why do most lithospheric plates contain both oceanic and continental crusts?

Lithospheric plate is composed of a layer of oceanic crust or continental crust superficial to an outer layer of the mantle. Containing both crust and the upper region of the mantle, lithospheric plates are generally considered to be approximately 60 mi (100 km) thick. Although containing only continental crust or oceanic crust in any one cross-section, lithospheric plates may contain various sections that exclusively contain either oceanic crust or continental crust and therefore lithospheric plates may contain various combinations of oceanic and continental crust.

The oceanic lithosphere is located under the Earth's bodies of water. Although generally thinner than its continental counterpart, some areas of oceanic lithosphere can be up to 200 kilometers thick. It is younger than continental lithosphere because most of it forms at mid-oceanic ridges, places on the ocean floor where new crust is formed and pushed out. Some areas are as young as 170 million years old. Oceanic lithosphere is composed of a very dense rock known as basalt. This rock is high in aluminum and magnesium and is often termed "sima." Part of the oceanic lithosphere is connected to the mantle; this segment is composed of peridotite.

Continental lithospheric plates are quite different than their oceanic counterparts. First, they are much older than the lithosphere below the ocean; some plates are billions of years old. The continental lithosphere is also much thicker than that in the ocean, at an average of 150 kilometers deep. Continental lithosphere is low in density, which allows it to remain buoyant and travel easily over the asthenosphere. The plates are composed of granite, which is high in aluminum and silica; because of this, the continental lithospheric plates are often called "sial" plates.