The first insight we get into what the Doom was is from the perspective of what the Doom is still doing to Valyria. From the description it sounds like there are active volcanoes both on land and under the sea so it is likely that volcanic activity occurred during the Doom.
Every man there knew that the Doom still ruled Valyria. The very sea there boiled and smoked, and the land was overrun with demons. It was said that any sailor who so much as glimpsed the fiery mountains of Valyria rising above the waves would soon die a dreadful death, yet the Crow's Eye had been there, and returned.
A Feast for Crows, The Reaver
Tyrion thinks about what he has read in the books about the Doom and from his description it sounds as if the Doom was Volcanoes erupting, earthquakes and the ground falling through. It also sounds like a tsunami might have occurred but it could just be a few big waves.
"Fourteen or fourteen thousand. What man dares count them? It is not wise for mortals to look too deeply at those fires, my friend. Those are the fires of god's own wrath, and no human flame can match them. We are small creatures, men."
"Some smaller than others." Valyria. It was written that on the day of Doom every hill for five hundred miles had split asunder to fill the air with ash and smoke and fire, blazes so hot and hungry that even the dragons in the sky were engulfed and consumed. Great rents had opened in the earth, swallowing palaces, temples, entire towns. Lakes boiled or turned to acid, mountains burst, fiery fountains spewed molten rock a thousand feet into the air, red clouds rained down dragonglass and the black blood of demons, and to the north the ground splintered and collapsed and fell in on itself and an angry sea came rushing in. The proudest city in all the world was gone in an instant, its fabled empire vanished in a day, the Lands of the Long Summer scorched and drowned and blighted.
An empire built on blood and fire. The Valyrians reaped the seed they had sown. "Does our captain mean to test the curse?"
A Dance with Dragons, Tyrion VIII
A description later on seems to confirm that a tsunami hit Valyria, though there could have been multiple of them.
On the day the Doom came to Valyria, it was said, a wall of water three hundred feet high had descended on the island, drowning hundreds of thousands of men, women, and children, leaving none to tell the tale but some fisherfolk who had been at sea and a handful of Velosi spearmen posted in a stout stone tower on the island's highest hill, who had seen the hills and valleys beneath them turn into a raging sea. Fair Velos with its palaces of cedar and pink marble had vanished in a heartbeat. On the north end of the island, the ancient brick walls and stepped pyramids of the slaver port Ghozai had suffered the same fate.
A Dance with Dragons, The Iron Suitor
The World of Ice and Fire also seems to hold that the Doom was volcanic eruptions and earthquakes though has some debates over what caused it.
And then, unexpected to all (save perhaps Aenar Targaryen and his maiden daughter Daenys the Dreamer), the Doom came to Valyria.
To this day, no one knows what caused the Doom. Most say that it was a natural cataclysm—a catastrophic explosion caused by the eruption of all Fourteen Flames together. Some septons, less wise, claim that the Valyrians brought the disaster on themselves for their promiscuous belief in a hundred gods and more, and in their godlessness they delved too deep and unleashed the fires of the Seven hells on the Freehold. A handful of maesters, influenced by fragments of the work of Septon Barth, hold that Valyria had used spells to tame the Fourteen Flames for thousands of years, that their ceaseless hunger for slaves and wealth was as much to sustain these spells as to expand their power, and that when at last those spells faltered, the cataclysm became inevitable.
Of these, some argue that it was the curse of Garin the Great at last coming to fruition. Others speak of the priests of R'hllor calling down the fire of their god in queer rituals. Some, wedding the fanciful notion of Valyrian magic to the reality of the ambitious great houses of Valyria, have argued that it was the constant whirl of conflict and deception amongst the great houses that might have led to the assassinations of too many of the reputed mages who renewed and maintained the rituals that banked the fires of the Fourteen Flames.
The one thing that can be said for certain is that it was a cataclysm such as the world had never seen. The ancient, mighty Freehold—home to dragons and to sorcerers of unrivaled skill—was shattered and destroyed within hours. It was written that every hill for five hundred miles split asunder to fill the air with ash and smoke and fire so hot and hungry that even the dragons in the sky were engulfed and consumed. Great rents opened in the earth, swallowing palaces, temples, and entire towns. Lakes boiled or turned to acid, mountains burst, fiery fountains spewed molten rock a thousand feet into the air, and red clouds rained down dragonglass and the black blood of demons. To the north, the ground splintered and collapsed and fell in on itself, and an angry sea came boiling in.
The World of Ice and Fire, Ancient History: The Doom of Valyria
The last paragraph in the above quote seems very similar to what Tyrion recalls so it seems as though Tyrion probably got his information from The World of Ice and Fire.
Later on Maester Yandel goes on to say the following, again saying fire played a part in the Doom.
The world has known ice in the Long Night, and it has known fire in the Doom. From the Frozen Shore to Asshai-by-the-Shadow, this world of ice and fire has revealed a rich and glorious history—although there is much yet to be discovered.
The World of Ice and Fire, The Glorious Reign
In conclusion the Doom of Valyria was a combination of many natural disasters occurring at the same time:
- Volcanic Eruptions
- Earthquakes
- Tsunamis
