As a volcano explodes, lightning is triggered by the buildup of static electricity in the ash cloud. Kowallis quotes Pliny the Younger, who recorded his observations of Mount Vesuvius’s eruption in A.D. 79: “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.”
Lightning was not particularly remarkable in Mesoamerica, but Nephi describes this particular lightning as abnormal—”exceedingly sharp” and “such as never had been known in all the land.” Neither Nephi nor anyone alive at that time had apparently witnessed an erupting volcano, and perhaps their records, even over six hundred years, did not include such a description. Volcanoes may lie dormant for hundreds of years before erupting, as Mount St. Helens proved.