Everybody knows about the great cataclysms of Greyhawk: the Devastation which was Invoked and the Rain of Fire Sans Color. But in the first published version of Greyhawk--that of Andre Norton's novel Quag Keep--she took a different tack with her conflagrations. As with much of her tome, it is obvious that she was working off a much earlier version of Greyhawk than was published in the 1980 folio. Likewise, she was not privvy to Big G's Brief History of Eastern Oerik. This is purely conjecture but I suspect it was not much developed at the time she sat down to write Quag Keep so we can give her some leeway for diverging from the script.
First up, the Plague of Fire: Not a rain of fire unleashed by Oeridian mages, rather the plague of Fire "breathed forth" from somewhere--I've conjectured elsewhere that it was a fleet of dragons, which isn't great but you're free to make up a better source. And the Plague of Fire didn't cause the Sea of Dust to come into existence, rather it destroyed the Kingdom of Kalastros which was located, I think, where the Dry Steppes would be in the Darlene maps. Indeed, the Sea of Dust is a much cooler locale in Nortonian Greyhawk, being a flowing body of fine particles that shift about in waves much like an ocean. Indeed, an ancient society once plied these sand waves on sailing vessels. If you don't think that's awesome then I doubt you're even reading this.
The second cataclysm is the Rieving of Keo the Less. The text describes this event even less than the plague of fire so it's very hard to determine what exactly this event entailed or even whether Keo is a place--as Greyhawkians will assume--or a personage of some sort; perhaps a maleficent demon or demigod of baleful intent. What we can decipher from the text is that, during a time of warfare, some really bad stuff went down resulting in the horrifying deaths of many, many people. It can be insinuated that the death and suffering meted out by this event was unspeakably terrifying. And the landscape is now devoid of human life. The city of Var, at least, is now lifeless ruin.
In neither case do we know who or why the cataclysm occurred. And they do not appear to be reciprocal events, as the Plague of Fire seems to be a thing of relative antiquity while the Rieving happened within living memory.
And the problem is somewhat exacerbated by the fact that "rieving" is not an English word. It does look kind of like "reave" which means to plunder, rieve is not a word. Perhaps Norton created her own word playing on grieving and reaving to create something new and awful sounding.
Though not quite a cataclysm in the sense that it doesn't seem to have altered the landscape or resulted in massive, widespread death, there is also a third major historical event of Nortonian G'hawk worth noting: the Harrowing of Ironnose. It was an airborne--and likely toxic--event involving the mighty dragon Lichis the Golden and the demon Ironnose which occurred over the Great Bay and continued on as far as the Wild Coast. The fight resulted in the defeat of Ironnose and retirement of Lichis to the Southern Mountains, where she retired from the activity of humans henceforth--at least until a gang of adventurers intent on finding the legendary Quag Keep--for no discernible reason--somehow coax her into aiding them.
"Harrowing" has a few possible meanings: to cultivate a field with a harrow--probably not what Norton had in mind--but also to plunder, and, strangely, to rieve! Not really, but it does mean to torment or vex, which seems the likeliest suspect.
Fantastical events events like the Harrowing--a public brawl between a dragon and a demon--are sadly lacking from Gygax's low fantasy version of Greyhawk. And so different are the cataclysms of Nortonian Greyhawk--I've seen it labelled Quaghawk somewhere--that it seems likely that she was unaware of Gygax's notion of great cataclysms and perhaps--dare I say it--Gygax was inspired by Norton to include such events in his history of the Flanaess.