Thorium is found at low levels in most rocks and soils, where it is about three times more abundant than uranium and about as abundant as lead. On average, soil commonly contains around 6 parts per million (ppm) thorium. Thorium occurs in several minerals ; the most common is the rare earth-thorium-phosphate mineral monazite, which contains up to 12% thorium oxide. Several countries have substantial deposits. Th decays very slowly (its half-life is about three times the age of the Earth). Other isotopes of thorium occur in the thorium and uranium decay chains. These are shorter-lived and hence much more radioactive than Th, though on a mass basis they are negligible.