Pacific States

Geologically, the Pacific States are a hodge-podge of elements created (and destroyed) along the subduction boundary that defines the West Coast. About 400 million years ago the West Coast of North America was several hundred miles to the east of where it is now. The Antler Orogeny, a proposed solution to the question of why there is a geological unconformity in northeastern Nevada, seems to have been scraped off an approaching tectonic plate beginning in the mid-Devonian and continuing into the Carboniferous. Events resulting from that collision produced numerous mineral hot springs along the face of the collision and we can now see some of the results of that along the Carlin Trend, a zone with a high concentration of minerals such as gold and silver. Northeastern Nevada has a series of these types of fault-block trends and geologists still haven't determined the extent of the Antler depositions along the West Coast but they extend north and south for several hundred miles.

During the Jurassic Period, 250 million years ago, Pangaea was breaking up and as Laurentia floated away, the Farallon Plate under the Pacific Ocean began to subduct along the West Coast. Above water, the Farallon Plate was composed mostly of island arcs, volcanic chains and assorted terranes. About 200 million years ago the Sonomia Terrane collided with and accreted to the western shoreline along the Golconda Thrust in central Nevada. The Intermontane Plate arrived at the coast of Alaska, Yukon, British Columbia and eastern Washington about 180 million years ago. The Smartville Block accreted to the continental margin in the area of California's Mother Lode country about 165 million years ago. About 115 million years ago the Insular Plate collided with the Pacific Northwest and added the Alexander Archipelago, Queen Charlotte Islands and Vancouver Island to the coastline. About 40 million years the Cascade stratovolcanoes began to erupt and, as today, they erupted regularly for more than 20 million years. There was a general gap in the eruptions in the mid-Miocene, during the time of the Columbia River Flood Basalts, then the eruptions began again and still continue.

About 200 million years ago the Sierra Nevada batholith began to form. The batholith added a second wave of plutons to its mass about 140 million years ago. About 90 million years ago the third (and last) wave of plutons were added. About 4 million years ago the Sierra Nevada batholith began to rise.

The Columbia River Basalt Group in Oregon, Washington, Idaho and Nevada began to form about 17 million years ago. In those days, the surface was hovering over what we now call the Yellowstone Hotspot. This was a flood basalt event that deposited most of the lava beds in the area between 17 and 14 million years ago. Lava flows continued on a much reduced scale up until about 6 million years ago.

About 43 million years ago the Pacific Plate changed its direction of movement from north to northwest. That change in the direction of compression also resulted in an expansion that first began to pull on the Rio Grande Rift about 35 million years ago. The San Andreas Fault Complex came into being about 20 million years ago as the mass of the North American Plate began to break the subducting Farallon Plate beneath it in two. As the Pacific Plate continued its northwest motion, strike-slip faults propagated throughout the western states. About 8 million years ago the Basin and Range Province began to form as the lithosphere below it expanded without rifting. About 3.5 million years ago the Pacific Plate changed direction again, moving about 13° east to a heading more north by northwest.

These days we hear the geologists and seismologists predicting "the Big One" will strike any moment now. And the reality is it could...

The coast of Oregon