The rocks of Saudi Arabia range in age from the Precambrian to the present day, forming part of a larger unit that includes the Arabian Peninsula and is known as the Arabian Plate. Some Precambrian rocks in this region date back to the Archean (nearly 3 million years ago) but most are Neoproterozoic (1000-540 Ma*). They originated as volcanic islands or as chains of volcanoes along spreading centers and subduction zones in a Neoproterozoic ocean and against ancient continental margins, and were folded and uplifted toward the end of the Precambrian as a large belt of mountains. The mountains existed between about 680-540 Ma and were part of one of the largest mountain belts ever known to have existed on Earth. By the end of the Precambrian, the mountains had been eroded and only their roots are preserved, exposed in western Saudi Arabia in the Arabian shield.
The younger rocks in Saudi Arabia belong to the Paleozoic (540-250 Ma), Mesozoic (250-65 Ma), and Cenozoic (65 Ma to Recent) (collectively referred to as Phanerozoic cover), and crop out as relatively flat lying beds of sedimentary rocks such as sandstone, siltstone, limestone, and evaporites (salt deposits), and volcanic rocks. The rocks were deposited unconformably on the underlying Precambrian basement, in riverbeds, in glacial valleys, and in shallow seas, or were extruded from subaerial volcanoes. The rocks north and east of the Arabian shield are referred to as the Arabian Platform; those on the shield are mainly harrat (fields of Cenozoic flood basalt); and those west of the shield are Cenozoic rocks that occupy the Red Sea basin. The youngest deposits in the region include coral limestone and unconsolidated sand, silt, gravel, and sabkhah, which accumulated in the sand seas of Ar Rub al Khali and An Nafud, filled dried-up lake beds and wadis, and fringed the coastlines.
The Precambrian contain most of Saudi Arabia’s known metal deposits of gold, silver, copper, zinc, iron, and magnesium. The Phanerozoic cover contains the oil resources and deposits of bauxite (the source of aluminum), phosphate, clay, limestone, silica sand, and lightweight aggregate that are of increasing importance to the industrial development of the Kingdom.