Continent

Earth's crust comes in two varieties, continental and oceanic. All crust consists primarily of silicate minerals. These contain silica (SiO₂), which consists of the elements silicon and oxygen bonded together, and a variety of other elements. Continental crust is silica-rich, or felsic. Oceanic crust is mafic, relatively rich in iron and magnesium and silica-poor. The mantle has silicate minerals with a greater abundance of iron and magnesium and even less silica than oceanic crust, so it is called ultramafic.

Continental crust is less dense and thicker (specific gravity = 2.7, thickness = 20-25 mi; 30-40 km) than oceanic crust (S.G. = 2.9, thickness = 2.5-3.75 mi; 6-7 km), and much less dense than the upper mantle (S.G. 3.3). Continents are therefore very unlikely to subduct at an oceanic trench, while oceanic crust subducts rather easily. Consequently, Earth's oldest oceanic crust is less than 200 million years old, while the oldest existing continental crust is 3.8 billion years old and most is more than two billion years old.