valley, which rounds Huaynaputina east and south of the volcano. Some tributary valleys join the Río Tambo from Huaynaputina; clockwise from the east these are the Quebradas Huaynaputina, Quebrada Tortoral, Quebrada Aguas Blancas and Quebrada del Volcán. The Río Tambo eventually flows southwestward into the Pacific Ocean. Geology The oceanic Nazca