The Basin and Range is a wide zone where plate-tectonic motions have stretched apart the crust as much as 50 percent in the last 15 million years or so. The uppermost, brittle rocks have cracked like breadcrust into long blocks that have foundered and tilted upon the softer crust beneath. These ranges shed sediment into the basins between them, marked in light gray. At the same time, magma burst up from below in widespread eruptions, leaving lavas marked in red and orange. The yellow areas are continental sedimentary rocks of the same age.
The dark gray areas are Proterozoic rocks, some 2 billion years old, that mark the eastern part of Mojavia, a large block of continental crust that was attached to North America and broken off during the breakup of the supercontinent Rodinia, about a billion years ago. Mojavia may have been part of Antarctica or part of Australiathose are the two leading theories, but there are other proposals as well. Arizona will provide rocks and problems for many generations of geologists to come.
The full-six (1250x1450 pixel) version of this map shows the age symbols for all the rocks plus the rivers and cities of Arizona.
To purchase a larger, more useful map of Arizona's rocks, see the Arizona Geological Survey's publications page.
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