Complex sentence: Evidence shows that the eruption was a single event, that lasted 2 to 4 days initiated by a Plinian eruption, triggered by abrupt changes in composition, properties and physical state in the melt or overpressure in the magma chamber.
Split into simpler sentences: Evidence shows that the eruption was a single event lasting 2 to 4 days. It was triggered by abrupt changes in composition, properties and physical state in the melt or overpressure in the magma chamber.