Radioactive potassium may be major heat source in Earth's core
From EurekAlert:
http://www.eurekalert.org/pub_releases/2003-12/uoc--rpm121203.php
San Francisco - Radioactive potassium, common enough on Earth to make potassium-rich bananas one of the "hottest" foods around, appears also to be a substantial source of heat in the Earth's core, according to recent experiments by University of California, Berkeley, geophysicists.
Radioactive potassium, uranium and thorium are thought to be the three main sources of heat in the Earth's interior, aside from that generated by the formation of the planet. Together, the heat keeps the mantle actively churning and the core generating a protective magnetic field.