Complex sentence: The caldera was created by a massive Phreatic eruption about 3100 years ago that generated about 5 cubic kilometers of pyroclastic flow and covered the surrounding area in volcanic ash up to 20 cm deep.
Split into simpler sentences: The caldera was created by a massive Phreatic eruption about 3100 years ago that generated about 5 cubic kilometers of pyroclastic flow. After the eruption, the area was covered by volcanic ash up to 20 cm deep.