Pacific Ranges - Geology

Geology

The Pacific Ranges are part of the southern portion of the Coast Plutonic Complex and has been characterized by rapid rates of uplift over the past 4 million years, which has led to relatively high rates of erosion.

The Garibaldi Volcanic Belt is within the Pacific Ranges, which is a volcanic belt formed by the subduction of the Juan de Fuca Plate (a remnant of the much larger Farallon Plate) under the North American Plate along the Cascadia subduction zone. The belt is the northern extension of the Cascade Volcanic Arc in the United States (which includes the volcanoes Mount St. Helens and Mount Baker) and contains the most explosive young volcanoes in Canada. The eruption styles in the belt range from effusive to explosive, with compositions from basalt to rhyolite. Morphologically, centers include calderas, cinder cones, stratovolcanoes and small isolated lava masses. Due to repeated continental and alpine glaciations, many of the volcanic deposits in the belt reflect complex interactions between magma composition, topography, and changing ice configurations. The most recent major catastrophic eruption in the Garibaldi Volcanic Belt was from Mount Meager 2,350 BP, which is Canada's most recent major catastrophic eruption.

The Garibaldi Volcanic Belt contains 2 extra volcanic fields, the Franklin Glacier Volcano and the Silverthrone Caldera, which lie 140 and 190 km northwest of the main volcanic belt.

The Cascadia subduction zone is a 680 mi (1,094 km) long fault, running 50 mi (80 km) off the west-coast of the Pacific Northwest from northern California to Vancouver Island. The plates move at a relative rate of over 0.4 inches (10 mm) per year at a somewhat oblique angle to the subduction zone.

Unlike most subduction zones worldwide, there is no oceanic trench present along the continental margin in Cascadia. Instead, terranes and the accretionary wedge have been uplifted to form a series of coast ranges and exotic mountains. A high rate of sedimentation from the outflow of the three major rivers (Fraser River, Columbia River, and Klamath River) which cross the Cascade Range contributes to further obscuring the presence of a trench. However, in common with most other subduction zones, the outer margin is slowly being compressed, similar to a giant spring. When the stored energy is suddenly released by slippage across the fault at irregular intervals, the Cascadia subduction zone can create very large earthquakes such as the magnitude 9 Cascadia earthquake of 1700.