By Richard I. Gibson
The cratonic core of South America is the Brazilian Shield, composed of Archean rocks modified by later tectonic events. The similar but smaller Guyana Shield occupies the northern part of the continent.
The two cratons are separated by the Amazon Rift – a long-standing break in what was probably originally a single large craton. That break follows a weak zone that has been around for hundreds of millions of years, and is the reason the Amazon River is where it is.
Map derived from U.S. Geological Survey
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