Geology of the Pacific Northwest
Most of the region began forming about 200 million years ago as the North American Plate started to drift westward during the rifting of Pangaea.
Since that date, the western edge of North America has grown westward as a succession of island arcs and assorted ocean-floor rocks have been added along the continental margin.
The Cascades Province forms an arc-shaped band extending from southwestern British Columbia to Northern California, roughly parallel to the Pacific coastline.
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.
This newly formed magma ascends upward through the crust along a path of least resistance, both by way of fractures and faults as well as by melting wall rocks.
Some of the melt rises toward the Earth's surface to erupt, forming a chain of volcanoes (the Cascade Volcanic Arc) above the subduction zone.
The probable explanation lies in the rate of convergence between the Juan de Fuca and North American Plates.
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 northernmost member, the Mount Meager massif, was responsible for a major catastrophic eruption that occurred about 2,350 years ago.
It is also the most unstable volcanic massif in Canada, which has dumped clay and rock several meters (yards) deep into the Pemberton Valley at least three times during the past 7,300 years.
It is a large shield volcano that covers an area of 1,800 km2 (690 sq mi) southwest of Dease Lake and north of Telegraph Creek.
The Chilcotin Group in southern British Columbia is a north–south range of volcanoes, thought to have formed as a result of back-arc extension behind the Cascadia subduction zone.
[3] The Wells Gray-Clearwater volcanic field in south-eastern British Columbia consists of several small basaltic volcanoes and extensive lava flows that have been active for the past 3 million years.
The event coincided with the arrival of the first European explorers to penetrate the uncharted coastal waters of northern British Columbia.
[6] More recently, in 2001, the Nisqually earthquake (magnitude 6.8) struck 16 km (10 mi) northeast of Olympia, Washington, causing some structural damage and panic.
The topography here is dominated by geologically young lava flows that inundated the countryside with amazing speed, all within the last 17 million years.
[8] The Snake River Plain stretches across Oregon, through northern Nevada, southern Idaho, and ends at the Yellowstone Plateau in Wyoming.
Looking like a great spoon scooped out the Earth surface, the smooth topography of this province forms a striking contrast with the strong mountainous fabric around it.
Calderas, great pits formed by explosive volcanism, and low shield volcanoes, and rhyolite hills are also part of the landscape here, but many are obscured by later lava flows.
Geologists know that beneath Hawaii and Iceland, a temperature instability develops (for reasons not yet well understood) at the boundary between the core and mantle.
[8] With the beginning of the Pleistocene time (about one million years ago), cooling temperatures provided conditions favorable for the creation of continental glaciers.
Over the centuries, as snowfall exceeded melting and evaporation, a great accumulation of snow covered part of the continent, forming extensive ice fields.
[10] Glacial Lake Missoula broke through the ice dam many times, allowing a tremendous volume of water to rush across northern Idaho and into eastern Washington.
[11] Such catastrophic floods raced across the southward-dipping plateau a number of times, etching the coulees which characterize this region, now known as the channeled scablands.
The eroding power of the water plucked pieces of basalt from the precipice, causing the falls to retreat 32 km (20 mi) and self-destruct by cutting through to the Columbia River valley near what is now the Grand Coulee Dam.
It started near Soap Lake in Washington State, where less resistant basalt layers gave way before the great erosive power of this tremendous torrent and waterfalls developed.
The record of this long history can be read in the many rock layers deposited over time through the forces of erosion, volcanic activity and plate subduction.
The disparate pieces of the North Cascade mosaic were born far from one another but subsequently drifted together, carried along by the tectonic plates that make up the Earth's outer shell or were uplifted, eroded by streams, and then locally buried in their own eroded debris; other pieces were forced deep into the Earth to be heated and squeezed, almost beyond recognition, and then raised again to view.
[12] The Coast Mountains are the western range of the North American mainland cordillera, covering the Alaska Panhandle and most of coastal British Columbia.
The plutonic complex is built on unusual island arc fragments, oceanic plateaus and continental margin assemblages accreted between the Triassic and the Cretaceous periods.
