What is the continuous external energy source powering geological activity on small moons?

Tidal heating delivered by their massive planetary hosts.

Geological activity, which includes processes like volcanism and tectonic shifts, requires a sustained source of internal heat. While large terrestrial bodies like Earth rely on the slow cooling after accretion and the ongoing decay of long-lived radioactive isotopes in their core and mantle, smaller celestial bodies lose internal heat much faster due to their high surface area to volume ratio. For these smaller moons orbiting giant planets, the key driver supplying the necessary massive, persistent energy is tidal heating. This external energy source, derived from the gravitational interactions with the massive planet, decouples the moon's geological vitality from its small physical size, keeping its interior active over vast timescales.