Oldest description of volcanic lightning

In regular thunderstorms, lighting results from the generation and separation of electrical charge owing to collisions between ice particles. In some volcanic eruptions, i.e., ones that include a significant ash plume, the electrical charge is generated by collisions among rock and ash in the plume. Water vapour released in eruptions may also play a part in this mechanism. Volcanic lightning is also known as "dirty thunderstorms". The oldest known reference to this phenomenon is found in the writings of Roman statesman and historian Pliny the Younger. As a teenager, Pliny witnessed the 79 CE eruption of Mount Vesuvius from his family home on the shores of the Bay of Naples. He had waited until it was almost too late before fleeing his home because he was waiting for the return of his uncle (and adopted father), Pliny the Elder. Pliny later learned that his uncle had been overcome by noxious fumes and died while organizing the evacuation of Stabiae, a town near Pompeii.

The description of volcanic lightning comes from a pair of letters written around 104 CE to his friend, the historian Publius Cornelius Tacitus. The second of these contains the following passage:

“The sea seemed to roll back upon itself, and to be driven from its banks by the convulsive motion of the earth; it is certain at least the shore was considerably enlarged, and several sea animals were left upon it. On the other side, a black and dreadful cloud, broken with rapid, zigzag flashes, revealed behind it variously shaped masses of flame: these last were like sheet-lightning, but much larger”.

Based on Pliny the Younger's correspondence, it has long been held that Vesuvius erupted on 24 August 79 CE. However, in October 2018, archaeologists at Pompeii announced they had discovered a charcoal inscription on an excavated wall dating to 17 October which implies the eruption may have taken place slightly later than we believed.