Complex sentence: At about 13:00 (local time) on 26 August, the volcano went into its paroxysmal phase, and by 14:00 observers could see a black cloud of ash high.
Split into simpler sentences: At about 13:00 (local time) on 26 August, the volcano went into its paroxysmal phase. By 14:00 observers could see a black cloud of ash high.