The Thumb is a

mountain A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited Summit (topography), summit area, and ...

located south of Sitchiada Mountain on the east side of Bear Lake, on the divide between the upper

Omineca River Omineca River is a river of the North American boreal forest, 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 La ...

and the basin of the Bear River in the

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 a ...

of the Central-North Interior of

British Columbia British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, ...

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...

. As the Omineca is part of the

Arctic Ocean The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximately and is known as the coldest of all the oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, a ...

drainage, via the

Peace Peace is a concept of societal friendship and harmony in the absence of hostility and violence. In a social sense, peace is commonly used to mean a lack of conflict (such as war) and freedom from fear of violence between individuals or groups. ...

and Mackenzie Rivers, and the Bear is in the basin of the

Skeena River The Skeena River is the second-longest river entirely within British Columbia, Canada (after the Fraser River). Since ancient times, the Skeena has been an important transportation artery, particularly for the Tsimshian and the Gitxsan—whose n ...

, which drains to the

Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

, The Thumb is located on the

Continental Divide A continental divide is a drainage divide on a continent such that the drainage basin on one side of the divide feeds into one ocean or sea, and the basin on the other side either feeds into a different ocean or sea, or else is endorheic, not ...

Geology

The Thumb is the largest in a cluster of roughly seven

volcanic plug A volcanic plug, also called a volcanic neck or lava neck, is a volcanic object created when magma hardens within a vent on an active volcano. When present, a plug can cause an extreme build-up of high gas pressure if rising volatile-charged mag ...

s. They are surrounded by the remains of eroded

cinder cone A cinder cone (or scoria cone) is a steep conical hill of loose pyroclastic fragments, such as volcanic clinkers, volcanic ash, or scoria that has been built around a volcanic vent. The pyroclastic fragments are formed by explosive eruptions o ...

s, lava flows and

dike Dyke (UK) or dike (US) may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), a subvertical sheet-like intrusion of magma or sediment * Dike (mythology), ''Dikē'', the Greek goddess of moral justice * Dikes, ...

s. Even though the plugs have not been dated, the existence of loose

scoria Scoria is a pyroclastic, highly vesicular, dark-colored volcanic rock that was ejected from a volcano as a molten blob and cooled in the air to form discrete grains or clasts.Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) ''G ...

and related intravalley lava flows to the current topography indicates they formed in the last 2.5 million years of the

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...

period. The vertical structure of The Thumb develops a prominent monument rising approximately above smoothly rising landscape along the ridge of the

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 t ...

. The Thumb is largely made of

columnar basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...

bounded by pockets of breccia comprising clasts of the basal

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) ...

that formed during the

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), E ...

period. The Thumb consists of alkali olivine basalt along with other Quaternary volcanic plugs in the

Omineca Mountains The Omineca Mountains, also known as "the Ominecas", are a group of remote mountain ranges in the Boreal Cordillera of north-central British Columbia, Canada. They are bounded by the Finlay River on the north, the Rocky Mountain Trench (here fil ...

. The basalt comprises

phenocryst 300px, feldspathic phenocrysts. This granite, from the Switzerland">Swiss side of the Mont Blanc massif, has large white plagioclase phenocrysts, triclinic minerals that give trapezoid shapes when cut through). 1 euro coins, 1 euro coin (diameter ...

s of

clinopyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...

and

labradorite Labradorite (( Ca, Na)( Al, Si)4 O8) is a calcium-enriched feldspar mineral first identified in Labrador, Canada, which can display an iridescent effect ( schiller). Labradorite is an intermediate to calcic member of the plagioclase series. It ...

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

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 ...

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

References

External links

* Volcanic plugs of British Columbia One-thousanders of British Columbia Omineca Country Great Divide of North America Quaternary volcanoes Omineca Mountains Quaternary British Columbia {{BritishColumbiaInterior-geo-stub

Geology

See also

References

External links