Geological Formation of Vancouver Island

How & when was Vancouver Island formed?

The story of Vancouver Island spans hundreds of millions of years. The island was formed through a combination of processes, including tectonic plate movement, volcanism, erosion and glaciation. The following is a brief summary of the more detailed information provided by the references listed at the bottom of this page.

Tectonic processes

Much of the rock that makes up Vancouver Island actually originated near the equator in the Pacific Ocean. It is part of a large piece of crust, or terrane, called Wrangellia, that also includes southeast Alaska, the Queen Charlotte Islands and part of the Coast Mountains. 380 million years ago, some of the oldest rock that makes up present day Vancouver Island was formed by undersea lava deposits. Later, the calcium shells of countless marine animals added layers that became limestone. Wrangellia slowly drifted northeastward (see the end section for a review of tectonic plate theory).

About 100 million years ago, Wrangellia collided with the North American continent. Slow but tremendous forces caused some regions to fold and buckle into mountain ridges and others to crumble and erode.

42 to 55 million years ago, two smaller pieces of crust called the Pacific Rim and Crescent terranes collided with and joined onto Vancouver Island. The Crescent terrane is composed of marine volcanic rock and forms the land of Sooke, Metchosin and Colwood. The region that the rest of Greater Victoria is built upon is granite and gneiss (metamorphic rock) that is thought to have formed from magma deep within the layers of rock. It was exposed during the uplift and erosion associated with the collisions of the Pacific Rim and Crescent terranes.

Glacial processes

The Earth has been subjected to many glacial (cold) and interglacial (warm) periods, throughout its history. These are governed largely by changes in the Earth’s orbit called Milankovich cycles, but also depend on other complex climate processes.

Between 15,000 and 29,000 years ago, the most recent glacial period or “ice age” developed; today it is referred to as the Fraser Glaciation. Ice accumulated on the mountain peaks of Vancouver Island and slowly grew into large glaciers. Eventually they joined with other glaciers and formed ice sheets that filled the major valleys. These ice sheets also joined others from the Coast Mountains and filled Georgia and Hecate Straits, and Queen Charlotte Sound.

The ice formed two lobes at the south end of Vancouver Island. One flowed south toward Seattle and terminated near Tacoma. The other flowed westward and northwestward through Juan de Fuca Strait and ended in a shelf some distance to the west of the Island.

Evidence of the direction of flow of these ice sheets can be seen in rocks around Victoria. Generally, striations in the rock are aligned with the direction of this flow. Although the overall flow was north to south, local variations occur where the ice moved over obstructions and where it turned eastward close to Juan de Fuca Strait. Some rocks are gradually and smoothly sloped on one side, and rough on the other side, somewhat like a rounded breaking wave. These are called roches moutonées, and the smoother side is the one the ice contacted first.

The ice sheets cut major features in the landscape, including U-shaped valleys such as Cowichan Valley, and the steep-sided fjords that are common all along the west coast of the island. Their weight was enough to depress the land mass of Vancouver Island by 150 to 300 metres. This caused the sea level at the time to be lower than today.

Between 15,000 years before present and today, the climate began to warm and the ice sheets slowly melted and retreated. The release of the weight of ice caused the land to rebound. This exposed areas of land that were previously submerged.

For about a thousand years, around 10,000 years ago, the rate of rebounding from the land exceeded the sea level rise caused by the melting glaciers. Eventually the sea reclaimed some of the recently exposed land, and stabilized at its current level.

In place of the ice sheets, large deposits of glacial till were left. This was formed from rock that was ground up under the ice sheets, deposited on the surface of the glaciers, or pushed up in front of advancing glaciers. Lakes, streams and rivers formed from melting ice, and rivers transported glacial sediment to new locations.