Complex sentence: It was long a dormant volcano and believed by many geologists to be extinct having not erupted in over 10,000 years until it entered a new phase of activity by erupting on September 17,2006.
Split into simpler sentences: It was long a dormant volcano and believed by many geologists to be extinct; having not erupted in over 10,000 years. However, this view changed when it entered a new phase of activity by erupting on September 17,2006.