The breaking of the rocks is most reasonably the result of the slipping of a fault line, which generated the earthquake, and perhaps was the impetus for the eruption of the volcano, or volcanos, at that point. The quaking of the earth lasted for three hours. Although we do not know exactly how the Nephites divided time within a day, three hours is clearly longer than earthquakes, by themselves, would last.
Assuming Mesoamerica as a possible location for the Book of Mormon lands, those lands are along the Ring of Fire where there is a line of active volcanos, many of which erupted around the time of Christ (dating is often not precise). Because there is destruction in the land southward and the land northward, there may have been more than one volcanic event. Because we are seeing a retrospective view of these destructions, it is also possible that there were two eruptions at similar times, but need not have been exactly at the same time.
The important part of verse 19 is the reference to darkness, for that is the feature that is the conceptual opposite of the light at Christ’s birth. The day without a night is paralleled to the darkness on the land.