Mormon tells us that the sign of the birth also confirms for many the validity of the signs of the coming of the Atoning Messiah to the New World. As Mormon describes this understanding, he does so in a formulaic way, citing “the whole earth from the west to the east, both in the land north and in the land south.” This phrase is a modifier of “the whole earth.” The statement of the directions had no more specific meaning than to indicate that this was visible in all the land.
How extensive was this “whole earth” that saw the sign? Of course we know that it really was not the “whole earth” as we conceive it. It was the whole earth as they conceived it. That earth was their known lands, defined by the directions. Even this is a cultural definition rather than a physical one. The best interpretation of this phenomenological event was that it was regionalized, but regionalized to the whole of the land that the Nephites considered “theirs.” The posited volcanic eruption would fit this description.