Sorry for any errors; I'm Italian.
I'm working on a story where the Earth is devastated by a great geological cataclysm (strong earthquakes worldwide, volcanism bursting everywhere, large tsunami on all continents etc. ...).
My conclusion is that only two things could cause a catastrophe like this: tidal forces or a strong increase in the internal temperature of the planet.
The tidal forces are not well adapted to the story (although it is the most plausible solution), because any object capable of generating tidal forces strong enough to affect the Earth so much would alter the orbit of the planet, or destroy it completely.
So I focused on the heating. At the beginning I worked on research from 1997 which suggested that large clusters of dark matter, crossing the solar system could heat it (the dark matter would be captured by the planet and the sun, eventually reaching a critical mass and begin to annihilate, generating intense heat inside the planets and the sun). In this case I would only exaggerate the density of the agglomeration of dark matter, to immediately reach the critical mass and generate huge amounts of energy.
But looking more, I realized that this idea is quite outdated and so I shelved it, and have since been blocked because I can not find a plausible solution to the cataclysm (I could still use the dark matter, but the idea is not convincing), and I categorically refuse to use the phlebotinum (like mutants neutrinos of 2012).
So I was wondering, is there a plausible mechanism to heat the interior of the Earth, or at least to cause a cataclysm like the one described above without destroying the Earth? Or is my idea simply impossible?
PS: The cataclysm should not be fast; it happens over several years (nearly a decade). It began as a simple and small increase of geological activity, but then degenerated into a catastrophe that can seriously alter the Earth's geography.