Complex sentence: The whole mountain has been completely eroded by glaciers to an elevation of twice during its lifetime and the current cone was built during the most recent major eruptive period 40,000-10,000 years ago.
Split into simpler sentences: The whole mountain has been completely eroded by glaciers to an elevation of twice during its lifetime. The current cone was built during the most recent major eruptive period 40,000-10,000 years ago.