3 Nephi 8:20. Thick Darkness
If these phenomena were real, should we not expect them to be described in such terms? The gospels tell us that there was darkness in Jerusalem at the time of the crucifixion (Matthew 27:45; Mark 15:33; Luke 23:44-45). Was that story borrowed from Exodus also?
The thick darkness and other cataclysmic phenomena recorded in 3 Nephi 8 are typical of volcanic eruptions. See the discussion in John A. Tvedtnes, “Historical Parallels to the Destruction at the Time of the Crucifixion,” in Journal of Book of Mormon Studies 3/1 (Spring 1994) and Bart J. Kowallis, “In the Thirty and Fourth Year: A Geologist’s View of the Great Destruction in 3 Nephi,” BYU Studies 37/3 (1997-98).