How do calamities work, again?

[Ilenya]

Elidibus mentions it at some point during 2.1 - 2.4, during the Warriors of Darkness' mini-arc in Heavensward. I don't have the exact line of dialogue, if that's what you want, but it's there.

Mostly what I am seeing is less "Flood is bad" and more "Flood is poorly timed" which for the Ascians amounts to the same thing, but it's useful to at least have some sort of clarification on it. I imagine as a whole the Floods are what they want, but it needs to be set up properly ahead of time. Make the cracks, then let the Flood happen. Ardbert and co just did their jobs too well.

Was it mentioned how exactly these cracks were made? Were they just events leading up to it that allowed it? So for Bahamut's, things like what became the Burn would be starting signs of the Seventh crack, while for the attempted eighth it'd be...Shinryu/Omega due to timing, I guess? Or is this all stuff that happens behind the scenes and not noticeable until the aether starts to change.

Edit: I realize that the cracking could have been the Battle at Silvertear Falls for the Seventh Calamity. Probably fits better in the timeline, but still hard to say.

[Daralii]

Mostly what I am seeing is less "Flood is bad" and more "Flood is poorly timed" which for the Ascians amounts to the same thing, but it's useful to at least have some sort of clarification on it. I imagine as a whole the Floods are what they want, but it needs to be set up properly ahead of time. Make the cracks, then let the Flood happen. Ardbert and co just did their jobs too well.

Was it mentioned how exactly these cracks were made? Were they just events leading up to it that allowed it? So for Bahamut's, things like what became the Burn would be starting signs of the Seventh crack, while for the attempted eighth it'd be...Shinryu/Omega due to timing, I guess? Or is this all stuff that happens behind the scenes and not noticeable until the aether starts to change.

Edit: I realize that the cracking could have been the Battle at Silvertear Falls for the Seventh Calamity. Probably fits better in the timeline, but still hard to say.

Likely the battles for Ala Mhigo and Doma, as well as the rebellions that followed in their wake. I think the thinning of aether was only brought up after 4.0.

Going back to Heavensward, Ardbert was pissed at Elidibus because the First didn't have time for the gradual ramp up toward a calamitous summoning, so Urianger proposed that there was no surer way to instantly swing the scale far enough than to kill the realm's beacon of hope. It was obviously to manipulate Ardbert's group to bring six crystal bearers together and stop the Flood, but considering Elidibus had arrived at the same conclusion by 4.56(after Gaius and his group coincidentally destroyed production facilities to push back the use of Black Rose), I doubt that was a flat out lie on Urianger's part.

[Cilia]

Was it mentioned how exactly these cracks were made? Were they just events leading up to it that allowed it? So for Bahamut's, things like what became the Burn would be starting signs of the Seventh crack, while for the attempted eighth it'd be...Shinryu/Omega due to timing, I guess? Or is this all stuff that happens behind the scenes and not noticeable until the aether starts to change.

Edit: I realize that the cracking could have been the Battle at Silvertear Falls for the Seventh Calamity. Probably fits better in the timeline, but still hard to say.

Crystal Exarch G'raha explains the process of Rejoining in detail after you collect Alphinaud and Alisae for the Shadowbringers campaign, but to put it shortly, there is no singular event on the Source or Shard that signifies the barriers as having cracked. Finding this out is simply a matter of observing aetherial activity, as the excess elemental aether creates disturbances on the shard and Source both. The excess Light on the First led to the Flood of Light (a century ago), the infestation of Sin Eaters, and the perpetual daytime that lasts until the Lightwardens are vanquished; on the Source it was observed by the thinning of aether, evidenced by deaspected crystals popping up (as excess Light aether essentially nullifies aetherial activity). The effects of that elemental aether are also magnified on the Source while the shard is imbalanced, which is why the Black Rose poison was so potent in the bad future Crystal Exarch G'raha hails from.

"Cracking" the barrier between Source and shard is simply a matter of letting the excess elemental aether from a shard trickle into the Source for long enough. This eventually causes a Calamity on the Source, which is a result of the shard collapsing and all the excess elemental aether being unleashed upon the Source.

To begin at the beginning, then... In the ancient past, a single star was divided into fourteen worlds. [...] Though physically separate, they retain a connection to each other, and the Source especially. Now, let us assume that a given element in one of the shards attains abnormal ascendancy. Just as water will flow from the highest point to the lowest, the excess energy will begin trickling into the Source... And such an influx of aether will of course exert a palpable influence. If the element in question were fire, then drought and wild fires might ensue. If it were ice, one might expect the weather to turn bitterly cold. As aether continues to pour in, such phenomena will become more and more extreme, until eventually, a single untimely event triggers a disaster which "cracks" the barrier dividing the two worlds. What was once a trickle now becomes a deluge, sweeping the shard along to be rejoined with the Source. At the same time, the element which held sway in the shard is unleashed in full, its energies amplifying the original disaster to truly catastrophic proportions. An earthquake thus magnified might strike with enough force to shatter continents; a tidal wave might swell to a size capable of drowning entire nations. These devastating events are what we refer to as "Umbral Calamities."

This is the clearest picture we have of the process of Rejoining.