Sangay - Geology

Geology

Sangay developed in three distinct phases. Its oldest edifice, formed between 500,000 and 250,000 years ago, is evidenced today by a wide scattering of material opening to the east, defined by a crest about 4,000 m (13,120 ft) high. This first Sangay, pockmarked by secondary ridges, is thought to have been 15–16 km (9–10 mi) in diameter, with a summit located 2 to 3 km (1 to 2 mi) southeast of the present summit. The curved shape of the remnants of this first structure indicates that it suffered a massive flank collapse, scattering the nearby forest lowlands with debris and causing a large part of its southern caldera wall to slide off the mountain, forming an embayment lower on its slopes. This 400 m (1,312 ft) thick block, the best preserved specimen of Sangay's early construction, consists of sequencely layered breccias, pyroclastic flows, and lahar deposits. Acidic andesites with just under 60% silicon dioxide dominate these flows, but more basic andesites can be found as well.

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Sangay's second edifice began to form anew after the massive sector collapse that damaged the first, being constructed between 100,000 and 50,000 years ago. Remnants of its second structure lie within the southern and eastern parts of the debris from its first collapse; some remnants of the volcano lie to the west and north as well. Sangay's second structure is believed to have had an east-to-west elongated summit, and like its first summit structure, it suffered a catastrophic collapse that created a debris avalanche 5 km (3 mi) wide and up to 20 km (12 mi) in length. It was likely less voluminous than the volcano's first version, and its summit lay near Sangay's current one.

Sangay currently forms an almost perfect glacier-capped cone 5,230 m (17,159 ft) high, with a 35° slope and a slight northeast-southwest tilt. Its eastern flank marks the edge of the Amazon Rainforest, and its western flank is a flat plain of volcanic ash, sculpted into steep gorges up to 600 m (1,970 ft) deep by heavy rainfall. It has a west-east trending summit ridge, capped by three active craters and a lava dome. Sangay has been active in its current form for at least 14,000 years, and is still filling out the area left bare by its earlier incarnations, being smaller than either of them. Uniquely, in its 500,000 years of activity, its magma plume has never changed composition or moved a significant distance.

Mainly andesitic in composition, Sangay is highly active. The earliest report of a historical eruption was in 1628; ash fell as far away as Riobamba, located 50 km (31 mi) northwest of Sangay, and was severe enough to cover pastures and starve local livestock. The volcano erupted again in 1728, remaining essentially continuously active through 1916, with particularly heavy activity in 1738–1744, 1842–1843, 1849, 1854–1859, 1867–1874, 1872, and 1903. After a brief pause, it erupted again on August 8, 1934, and has not completely quelled ever since, with heavy eruptive periods occurring in 1934–1937 and 1941–1942.

Eruptions at Sangay exhibit strombolian activity, producing ashfall, lava flows, pyroclastic flows, and lahars. All known eruptions at the volcano have had a Volcanic Explosivity Index (VEI) of 3. Despite its activity, Sangay is located in a remote, uninhabited region; only a large Plinian eruption could threaten occupied areas 30–100 km (19–62 mi) to its west. Nonetheless, a flank collapse on its eastern side, possible given the volcano's construction and history, could displace nearby forest and possibly affect settlements. Access to the volcano is difficult, as its current eruptive states constantly peppers the massif with molten rock and other ejecta. For these reasons, it is not nearly as well-studied as other, similarly active volcanoes in the Andes and elsewhere; the first detailed study of the volcano was not published until 1999.