Now before everyone screams ASB, my question is actually about whether or not a natural disaster such as Krakatoa could be butterflied away, or if it would still happen, albeit differently in an altered TL because a natural disaster is just that, something which cannot really be controlled or stopped.
So of course, depending on how far back the POD is, lets say mid-1800s, such as 1850s, could Krakatoa be averted, or would it happen earlier or later, and varying in strength, perhaps. Or would it happen at the same time regardless?
In general, I think it would be difficult to butterfly a
geological disaster as opposed to a
meteorological one. Weather patterns have to account for things like temperature, air pressure, ocean currents (for hurricanes) and man-made factors such as pollutants and greenhouse gases from industry and agriculture, etc., in addition to many other factors I'm probably missing.
Volcanoes, if I remember my geology class correctly, tend to erupt when "sticky" silicates build up inside of the volcano, creating pressure which is eventually later in a violent eruption. To avert those entirely or for a significant length of time would be ASB to me.
Earthquakes occur when faults slip, though they can also happen when fluids are pumped into the ground (see the disposal of chemicals from the Rocky Mountain Arsenal, the controversy over "fracking", etc.). Unless you are setting a story with a 20th century or later POD, I would think changing earthquake would be ASB too. Even then would be stretching it too.
I think your Krakatoa story would be best suited for the ASB forum, but feel free to dispute me. I'm going off from half-remembered science classes.
Remember, Alien Space Bats is
not an insult, just a measure of plausibility.