Pacific - Cascades volcanic province
Explore: Sierra Nevada | Cascade Volcanoes | North Cascades
Ring of Fire
The Cascades volcanoes define the Pacific Northwest section of the Ring of Fire', a fiery array of volcanoes that rim the Pacific Ocean. As if volcanic hazards were not enough, the Ring of Fire is also infamous for its frequent earthquakes. In order to understand the origins of this concentrated band of Earth hazards we have to take a peek beneath our feet.
If we could slice deep into the Earth from the Pacific Ocean through the Pacific Northwest, we might see something like the image below. Beneath the Cascades, a dense oceanic plate plunges beneath the North American Plate; a process known as subduction. As the oceanic slab sinks deep into the Earth's interior beneath the continental plate, high temperatures and pressures allow water molecules locked in the minerals of solid rock to escape. The water vapor rises into the pliable mantle above the subducting plate, causing some of the mantle to melt. This newly formed magma rises toward the Earth's surface to erupt, forming a chain of volcanoes (the Cascade Range) above the subduction zone.
A dense oceanic plate sinks beneath a more buoyant continental plate. Click here to learn more.
Living on the edge
A close-up look at the Cascades (below) reveals a more complicated picture than the simple subduction zone shown in the image on the right.
Not far off the coast of the North Pacific lies a spreading ridge; a divergent plate boundary made up of a series of breaks in the oceanic crust where new ocean crust is created. On one side of the spreading ridge new Pacific Plate crust is made, then moves away from the ridge. On the other side of the spreading ridge the Juan de Fuca Plate and Gorda Plate move eastward.
There are some unusual features at the Cascade subduction zone. Where the Juan de Fuca Plate sinks beneath the North American Plate there is no deep trench, seismicity (earthquakes) are fewer than expected, and there is evidence of a decline in volcanic activity over the past few million years. The probable explanation lies in the rate of convergence between the Juan de Fuca and North American Plates. These two plates converge at 3-4 centimeters per year at present. This is only about half the rate of convergence of 7 million years ago.
Plate tectonic setting of the Cascades. Yellow rings indicate recorded earthquakes. Black lines show faults active within past 10,000 years.
The small Juan de Fuca Plate and two platelets, the Explorer Plate and Gorda Plate are the meager remnants of the much larger Farallon oceanic plate. The Explorer Plate broke away from the Juan de Fuca about 4 million years ago and shows no evidence that it is still being subducted. The Gorda platelet split away between 18 and 5 million years ago and continues to sink beneath North America.
The Cascade Range made its first appearance 36 million years ago, but the major peaks that rise up from today's volcanic centers were born within the last 1.6 million years (Pleistocene). More than 3000 vents erupted during the most recent volcanic episode that began 5 million years ago. Are there more eruptions in our future? As long as subduction continues, new Cascade volcanoes will continue to rise.