Ubinas 1.55

Ubinas is an active 5672m stratovolcano in the Andes of southern Peru. Until 2006, this stratovolcano had not erupted for about 40 years.DescriptionUbinas is located in southern Peru. The upper slopes of the volcano, composed primarily of Pleistocene andesitic lava flows, steepen to nearly 45 degrees. The steep-walled, 1.4km wide and 150m deep summit caldera contains an ash cone with a 500m-wide funnel-shaped vent that is 200m deep. Debris-avalanche deposits from the collapse of the SE flank of Ubinas extend 10km from the volcano.The volcano is a composite cone built on a high plateau formed from Oligocene-Miocene (Neogene) ignimbrites and intrusive rocks. Ubinas comprises two edifices that divide two major periods of its eruptive behaviour. The first is the lower volcano under 600m high, known as Ubinas 1 (eruptive period >376 ka), which collapsed and formed a debris-avalanche deposit as far as 12km downstream Rio Ubinas. This collapse was followed by an eruption of non-welded ignimbrites, ponding to a thickness of 150m over 5km from the summit, and in turn was covered by a 100m thick ash/pumice deposit. Ubinas 1 is overlain by a steeper cone more than 900m high, known as Ubinas 2 (eruptive period <376 ka). Ubinas 2 formed the summit caldera, with walls up to 300m high that consist of hydrothermally altered lava flows and floored by a 20m thick pile of lapilli and ash-fall layer. This morphology was created by frequent eruptions in the Late Holocene from the younger inner crater. The inner crater is under 200m high and shows pervasive hydrothermal alteration and abundant fractures. Countless explosive events have taken place inside the summit caldera during the last 9.7 ky, and it has been suggested by Thouret et al. that the caldera be considered gravitationally unstable.