New Lands Along the Old Coast: Building the Pacific Northwest
As Pangaea ruptured during the birth of the Atlantic Ocean about 200 million years ago, the North American plate began drifting westward. Since that date, the western edge of the continent has grown westward as a succession of volcanic island chains and assorted ocean-floor rocks have been added along the continental margin.
These “terranes” were then welded to the continent by a series of volcanic regimes which have developed along its western edge, in a process which continues today. The events of these last 200 million years can be organized into four distinct episodes.
This section reconstructs the four great geologic episodes during which our land of lofty mountains and high plateaus evolved after the breakup of Pangaea.
The Pacific Northwest as a geological province extends north from Washington State through British Columbia to Alaska. Most of this region has been added to North America over the last 200 million years. (Image: NASA)
Reeling from the Breakup of Pangaea
Exotic Terranes and Continental Arcs:
The Pieces of the Puzzle
The Key to Pacific Northwest Geology: The 4X4 History
The combination of these two processes – the accretion of four exotic terranes and the subsequent intrusion of four belts of molten rocks -- are the benchmarks along which Washington’s geologic history is organized over the last 200 million years. To unravel Washington’s complex geologic history, you must first understand each of the four terrane belts and each of the four continental arcs. We believe this “4X4” approach is the key to deciphering and understanding Washington’s complex geologic history.
Each of the four episodes of geologic history over the last 200 million years is named after the continental arc that dominated the episode.
1. The Omineca Episode (180-115 million years)
2. The Coast Range Episode (115-57 million years)
3. The Challis Episode (57-37 million years)
4. The Cascade Episode (37 million years to the present).
To appreciate how these chapters of Washington’s history formed, we suggest you tackle them one at a time from oldest to youngest.