In geologic terms a continent is defined as
an area of the crust in which the depth to the
underlying mantle exceeds 25 km. In the ocean
basins, the crust (called oceanic crust) is only
about 8 km thick. But in CONTINENTS the crust is
on average 35 km thick. It can be as thick as 75
km as beneath mountain belts like the Andes and it
can be as thin as ~ 20 - 25 km as beneath the
great Basin in the US (Nevada).
North America and South America have not
always been connected. The connection is a recent
development, so as far as ancient cratons go the
two areas really are distinct continents.
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