Complex sentence: Most if not all geologists now accept that the tephra or pumice fall from the eruption was far greater than previously thought (i.e.150 cubic km instead of 14); this means the event would have been visible from China and Rome.
Split into simpler sentences: Most if not all geologists now accept that the tephra or pumice fall from the eruption was far greater than previously thought, approximately instead of. This means the event would have been visible from China and Rome.