The half life of uranium is 760 million years. If the age of the earth is 4.2 billion years, then 4.2/.76 = about 6 doublings or about 64 times as much uranium existed on earth at creation as exists now. Uranium is rather heavy. Why doesn't it sink and become more concentrated at the core? Would this high concentration uranium cause early Earth's volcanoes?

rocky Blaine DeSantis

Dear Blaine,

There are three different isotopes of uranium. They are:

uranium-234: half life = 244 thousand years, 0.0055% of all uranium.
uranium-235: half life = 704 million years, 0.72% of all uranium.
uranium-238: half life = 4.5 billion years, 99.28% of all uranium.

Half-life is the length of time required for half of a given number of initial number of atoms of that isotope to decay.

Elements with short half-lifes, like uranium-235 and potassium-40 (half-life = 1.25 billion years), were important in the early thermal history of the Earth. About 2.5 billion years ago, elements with longer half-lives, like uranium-238, became more important. Heat production was about 4 times greater 4 billion years ago than it is today. Part of that high heat production is from the decay of the elements with short half-lifes. The other part of that heat was from the accretion of the Earth and the formation of the core. Estimates suggests heat production was divided equally between radioactive decay and accretion.

You are correct that the atomic weight of uranium is high. Other factors are equally if not more important in controlling the distribution of elements in the Earth. The size and charge of the uranium atom makes it difficult for the atom to fit in most minerals that are common in the mantle. Thus, when a part of the mantle melts, the uranium goes in the melt. The melt migrates towards the surface. So most of the uranium is carried to the crust by magma. Roughly two-thirds of the mantle has been cycled through volcanic systems so most of the uranium (and potassium) has been removed from the mantle and placed in the crust. Heat production estimates are 100-500 times higher for crustal rocks compared to mantle rocks.

With all the extra heat volcanism on the early Earth must have been incredible. Some models propose a surface covered by a magma ocean. Some types of lava were only erupted early in earth history when temperatures were higher; once the Earth cooled passed a critical temperature they were no longer erupted. Since very old rocks are rare we don't know as much as we'd like about this early period of Earth's history.

Thanks for a good question.

Steve Mattox, University of North Dakota

Sources of Information:
Francis, P., 1994, Volcanoes a planetary perspective: Oxford University Press, New York, 443 p.

Henderson, P., 1982, Inorganic Geochemistry: Pergamon Press, New York, 353 p.

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