Complex sentence: During the late Pleistocene and Holocene periods, there have been six eruptive episodes: four of them rhyolitic, on the eastern half of the caldera; and two basaltic, on the flanks.
Split into simpler sentences: During the late Pleistocene and Holocene periods, there have been six eruptive episodes. Four of them rhyolitic, on the eastern half of the caldera, and two basaltic, on the flanks.