All of Saturn's larger satellites are made mostly of water ice with some lesser rock component. With the exception of Enceladus, whose surface appears to be virtually pure ice, these moons' surfaces are made of dirty ice, with models suggesting increasing amount of rocky material toward their centers. Enceladus, Tethys, and Dione all exhibit widely varied crater densities and large canyons, indicating that they have all been substantially affected by geologic activity. A likely geologic process on these icy bodies is cryovolcanism, which involves partial melting of an icy satellite's interior and resultant outpourings of icy liquids. Ammonia-water cryovolcanism is widely suspected on all three objects, because the amount of heat available to drive geologic activity there is very meager, and ammonia-water ices (an expected component of Saturnian satellites) have a very low melting point, 176 K (-97 oC).Enceladus has a density only slightly more than that of pure water and is the least dense Saturnian satellite, therefore the least rocky of all the icy satellites yet known. Evidence of geologic activity abounds; its ridged terrain, for example, is so sparsely cratered (of the icy satellites, only Europa is less cratered) that some geologists believe it may still be active. Enceladus also has some very heavily cratered terrain, and terrain with intermediate crater densities, indicating that it has had a protracted history of geologic activity. Tectonic activity is evidenced by mountains 1 to 2 km high, strike-slip faults, normal faults, and folded strata. All these indicate that large blocks of Enceladus' surface have shifted relative to one another. As with Io and Europa, the driving energy for this activity may be the tidal interactions from orbital resonance-in this case Dione and Saturn causing the tidal heating that led to partial melting and softening of the interior.