How do calamities work, again?

[Zohar_Lahar]

I don't have quotes at hand, but from what I understand the Seventh Umbral Calamity was indeed a Calamity with the associated Rejoining.

I think the Rejoining occurred when Bahamut started breaking free of Dalamud. For millenia it had been fettered by the neurolinks, clearly designed to withstand the Dreadwyrm's power. Once the Seventh world started rejoining, that cascade of aether probably enhanced his megaflare to the point of overwhelming the satellite, causing Dalamud to violently disintegrate. And since the Seventh shard was overaspected to Darkness -- all six elements astrally charged -- this also augmented the secondary effects of the impacting debris and rampaging primale to causing powerful megatsunamis, rifts opening up in the earth, forming glaciers, and other crazy effects.