I've been doing some speculating, and came up with this in Blender:
The Unobtanium conducts electricity, and therefore via geodynamo theory, creates a magnetic field, protecting Kerbals. Its ultra high density results in it sinking to the core. The Mantle of Explodium helps explain the abundance of rocket fuel on Kerbin, and possibly the theorized vulcanism! The Ore Crust is a result of an earlier impact, early when Kerbin resembled Dres, and was seismically inactive, when an asteroid impacted Kerbin, disturbing a large mass east of the 'Keurasia' continent, antipodal to the new huge crater, which formed into a new body, Mun. The debris cloud formed a lithobraking medium to slow down a passing comet, hence known as 'Minmus'. The asteroid liquefied instantly, and was spread in the upper mantle, and lower crust. The seismic consequences of this impact disturbed the inert interior, kick-starting seismic activity.
(Yes, that is Kerbin, you can see the KSC continent and the spain analog on the right. If anyone wants the blend file, or the original image at a different res, just ask.)