Weeping Wall, Umnak Island

Weeping Wall, Umnak Island

by | Sep 16, 2021

The Weeping Wall is a cliff on the northwest side of Okmok Volcano on the northeastern part of Umnak Island, one of the Fox Islands in the Eastern Aleutians, about 73 miles (117 km) west-southwest of Dutch Harbor and 49 miles (79 km) north-northeast of Nikolski, Alaska. The descriptive name was given in 1939 by the U.S. Coast and Geodetic Survey for several cracks in the cliff rocks that emit spouts and waterfalls. Umnak Island is the third largest of the Aleutian Islands after Unimak and Unalaska Islands and is separated from Unalaska Island by Umnak Pass. The island is home to several stratovolcanoes and a large shield volcano. The stratovolcanoes of Umnak Island include Mount Vsevidof which is the highest peak on the island with an elevation of 7,051 feet (2,149 m) and Mount Recheshnoi with an elevation of 6,509 feet (1,984 m). The shield volcano is called Okmok Volcano and has a summit truncated by a caldera 5.8 miles (9.3 km) wide. The Unangan Aleut name for the volcano is Unmagim Anatuu. Mount Okmok is the highest point on the rim of the Okmok Caldera with an elevation of 3,519 feet (1,073 m). Several satellite cones and domes have formed on the sides of the caldera including Mount Tulik on the southeastern slope with an elevation of 4,111 feet (1,253 m), Mount Idak on the northeast slope with an elevation of 1,919 feet (585 m), and Jag Peak on the southwestern flank with an elevation of 2,880 feet (878 m).

On July 12, 2008, Okmok Caldera exploded without warning, sending a plume of ash to 50,000 feet (15,000 m) into the air. The eruption took place at a new vent on the northeast part of the caldera creating a cone about 800 feet (244 m) high. The eruption dramatically altering the caldera hydrology and discharged huge lahars, or volcanic mudflows, from the caldera to the coast. At least 17 eruptions of Okmok have been reported since 1805, and the 2008 eruption was by far the largest. Shield volcanos are formed by the eruption of low viscosity lava that travels farther and forms thinner flows than the more viscous lava erupted from stratovolcanos. Repeated eruptions result in the steady accumulation of broad sheets of lava, building up the distinctive low profile ‘shield’ shape. Shield volcanoes are found wherever highly fluid low-silica lava reaches the surface of the Earth, and are most characteristic of ocean island volcanism associated with hot spots or with continental rift volcanism. They are distinguished from the three other major volcanic types—stratovolcanoes, lava domes, and cinder cones—by their structural form, a consequence of their particular magmatic composition. Whereas stratovolcanoes and lava domes are the product of highly viscous flows, and cinder cones are constructed of explosively eruptive tephra, shield volcanoes are the product of gentle effusive eruptions of highly fluid lavas. Active shield volcanoes experience near-continuous eruptive activity over extremely long periods of time, resulting in the gradual build-up of edifices that can reach large dimensions. With the exclusion of flood basalts, mature shield volcanoes are the largest volcanic features on Earth. For example, Mauna Loa is the largest shield volcano in the world with an elevation of 13,678 feet (4,169 m), and a base over 60 miles (100 km) wide.

Basaltic rocks can be productive aquifers which are hydrogeologic formations that contain sufficient saturated permeable material to yield significant quantities of stored water. Lava flows extruded from Okmok Volcano formed a massive complex of interbedded tholeiitic basaltic rocks characterized by tension faulting caused by the cooling magma. Numerous basalt flows commonly overlap and the individual flows can be separated by layers of material that are permeable to water. Columnar joints also develop in the central parts of the basalt flows creating passages that allow water to move vertically through the basalt. The highly porous basalt of Okmok is called scoria, which formed when the magma cooled and released pressurized volcanic gases. This created highly vesiculated rock, and the porosity of the rock is determined by the size and number of vesicles and connections among vesicles. The porous rocks on Okmok Volcano create groundwater aquifers fed by rainfall and snowmelt. Typically the aquifer volume will peak during or shortly after a summer rain event when snowmelt is at a maximum. The Weeping Wall is a series of natural waterfalls created by the intersection of the cliff face and the groundwater aquifer. Read more here and here. Explore more of Okmok Volcano and Umnak Island here:

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