Complex sentence: Less likely would be a resumption of cone-building activity and the creation of a coalescing chain of small stratovolcanoes, which could, in time, merge to reform Mazama to an approximation of its former size.
Split into simpler sentences: Less likely would be the resumption of cone-building activity and the creation of a coalescing chain of small stratovolcanoes. These could, in time, merge to reform Mazama to an approximation of its former size.