As a volcano explodes, lightning is triggered by the buildup of static electricity in the ash cloud (Russell H. Ball. “An Hypothesis concerning the Three Days of Darkness among the Nephites.” Journal of Book of Mormon Studies, 1993, 2:1).
“One of the earliest accounts of these types of volcanic lightning displays comes from Pliny the younger, who observed the eruption of Mount Vesuvius in A.D. 79. Pliny writes:
“A fearful black cloud was rent by forked and quivering bursts of flame, and parted to reveal great tongues of fire, like flashes of lightning magnified in size.” (Bart J. Kowallis. “In the Thirty and Fourth Year: A Geologist’s View of the Great Destruction in 3 Nephi.” BYU Studies. 37, no. 3, 1997-98, pp. 155).
Lightning in and of itself would not have been particular remarkable in Mesoamerica. What is being noted in the Book of Mormon is something quite apart from the normal. Nephi describes this particular lightning as “exceedingly sharp,” and “such as never had been known in all the land.” Neither Nephi nor anyone alive at that time would have seen such an explosive volcano, and there may not have been one described in the Nephite records, as these volcanoes may lie dormant for hundreds of years before erupting, and the Mount St. Helens proved.