|MadSci Network: Earth Sciences|
You ask a very good question. Lets clarify some terms before we discuss your question.
Geologists use two general terms when discussion molten rock - magma, which is molten rock that is not of has not been erupted onto the earthís surface, and lava- molten rock which is or has been erupted onto the earthís surface. And, as you are probably aware, the earth is made up of at least four concentric zones; the solid inner core (780 miles thick), the liquid outer core (2100 miles thick), the partially solid - partially liquid mantle (3900 miles thick), and the outer-most layer, the crust (10 to 25 miles thick).
The production of internal heat on the earth is actually thought to be due to two very different processes. The first, as you have asked in your question, is radioactive decay. The second is friction, due to flexing of the earth due to tidal forces caused by the gravitational attraction between the moon and the earth. So the process of creating magmas and lavas is not completely be due to radioactive decay. Many different and not well understood processes come into play when magmas and lavas are produced. One of the most controlling is due to the very high pressures exerted by having thousandís of miles of rock thickness overlying these rocks. High pressures can cause rock material to stay solid at temperatures that, if the rocks were at the earthí s surface, they would melt.
Many unstable elements can undergo spontaneous radioactive decay, they are called radioisotopes. This decay produces heat. This heat may or may not be radioactive. In the earthís crust, radioisotopes are concentrated in the outermost zone of the solid earth. Here they generate heat to a degree that can maintain deep-seated rock close to its melting point. Unequal heating produces differences in rock density and these density differences cause some of these huge rock masses to rise or sink. Local pockets of magma are formed due to changes in the pressures and temperatures and are forced up to the earthís surface, making volcanic eruptions (extruding lavas) and making new rocks. Because of the very thick layers of rocks that a magma has to travel through to get to the earthís surface (very long times of travel), most residual radioactivity (from radioisotopes contained in the magma) has decreased by the time the magma has reached the earthís surface. In addition, many lavas actually do have some low level of radioactivity.
You may wish to do further research on your topic. Any good public, high school, or college library will have a variety of Physical Geology textbooks that will have good information on the Earthís internal production of heat.
Try the links in the MadSci Library for more information on Earth Sciences.