A geology question

[einsteinsdog]

Why hasnt the uranium in the earths crust sunk to the core over millions of years as it is heavier than iron which makes up the core

[Clanad]

Of the three different types of Uranium, (234, 235 and 238), 234 and 235 have fairly low half lives, 244 thousand years and 704 million years respectively. The early decay of the former two result in lead isotopes, which are found at or near the Earth's core. In fact they are used to help determine the age of the Earth and contribute to the high temperatures found at the core, according to current thought.
Decay of U-238 contributes to heat current production . Because of the heat forming circulating magma and because of the unique atomic structure of U, most of the U is cycled upwards in the magma flow. Over the eons, roughly 65 percent of the mantle has been cycled through volcanic systems magma, so most of the U has been removed from the mantle and placed in the crust. Good question...