Semisopochnoi Island or Unyak Island (obsolete , modern Semisopochny "having seven hills"; ) is part of the

Rat Islands The Rat Islands (,) are a group of American High islands, volcanic islands in the Aleutian Islands in southwestern Alaska, between Buldir Island and the Near Islands group to its west, and Amchitka Pass and the Andreanof Islands group to its e ...

group in the western

Aleutian Islands The Aleutian Islands ( ; ; , "land of the Aleuts"; possibly from the Chukchi language, Chukchi ''aliat'', or "island")—also called the Aleut Islands, Aleutic Islands, or, before Alaska Purchase, 1867, the Catherine Archipelago—are a chain ...

Alaska Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...

. The island is uninhabited and provides an important nesting area for maritime birds. The island is of volcanic origin, containing several volcanoes including Mount Young. It has a land area of 85.558 square miles (221.59 square km), measuring 11 miles (18 km) in length and 12 miles (20 km) in width. At 179°46' East (+179.7667) (in the

Eastern Hemisphere The Eastern Hemisphere is the half of the planet Earth which is east of the prime meridian (which crosses Greenwich, London, United Kingdom) and west of the antimeridian (which crosses the Pacific Ocean and relatively little land from pole to p ...

), the easternmost tip of Semisopochnoi is, by longitude, the easternmost land location in the United States and

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

. Semisopochnoi is located 14 minutes (0.2333 degrees) or west of the

180th meridian The 180th meridian or antimeridian is the meridian (geography), meridian 180° both east and west of the prime meridian in a Geographic coordinate system, geographical coordinate system. The longitude at this line can be given as either east ...

Wildlife

Semisopochnoi has no known native land mammals. Arctic foxes were introduced to the island during the 19th century for fur farming and removed in 1997. Most ground-nesting bird species ( Aleutian cackling goose,

rock ptarmigan The rock ptarmigan (''Lagopus muta'') is a medium-sized game bird in the grouse family. It is known simply as the ptarmigan in Europe. It is the official bird for the Canadian territory of Nunavut, where it is known as the ''aqiggiq'' (ᐊᕿ� ...

) and most burrow-nesting seabirds ( storm-petrels,

ancient murrelet The ancient murrelet (') is a bird in the auk family. The English term "murrelet" is a diminutive of "murre", a word of uncertain origins, but which may imitate the call of the common guillemot. Ancient murrelets are called "ancient" because t ...

s, Cassin's auklets, tufted puffins) were

extirpated Local extinction, also extirpation, is the termination of a species (or other taxon) in a chosen geographic area of study, though it still exists elsewhere. Local extinctions are contrasted with global extinctions. Local extinctions mark a chan ...

by foxes; Semisopochnoi Island is currently in the early stages of recovery. The island has remained free of Norway rats. The large least and crested auklet colony near Sugarloaf Head is one of the largest among the nine auklet colonies in the Aleutian Islands. Crested auklets and least auklets breed in one colony, located on the south facing slopes of Sugarloaf Peak volcano and its associated cinder cones near Sugarloaf Head. Semisopochnoi Island also supports a significant population of

red-faced cormorant The red-faced cormorant (''Urile urile''), red-faced shag or violet shag, is a bird species of the family Phalacrocoracidae. Its range spans from the eastern tip of Hokkaidō in Japan, northern korean peninsula, via the Kuril Islands, the sout ...

s. In total, Semisopochnoi supports well over a million seabirds, most of which are least auklets or crested auklets.

Geology

Semisopochnoi, the largest

subaerial In natural science, subaerial (literally "under the air") has been used since 1833,Subaerial
in the Merriam- ...

volcano A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most oft ...

of the western Aleutians, is 20 km wide at sea level and contains a caldera 8 km wide that formed as a result of the collapse of a low-angle, dominantly basaltic volcano following the eruption of a large volume of dacitic

pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of extremely vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicula ...

. The high point of the island is 1,221 meter Anvil Peak, a double-peaked cone that forms much of the island's northern part. The three-peaked, 774-meter high Mount Young (formerly named Mount Cerberus) is a volcano within the caldera. Each of the peaks contains a summit crater; lava flows on the northern flank of Mt. Young appear more recent than those on the southern side. Other post-caldera volcanoes include the symmetrical 855 m high Sugarloaf Peak south-southeast of the caldera and Lakeshore Cone, a small cinder cone at the edge of Fenner Lake in the northeast part of the caldera. Most documented historical eruptions have originated from Mt. Young, although Coats (1950) considered that both Sugarloaf and Lakeshore Cone within the caldera could have been active during historical time. Semisopochnoi's last known volcanic eruption took place from February 2021 to May 2023. A historic eruption of Semisopochnoi was reported in 1873, and at least four others may have occurred in the previous hundred years, but documentation is scant. These eruptions apparently emanated from the flanks of Mount Young; the most recent flow appears to be less than a century old.

Easternmost North America location debate

Semisopochnoi's position in the Aleutian Island Chain gives it the distinction of being the Aleutian island most immediately West of the 180th meridian, giving it longitudinal coordinates in the Eastern Hemisphere. Therefore, some have argued it is technically the easternmost location in North America. This is a counterintuitive argument considering its location near the extreme western boundary of the United States and depends on a series of assumptions about the definition of boundaries between hemispheres, boundaries between continents, as well as whether island locations should even be considered part of a continent. The other contenders for the title of easternmost North American location are Nordost Rundingen,

Greenland Greenland is an autonomous territory in the Danish Realm, Kingdom of Denmark. It is by far the largest geographically of three constituent parts of the kingdom; the other two are metropolitan Denmark and the Faroe Islands. Citizens of Greenlan ...

at 12°08' West (-12.1333) and Cape Spear,

Newfoundland Newfoundland and Labrador is the easternmost province of Canada, in the country's Atlantic region. The province comprises the island of Newfoundland and the continental region of Labrador, having a total size of . As of 2025 the population ...

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

at 52°37' West (-52.6167) —which, along with Semisopochnoi, are all island locations. Cape St. Charles at 55° 37' West (-55.6167) on the

Labrador Peninsula The Labrador Peninsula, also called Quebec-Labrador Peninsula, is a large peninsula in eastern Canada. It is bounded by Hudson Bay to the west, the Hudson Strait to the north, the Labrador Sea to the east, Strait of Belle Isle and the Gulf of ...

is the easternmost point of mainland North America. Cape Spear is the easternmost location relative to the geographic center of North America if Greenland is excluded because it is not politically part of North America. For this and other reasons, Cape Spear is traditionally considered to be the easternmost location in North America.

Renaming of Mount Cerberus

On December 12, 2022, a Congressional bill authored by Senator

Lisa Murkowski Lisa Ann Murkowski ( ; born May 22, 1957) is an American attorney and politician serving as the Seniority in the United States Senate, senior United States senator from the state of Alaska, having held the seat since 2002. She is the first woman ...

was passed by the 117th Congress renaming Mount Cerberus " Mount Young" in honor of long-serving Alaska U.S. Representative

Don Young Donald Edwin Young (June 9, 1933 – March 18, 2022) was an American politician from Alaska. He is the List of members of the United States Congress by longevity of service, longest-serving Republican Party (United States), Republican in House ...

, who died on March 18, 2022.

References

External links

Semisopochnoi Photos
Photographs from Semisopochnoi Island, July 2008
Google Earth view
{{authority control Rat Islands Volcanoes of the Aleutian Islands Stratovolcanoes of the United States Uninhabited islands of Alaska Calderas of Alaska Cinder cones of the United States Islands of Alaska Islands of Unorganized Borough, Alaska Volcanoes of Unorganized Borough, Alaska Landforms of Aleutians West Census Area, Alaska VEI-7 volcanoes