Original: From December 7, 1989 to January 6, 1990, and from November 5, 1990 to February 14, 1991, the mountain erupted with sometimes huge clouds of ash.
Simplified: From December 7, 1989 to January 6, 1990, and from November 5, 1990 to February 14, 1991, the volcano erupted with sometimes huge clouds of ash.