Geothermal activity

The largest known meltdown on earth is not what happened in nuclear plant accidents, but it is the earth itself. Since the formation of the earth 4.5 billion years the core of the planet is releasing energy to the outer space.
On average it is estimated that the geothermal heat flow is 0.082 W m^-2 at the surface of the earth, with geographical differences between 0.065 and 0.101 W m^-2 (Pollack et al.).
While this geothermal heat flow is very small as compared to solar input, it is 2.6 times more than what humans currently release as primary energy. Also, it plays a role in the movement of glaciers: the mean geothermal heat flux can melt approximately 8 litres m^-2 a^-1, a kind of lubrication for the flow of glaciers towards warmer valleys and oceans.

Volcanic eruptions release earth's internal energy in rare outbursts, with relatively limited thermal impact, but important emissions of dust. These particulates remain in the atmosphere and block a part of the solar irradiation before it can hit the surface. A median transmittance is recovered after a few years, without long term variations, as seen in Figure 13.

Transmittance of solar radiation in the atmosphere with indication of major volcanic events.
Source: NOAA

Tectonic movements

Continental plates are constantly moving and crushing into each other.
Horizontal motions are of the order of a few cm per year, vertically shift of up to 10 mm a^‑1 can be observed.
By itself, an impact on climate cannot be expected, but it may have an effect on sea level, independently of other factors.

Meteorites

The earth is constantly hit by small meteorites. As long as their size remains modest they volatilize in the atmosphere or hit the ground with some local damages. But a major event may change the whole picture, as it is commonly assumed to have been the case 66 million years ago, at the end of the Cretaceous, when dinosaurs became suddenly extinct.