oldest rocks on Arrakis have been dated, isotopically, as being 1 1 eons in age (1 eon = 1 billion standard years) These older rocks have been found only by deep drilling ami furthermore have been located only in a few places, existing as small fragments contained in much younger rock Almost all other planets of the same classification have rock exposed at the surface with ages greater than 2 I eons The evidence that geologic processes on Arrakis are much more dynamic in the destruction of ancient rock and the formation of new rock and surface features is thus convincing The ancient Terrans held a simplistic view of planetary genesis still accepted today in most respects Thev believed that planets were formed by accretion of smaller fragments dust, gas and ices remaining about a star after or during stellar formation As each planet grew, its interior warmed through gravitational contraction with heat released by radioactive decay The surfaces and mten ors of Neta-class (modern terminology) plan ets soon became molten After about one eon sufficient cooling occurred to form a solid crust Convection in the planet's mol ten mtenor caused continued breakup of the crust and formation of new crust i^mubt like scum floating on the surface of a bubbling pot of molten fanmetal) As cooling contin ued the crust thickened to the pomt where it became stable and a basically permanent feature This typically occurred about 1 5 eons after formation Eventually the mtenor became only quasi molten However, heat generated by radioac live decay continued to cause a slow convec tion which m turn produced on-going dis turbances within the crustal material These included volcanic eruptions quakes and move ment of portions of the crust with respect to the other portions such phenomena coatmu mg to the present The Terrans called these crustal motions piate tectonics and, as noted above, ascribed the heat energy from radioactivity to be the driving energy source From the relatively few scientific records remaining from Terran times it appears that this explanation was universally accepted However, oi the tojr Neta-class planets in their planetary system only one exhibits plate tectonics behavior Today we know exactly what causes plate tectonics, why only some Neta planets exhibit plate tectonics behavior, and hence why Arrakis is more technically active man any other ARRAKIS GEOLOGY 36 ARRAKIS GEOLOGY Neta planet Bnaceret has summanzed cur rent understanding Heat from radioactive decay by itself iis insufficient to cause crusta] (plate) movements Also, internal heat is not necessary although in all cases internal heating results from action of the mechanism primarily responsible for plate motions Two mechanisms are known convection currents resulting from severe internal heating and external gravitational torques In some cases both mechanisms play a major role The former is the sole mechanism for only a few planets, none of which exist in the T