Outer Layers
Earth's crust and the uppermost part of the mantle are known as the lithosphere. This is a fairly rigid zone that extends about 100 kilometers below the surface of the earth. The crust extends some 60 kilometers or so under the continental surface, but only about 10 kilometers below the ocean floor. The continental crust has a lower density than the oceanic crust does. It is primarily a light granitic rock rich in the silicates of aluminum, iron, and magnesium. In a simplified view the continental crust can be thought of as layered: On top of a layer of igneous rock (molten rock that has hardened) lies a thin layer of sedimentary rocks (rocks formed by sediment and fragments that water deposited); there is also a soil layer deposited during past ages in the parts of continents that have had no recent volcanic activity or mountain building.
Sandwiched between the lithosphere and the lower mantle is the partially molten material of the asthenosphere, about 150 kilometers thick. It consists primarily of iron and magnesium silicates that readily deform and flow under pressure.
In efforts to date various regions of the continents, geochemists have shown with radioactive-dating techniques that the oldest rock formations on the continents are between 3.8 and 3.5 billion years old. For North America the oldest part, called a continental shield, is a crescent-shaped region bordering the west and south sides of Hudson's Bay. A younger crescent lying roughly to the west and south surrounds this oldest region, and the westernmost and southernmost parts of the continent are even younger. A somewhat similar pattern exists for the other continents.
Antarctic Plate
The inference is that the continents are not original with the formation of the earth 4.6 billion years ago but are a secondary aspect that continues to grow. We know that the continental margins, particularly the western edge of North America, are new additions to the continents. The coastal regions are building due to the deposition of sediments washed down by rivers from the interior of the continent. In striking contrast the oldest known parts of the oceanic crust are about 200 million years old or almost twenty times younger than the oldest parts of the continents.
