Complex sentence: Intervals between eruptions can range from 35 to 120 minutes, averaging 66.5 minutes in 1939, slowly increasing to an average of 90 minutes apart today, which may be the result of earthquakes affecting subterranean water levels.
Split into simpler sentences: Intervals between eruptions can range from 35 to 120 minutes, averaging 66.5 minutes in 1939, slowly increasing to an average of 90 minutes apart today. Over the years, the length of the interval has increased, which may be the result of earthquakes affecting subterranean water levels.