Our current laws of Physics recognize that heat is transferred by one of ...
conduction
convection
radiation.
Radiation is a mechanism for transferring heat through space. That mechanism doesn't really apply inside the "solid" Earth. While a certain amount of heat is transferred by conduction, the rock making up the bulk of the Earth is a poor conductor of heat.
The majority of the heat is transferred by the hot rock physically moving from the center of the earth toward the surface in the process called convection.
