The Canadian portion of the Pacific Northwest began forming during the early Jurassic period when a group of active volcanic islands collided against a pre-existing continental margin and coastline of Western Canada.
These volcanic islands, known as the Intermontane Islands by geoscientists, were formed on a pre-existing tectonic plate called the Intermontane Plate about 245 million years ago by subduction of the former Insular Plate to its west during the Triassic period.
This subduction zone records another subduction zone called the Intermontane Trench under an ancient ocean between the Intermontane Islands and the former continental margin of Western Canada called the Slide Mountain Ocean.
This arrangement of two parallel subduction zones is unusual in that very few twin subduction zones exist on Earth; the Philippine Mobile Belt off the eastern coast of Asia is an example of a modern twin subduction zone.
As the Intermontane Plate drew closer to the pre-existing continental margin by ongoing subduction under the Slide Mountain Ocean, the Intermontane Islands drew closer to the former continental margin and coastline of Western Canada, supporting a volcanic arc on the former continental margin of Western Canada.
As the North American Plate drifted west and the Intermontane Plate continued to drift east to the ancient continental margin of Western Canada, the Slide Mountain Ocean began to close by ongoing subduction under the Slide Mountain Ocean.
This subduction zone eventually jammed and shut down completely about 180 million years ago, ending the arc volcanism on the ancient continental margin of Western Canada and the Intermontane Islands collided, forming a long chain of deformed volcanic and sedimentary rock called the Intermontane Belt, which consists of deeply cut valleys, high plateaus, and rolling uplands.
This collision also crushed and folded sedimentary and igneous rocks, creating a mountain range called the Kootenay Fold Belt which existed in far eastern British Columbia.

After the sedimentary and igneous rocks were folded and crushed, it resulted in the creation of a new continental shelf and coastline.
The Insular Plate continued to subduct under the new continental shelf and coastline about 130 million years ago during the mid Cretaceous period after the formation of the Intermontane Belt, supporting a new continental volcanic arc called the Omineca Arc.
Magma rising from the Omineca Arc successfully connected the Intermontane Belt to the mainland of Western Canada, forming a chain of volcanoes in British Columbia that existed discontinuously for about 60 million years.
The ocean lying offshore during this period is called the Bridge River Ocean.
It was also during this period when another group of active volcanic islands existed along the newly built continental shelf and coastline.
These volcanic islands, known as the Insular Islands, were formed on the Insular Plate by subduction of the former Farallon Plate to its west during the early Paleozoic era.
As the North American Plate drifted west and the Insular Plate drifted east to the continental margin of Western Canada, the Bridge River Ocean began to close by ongoing subduction under the Bridge River Ocean.
This subduction zone eventually jammed and shut down completely 115 million years ago, ending the Omineca Arc volcanism and the Insular Islands collided, forming the Insular Belt.
Compression resulting from this collision crushed, fractured and folded rocks along the continental margin.
The Insular Belt then welded onto the continental margin by magma that eventually cooled to create a large mass of igneous rock, creating a new continental margin.
This large mass of igneous rock is the largest granite outcropping in North America.

The Farallon Plate continued to subduct under the new continental margin of Western Canada after the Insular Plate and Insular Islands collided with the former continental margin, supporting a new chain of volcanoes on the mainland of Western Canada called the Coast Range Arc about 100 million years ago during the Lat
The Canadian portion of the Pacific Northwest began forming during the early Jurassic period when a group of active volcanic islands collided against a pre-existing continental margin and coastline of Western Canada. These volcanic islands, known as the Intermontane Islands by geoscientists, were formed on a pre-existing tectonic plate called the Intermontane Plate about 245 million years ago by subduction of the former Insular Plate to its west during the Triassic period. This subduction zone records another subduction zone called the Intermontane Trench under an ancient ocean between the Intermontane Islands and the former continental margin of Western Canada called the Slide Mountain Ocean. This arrangement of two parallel subduction zones is unusual in that very few twin subduction zones exist on Earth; the Philippine Mobile Belt off the eastern coast of Asia is an example of a modern twin subduction zone. As the Intermontane Plate drew closer to the pre-existing continental margin by ongoing subduction under the Slide Mountain Ocean, the Intermontane Islands drew closer to the former continental margin and coastline of Western Canada, supporting a volcanic arc on the former continental margin of Western Canada. As the North American Plate drifted west and the Intermontane Plate continued to drift east to the ancient continental margin of Western Canada, the Slide Mountain Ocean began to close by ongoing subduction under the Slide Mountain Ocean. This subduction zone eventually jammed and shut down completely about 180 million years ago, ending the arc volcanism on the ancient continental margin of Western Canada and the Intermontane Islands collided, forming a long chain of deformed volcanic and sedimentary rock called the Intermontane Belt, which consists of deeply cut valleys, high plateaus, and rolling uplands. This collision also crushed and folded sedimentary and igneous rocks, creating a mountain range called the Kootenay Fold Belt which existed in far eastern British Columbia.
After the sedimentary and igneous rocks were folded and crushed, it resulted in the creation of a new continental shelf and coastline. The Insular Plate continued to subduct under the new continental shelf and coastline about 130 million years ago during the mid Cretaceous period after the formation of the Intermontane Belt, supporting a new continental volcanic arc called the Omineca Arc. Magma rising from the Omineca Arc successfully connected the Intermontane Belt to the mainland of Western Canada, forming a chain of volcanoes in British Columbia that existed discontinuously for about 60 million years. The ocean lying offshore during this period is called the Bridge River Ocean. It was also during this period when another group of active volcanic islands existed along the newly built continental shelf and coastline. These volcanic islands, known as the Insular Islands, were formed on the Insular Plate by subduction of the former Farallon Plate to its west during the early Paleozoic era. As the North American Plate drifted west and the Insular Plate drifted east to the continental margin of Western Canada, the Bridge River Ocean began to close by ongoing subduction under the Bridge River Ocean. This subduction zone eventually jammed and shut down completely 115 million years ago, ending the Omineca Arc volcanism and the Insular Islands collided, forming the Insular Belt. Compression resulting from this collision crushed, fractured and folded rocks along the continental margin. The Insular Belt then welded onto the continental margin by magma that eventually cooled to create a large mass of igneous rock, creating a new continental margin. This large mass of igneous rock is the largest granite outcropping in North America.
The Farallon Plate continued to subduct under the new continental margin of Western Canada after the Insular Plate and Insular Islands collided with the former continental margin, supporting a new chain of volcanoes on the mainland of Western Canada called the Coast Range Arc about 100 million years ago during the Lat
Volcanism of Canada