دانلود کتاب ذخایر و رخدادهای معدنی در سپر عرب-نوبی

There is a general consensus that the different parts of the continental crust merged together in their entirety three times throughout the Earth’s history, forming three giant continents, namely “Columbia,” “Rodinia,” and “Pangaea.” In addition to these three giant continents (Supercontinents), the process of assemblies had taken place several times during the geologic history, which assembled “some” (but not all) parts of the continental crust and resulted in the formation of large “Cratons” and continental blocks in a period of time prior to the formation of the giant “Colombia” supercontinent. These “Supercontinents” and “Cratons” constitute themajority of the ancient Earth’s surface and play an important role in Earth’s history. The continental “Cratons,” also called “proto-continents,” are large and stable continental masses that have been stable over a billions of years. Cratonic areas have a thick crust and deep lithospheric roots that reached few hundreds kilometers into the Earth’s mantle to make the cratons have long-time stability. The exposed Precambrian crystalline basement and high-grade metamorphic rocks of the cratonic areas are called “Shields,” whereas the extensive sedimentary cover of these areas is called “Platforms.” Both shields and platforms of specific continental masses form the cratons. Cratonic areas are economically important where most of the world’s diamonds and many other valuable mineral deposits are produced from cratonic areas. Furthermore, these old cratonic masses store a complete record of the long Earth’s history, which provide us with great information.