Uranium is a naturally occurring element that can be found in low levels within all rock, soil, and water. Uranium is the 51st element in order of abundance in the Earth's crust. Uranium is also the highest-numbered element to be found naturally in significant quantities on Earth and is almost always found combined with other elements.[10] Along with all elements having atomic weights higher than that of iron, it is only naturally formed in supernovae.[51] The decay of uranium, thorium, and potassium-40 in the Earth's mantle is thought to be the main source of heat[52][53] that keeps the outer core liquid and drives mantle convection, which in turn drives plate tectonics.
Uranium's average concentration in the Earth's crust is (depending on the reference) 2 to 4 parts per million,[9][20] or about 40 times as abundant as silver.[15] The Earth's crust from the surface to 25 km (15 mi) down is calculated to contain 1017 kg (2×1017 lb) of uranium while the oceans may contain 1013 kg (2×1013 lb).[9] The concentration of uranium in soil ranges from 0.7 to 11 parts per million (up to 15 parts per million in farmland soil due to use of phosphate fertilizers), and its concentration in sea water is 3 parts per billion.[20]
Uranium is more plentiful than antimony, tin, cadmium, mercury, or silver, and it is about as abundant as arsenic or molybdenum.[10][20] Uranium is found in hundreds of minerals, including uraninite (the most common uranium ore), carnotite, autunite, uranophane, torbernite, and coffinite.[10] Significant concentrations of uranium occur in some substances such as phosphate rock deposits, and minerals such as lignite, and monazite sands in uranium-rich ores[10] (it is recovered commercially from sources with as little as 0.1% uranium[15]).
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