Unga Island Petrified Forest, Shumagin Islands

Unga Island Petrified Forest, Shumagin Islands

by | Jun 1, 2023

Unga Island is part of the Shumagin Islands situated south of the Alaska Peninsula in the Gulf of Alaska, and the site of a petrified forest exposed along the eroding northern shore, about 64 miles (103 km) northeast of King Cove and 11 miles (18 km) northwest of Sand Point, Alaska. The name is derived from the Unangan Aleut language and was first reported as ‘Ounga’ in 1827 by Admiral Adam J. von Krusenstern of the Imperil Russian Navy, and in 1840 as ‘Unga’ by Ivan P.Veniaminov, a priest with the Russian Orthodox Church. Unga Island is the largest of 20 islands in the Shumagin archipelago. The other large islands include Popof, Korovin, and Nagai islands. Smaller islands include Andronica, Big Koniuji, Little Koniuji, Simeonof, Chernabura, and Bird. The archipelago was discovered during the Great Northern Expedition led by Vitus Bering in 1741 on Saint Peter. The expedition stopped to fill barrels with water from a brackish pool near the beach. Georg W. Steller, as the expedition physician and naturalist, knew that this water was unsuitable and protested strongly against taking it aboard, warning that its use would lead to a rapid increase of scurvy. Ten men were soon sick with scurvy and taken ashore for rest and fresh air, but Nikita Shumagin died soon after landing and was buried on Nagai Island. In his memory the island was named Shumagin, a name now applied to the entire archipelago.

The Alaska Peninsula and the Shumagin Islands are part of the Aleutian Range volcanic belt that extends 1,000 miles (1610 km) from the west side of the Cook Inlet to the western end of the Aleutian Island chain. Unga Island formed as a result of volcanic activity, although there are no active volcanoes there at present. The eastern half of the island is mountainous and consists largely of andesitic and dacitic lava flows. Fragmental rocks resulting from explosive volcanic activity are common and include coarse volcanic breccia and fine-grained tuffs. The western half of the island is low, with rolling hills and grassland covering rocks of the Unga Conglomerate, a sedimentary rock formation several hundred feet thick that buries the volcanic rocks. The deeper part of the formation consists of thin layers of sandstone, conglomerate, shale, and lignite that developed during the Miocene, or from 25 to 11 million years ago. The upper portion consists of coarse fragments of volcanic breccia with embedded petrified trees. The topography of the entire island has been affected by glaciers which scoured the surface during the last ice age. Unga Island has risen about 100 feet (31 m) since the Pleistocene and the lower, flatter parts of the island are old sea terraces that formed when sea levels were lower and the outer coast of the Shumagin Islands was the south shore of the Alaska Peninsula. Archaeological investigations and radiocarbon dates of artifacts collected from these ancient terraces suggest that Aleut people came from the south and arrived about 4,000 years ago. Today, almost the entire population of the Shumagin Islands lives in the city of Sand Point on Popof Island.

Petrified logs and stumps are exposed mostly in the lower intertidal and subtidal along about 5 miles (8 km) of the northern shore of Unga Island between Unga Spit to the east and Bay Point to the west. The stumps range in diameter from 2 feet (0.6 m) to about 9 feet (2.7 m) and the longest log is 59 feet (18 m). These are remnants of a Sequoia or Metasequoia forest buried by a sudden deposition of volcanic breccia derived from nearby volcanic eruptions during the early Cenozoic, or about 65 to 2 million years ago. The trees were subsequently preserved by permineralization, where groundwater dissolves silica from volcanic rocks which then fills the pore spaces in the wood and the silica is remineralized. Silicification also replaces the organic cellular tissue so thoroughly that many silicified trees consist of close to 100 percent silica, preserving the smallest and most delicate structural details of the wood cells. Any substrate that is able to bring elevated silica into solution is also capable of silicifying plant material. This can be a volcanic tephra rich in glassy material or a silica-rich hydrothermal solution from a hot spring. Plant tissue apparently has the ability to attract silica from aqueous solutions and precipitate the mineral in the cells as opal and chalcedony. Without this ability, wood and all other plant tissues would rapidly decompose in an oxygenated environment and this may be the single most important process involved in the preservation of plants in the fossil record which allows insight into past ecological and climatic conditions on ancient land surfaces. The Unga Island petrified forest is part of the Alaska Maritime National Wildlife Refuge and had been designated as a National Natural Landmark. Read more here and here. Explore more of Unga Strait and the Shumagin Islands here:

About the background graphic

This ‘warming stripe’ graphic is a visual representation of the change in global temperature from 1850 (top) to 2022 (bottom). Each stripe represents the average global temperature for one year. The average temperature from 1971-2000 is set as the boundary between blue and red. The color scale goes from -0.7°C to +0.7°C. The data are from the UK Met Office HadCRUT4.6 dataset. 

Credit: Professor Ed Hawkins (University of Reading). Click here for more information about the #warmingstripes.

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