The earliest eruptions of the volcano occurred around one million years ago, explosive activity producing ignimbrite. The explosive eruption phase finished around 830,000 years before present, when the nature of the activity changed to a mix of pyroclastic and effusive, lava eruptions which built up the mountain and which have been dated to around 500,000 years ago. One of the explanations advanced for the Valle dei Grigi is that it is the result of the collapse of a sector of the mountain, as occurred during the 1980 eruption of Mount St. Helens and the 1792 eruption of Mount Unzen.