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.