Eight hundred million years ago, the North American continent's western border included the southeastern part of California but not Washington or Oregon. Geologists attribute the accretion (addition) of land, such as Washington and Oregon, to various phenomena:
In the 1980s a theory was proposed, based on Alaska's pattern of rocks, that terranes are responsible for areas in which a particular formation or group of predominant rocks is different from those that are native to the area. Terranes are bordered by fractures in the crust (fault lines) and are made up of rocks different than those in the area to which the terrane has become accreted.
Terranes that have accreted to North America may have arrived from Asia or Australia by way of island microcontinents in the Pacific Ocean.
A terrane is said to be exotic if it has been transported into its present location from another place a great distance away. It originates somewhere else and is transported by plate movement and added on to a continent's coast when it collides with it. Two ways geologists determine where terranes came from are:
Terranes were added to the continent either separately or as an amalgamation of other terranes (a super-terrane). Geologists use "overlap sequences" and "relative" ages to figure out the story.
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