After the breakup, the continents traveled in spurts rather than drifting
apart at a constant speed. The rate of seafloor spreading in the Atlantic was
matched by plate subduction in the Pacific, where one plate dives under
another, forming a deep trench. This is why the oceanic crust of the Pacific
Basin dates back no farther than the early Jurassic. A high degree of geologic
activity around the rim of the Pacific Basin produced practically all the moun-
tain ranges facing the Pacific and the island arcs along its perimeter.
Much of western North America assembled from island arcs and other
crustal debris skimmed off the Pacific plate as the North American plate con-
tinued heading westward. Northern California is a jumble of crustal fragments
assembled within about 200 million years ago. A nearly complete slice of
ocean crust, the type shoved up on the continents by drifting plates, sits in the
Figure 155The breakup
and drift of the continents
to their present localities.JURASSIC BIRDS225 million years agoTethys
SeaP a n g a e aPANGAEALAL AL AURASIAURASIAU R A S I AGG ONDGONDWANAONDWWANAANA180 million years ago135 million years ago 65 million years ago