Vanisle Formation

HOW VANCOUVER ISLAND WAS FORMED

Plate Tectonics and Continental Drift
Glaciation
Earthquakes
Erosion by Water and Wind
Glossary of Terms Used

If you look at a map of North and South America you will see that there are a series of mountain ranges running north and south along the Pacific coast of both continents. One of those ranges, the Insular Mountains, has not fully emerged above the level of the sea, and it is the higher elevations of this range that form the series of island's which parallel the coast line in British Columbia and Northern Washington. Vancouver Island is the largest of this chain of islands.

This mountainous area as a whole is known as the Western Cordillera. The mountains are volcanic in origin and there are a few that are still active. It is also a region where there is a lot of earthquake activity. A map showing the seismic activity in Canada and then more particularly around Vancouver and Vancouver Island illustrates the point. Observing that seismic and volcanic activity seemed to be concentrated in particular parts of the earth, geologists have long sought an explanation for this. They now think they have an answer.

If this is just too much for you, click on the banner for my version.

CONTINENTAL DRIFT AND PLATE TECTONICS

In the 16th century, Francis Bacon noted that it looked like the coast lines of the continents could be fitted together like pieces of a jigsaw puzzle. By the twentieth century, geologists had further evidence which suggested that in prehistoric time there had been only one super continent. Rock formation, mountain ranges and fold belts all suggest that the continents were once joined but have drifted apart. By the 1960's geologists had developed a theory which they felt explained the observations they had been able to make. They hypothesized that perhaps the earth's crust consisted of huge, dense plates on which the lighter continents were perched. These plates seemed to be moving and colliding.

Where two plates converged and collided, immense compression forces were generated and buckled the continental rock, producing folded mountains.

They also felt that this theory explained volcanic and seismic activity found in these regions since when a plate sinks into the earth's mantle there is a massive increase in pressure and temperature. At about 400-600km below the surface of the earth, this plate is melted and absorbed back into the core of the earth. Melting and friction release pockets of magma which rise through the plate above creating volcanoes. The friction as one plate slides beneath the other causes some "jerky" movements, and hence the number of earthquakes experienced in these zones. This theory which describes the motion of the plates is called plate tectonics.

How This Relates to Our Island

Evidence suggests that the Pacific Plate which I have shown in these illustrations in blue is being subducted into the earth's mantle. The plate that is sinking like this is called the slab. The Continental plate which is overriding this one is called the wedge. Sometimes volcanic pressure builds but the upswell of magma does not break through the earth's crust. The magma which is still below the earth's surface cools slowly forming large granite crystals. These underground formations are called intrusions. A batholith is a huge intrusion at least 100 km² (60 miles²). These batholiths are initially covered by the sedimentary rock of the earth's crust but when they are shoved up by the type of plate action we have described here, they become exposed. The Insular Mountains of Vancouver Island have this type of volcanic origin.

To illustrate the process in a very simplified way:

That then, was the start of our island, but there is more to come.

For more detail on the movement of plates and the formations which make up Vancouver Island see our page on Wrangellia

Glaciation

About 18,000 years ago there was a great ice age. The ice fields covered most of the mountains. Glaciers, descending down the valleys to the sea, steepened the valley sides and eroded their floors. When the ice melted and the sea level rose, these valleys became the great fiords which characterize the coasts of both Vancouver Island and the mainland.

Sometimes where hollows existed in the land, the ice fields settled in basins. Seasonal temperature changes caused a pattern of partial thawing and refreezing which was accompanied by some rotation of the ice within these basins. The term used for these formations is cirques. The Comox Glacier which still dominates the skyline of the towns of Courtenay and Comox has created a cirque. A close up of the glacier hints at its size.

When several of these cirques formed, the divides between them became great horns when the ice cap receded, gouging away the sides of the mountain in the process. The result of this type of ice activity is evident on Mt. Elkhorn.

Seismic Activity

Seismic activity on Vancouver Island right up to recent times has continued to reshape the land masses. Significant quakes have led to crumbling mountains, land slides, and the formation of fissures and crevices.

In this picture of Mt. Colonel Foster, evidence of the quake of 1946 can be seen in the form of the land slide to the right of the photo. This slide took away about a quarter of the mountain.

Historians believe that the Forbidden Plateau region of Strathacona Park, just outside of Courtenay derived its name from native people who had reported the strange and frightening phenomena they had witnessed there. The area is marked by bottomless crevices that have been formed by earthquakes in fairly recent times. It seems likely that some native people may have been present in the area when this quake opened these fissures. If they then reported the experience it is not surprising that the region was avoided thereafter for a number of years.

Erosion by Water and Wind

There are a number of rivers on the island and they have each eked out their path through the mountainous terraine. They often find their source high in the mountains and their descent is steep and waterfalls are common. Sandstone shelves with rocks of differing densities are eroded to form bowls.

The ocean waves and winds erode the shore lines of Vancouver Island. Because much of the rock is hard granite, steep cliffs have been formed. The folding action that formed the islands means that some softer sedimentary rock is mixed in with the granite. This erodes more quickly so that you get islands formed where a harder rock mass remains when the less stable sedimentary rock around it has been washed away. When and island has particularly steep sides, it is known as a seas stack. This photo was taken from a point on the West Coast Trail where the trail runs parallel to the edge of a cliff and a shows a good example of stack formation.

Glossary of Terms Used

batholith: a large scale upswelling of magma, typically formed of granite rock.
cirque: a basin shaped hollow where snow can collect and survive summer melting.
glaciation: the scouring or erosion of rocks by glaciers
hypothesis: something assumed or put forth as an idea to be tested
intrusion: rock (usually granite) formed from magma cooling within the crust of the earth.
plate tectonics: movement of the plates that form the crust of the earth
seismic: relating to earthquakes
slab: a portion of the earth's crust which is being subducted
subduction: the movement of one global plate down under the edge of another
theory: a more or less plausible or scientifically accepted principle that is offered to explain observed phenomena. Not proven yet.
wedge: the portion of the earth's crust that rides over subducted crust and through which magma rises to creat volcanoes