The continent's are composed of mainly granitic rock whose thickness varies between 10 and 70 kilometers. The thickest portions of continental crust are found under the various mountain ranges. Continental crust is also lighter than oceanic crust having a density of about 2.7 grams per cubic centimeter. Oceanic and continental rocks also differ from each other in terms of age. Continental crust contains some very old rocks that were formed during the Precambrian between 3 and 4 billion years ago.
Oceanic rocks are normally quite young deposits.
Variations in the age, density, and chemical composition of oceanic and continental crust suggest that these lithospheric deposits were created by different processes. The following discussion describes these differences. Continental Crust All of the Earth's continents have a core foundation that is made of mixtures of very old granite , gneiss , schist , sedimentary , and volcanic rocks. This core foundation is often referred to as a shield or basement rock . Rocks found in the shields were formed during the Precambrian and are some of the oldest rocks found on the Earth.