The Thumb (Omineca) - AbsoluteAstronomy.com

The Thumb is a mountain

Mountain

Image:Himalaya_annotated.jpg|thumb|right|The Himalayan mountain range with Mount Everestrect 58 14 160 49 Chomo Lonzorect 200 28 335 52 Makalurect 378 24 566 45 Mount Everestrect 188 581 920 656 Tibetan Plateaurect 250 406 340 427 Rong River...

located 7 km (4 mi) south of Sitchiada Mountain on the east side of Bear Lake

Bear Lake (Bear River)

Bear Lake is a large lake in the northwestern Omineca Country of the North-Central Interior of British Columbia, Canada, located north of Babine and Takla Lakes. Unlike those two lakes, which are part of the Fraser River drainage, Bear Lake is tributary to the Skeena River via the Bear River,...

, on the divide between the upper Omineca River

Omineca River

Omineca River is a river in northern British Columbia, Canada. It flows into the Williston Lake, and is part of the Peace River basin. It was originally a tributary of the Finlay River before the creation of Lake Williston.-References:...

and the basin of the Bear River

Bear River (Sustut River)

The Bear River is a river in northwestern British Columbia, Canada. It flows northwest into the Sustut River, which flows southwest into the upper Skeena River....

in the Omineca Country

Omineca Country

The Omineca Country, also called the Omineca District or the Omineca, is a historical geographic region of the Northern Interior of British Columbia, roughly defined by the basin of the Omineca River but including areas to the south which allowed access to the region during the Omineca Gold Rush of...

of the Central-North Interior of British Columbia

British Columbia

British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...

, Canada

Canada

Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

. As the Omineca is part of the Arctic Ocean

Arctic Ocean

The Arctic Ocean, located in the Northern Hemisphere and mostly in the Arctic north polar region, is the smallest and shallowest of the world's five major oceanic divisions...

drainage, via the Peace

Peace River (Canada)

The Peace River is a river in Canada that originates in the Rocky Mountains of northern British Columbia and flows to the northeast through northern Alberta. The Peace River flows into the Slave River, a tributary of the Mackenzie River. The Mackenzie is the 12th longest river in the world,...

and Mackenzie River

Mackenzie River

The Mackenzie River is the largest river system in Canada. It flows through a vast, isolated region of forest and tundra entirely within the country's Northwest Territories, although its many tributaries reach into four other Canadian provinces and territories...

s, and the Bear is in the basin of the Skeena River

Skeena River

The Skeena River is the second longest river entirely within British Columbia, Canada . The Skeena is an important transportation artery, particularly for the Tsimshian and the Gitxsan - whose names mean "inside the Skeena River" and "people of the Skeena River" respectively, and also during the...

, which drains to the Pacific

Pacific Ocean

The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...

, The Thumb is located on the Continental Divide.

Geology

The Thumb is the most abundant feature in a cluster of approximately seven volcanic plug

Volcanic plug

A volcanic plug, also called a volcanic neck or lava neck, is a volcanic landform created when magma hardens within a vent on an active volcano. When forming, a plug can cause an extreme build-up of pressure if volatile-charged magma is trapped beneath it, and this can sometimes lead to an...

s combined with dike

Dike (geology)

A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...

s, lava flows, and leftovers of eroded cinder cone

Cinder cone

According to the , Cinder Cone is the proper name of 1 cinder cone in Canada and 7 cinder cones in the United States:In Canada: Cinder Cone In the United States:...

s. Even though the plugs have not been dated, the current form of loose scoria

Scoria

Scoria is a volcanic rock containing many holes or vesicles. It is most generally dark in color , and basaltic or andesitic in composition. Scoria is relatively low in mass as a result of its numerous macroscopic ellipsoidal vesicles, but in contrast to pumice, all scoria has a specific gravity...

and related intravalley lava flows to the current topography indicates they formed in the past two million years of the Quaternary

Quaternary

The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...

period.

The vertical structure of The Thumb develops a prominent monument rising approximately 189 m (620 ft) above smoothly rising landscape along the ridge of the Connelly Range

Connelly Range

The Connelly Range is a subrange of the Hogem Ranges of the Omineca Mountains, located between Bear Lake and the headwaters of the Omineca River in northern British Columbia, Canada. As the Omineca is in the Arctic Ocean drainage and Bear Lake that of the Pacific, the range is part of the...

. The Thumb is largely made of columnar basalt bounded by pockets of breccia comprising clasts of the basal sandstone

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

that formed during the Paleocene

Paleocene

The Paleocene or Palaeocene, the "early recent", is a geologic epoch that lasted from about . It is the first epoch of the Palaeogene Period in the modern Cenozoic Era...

period.

The Thumb consists of alkali olivine basalt along with other Quaternary volcanic plugs in the Omineca Mountains

Omineca Mountains

The Omineca Mountains, also known as "the Ominecas", are a group of remote mountain ranges in north-central British Columbia, Canada. They are bounded by the Finlay River on the north, the Rocky Mountain Trench on the east, the Nation River on the south, and the upper reaches of the Omineca River...

. The basalt comprises phenocryst

Phenocryst

thumb|right|300px|[[Granite]]s often have large [[feldspar|feldspatic]] phenocrysts. This granite, from the [[Switzerland|Swiss]] side of the [[Mont Blanc]] massif, has large white [[plagioclase]] phenocrysts, [[triclinic]] [[mineral]]s that give [[trapezium|trapezoid]] shapes when cut through)...

s of clinopyroxene and labradorite

Labradorite

Labradorite , a feldspar mineral, is an intermediate to calcic member of the plagioclase series. It is usually defined as having "%An" between 50 and 70. The specific gravity ranges from 2.68 to 2.72. The streak is white, like most silicates. The refractive index ranges from 1.559 to 1.573....

. Volcanic plugs in the Omineca Mountains, such as The Thumb, are located at the outermost boundary of all major volcanic belt

Volcanic belt

A volcanic belt is a large volcanically active region. Other terms are used for smaller areas of activity, such as volcanic fields. Volcanic belts are found above zones of unusually high temperature where magma is created by partial melting of solid material in the Earth's crust and upper mantle....

s in British Columbia, and their origins are not well-defined.

Geology

See also