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At the end of the desert's westward Great River (think the Nile in Egypt) there is a volcanic island called Harek-Set. Its black mountain is a hotspot volcano which produces obsidian and makes the soil of the island highly fertile. The island is the jewel of the the native Sinis people who erected their capital city there as they conquered the Riverlands. On the northern bank of the River, overlooking Harek-Set, is the rival city of Gloriena, formerly known as Harek-Ra. To summarize the long and complicated political history, Moram, the new supposedly immortal pharaoh of the Sinis blew out the north side of the volcano with supposedly divine powers in order to destroy her enemies in Gloriena.

What I want to know is how plausible it is to have a volcanic eruption that does each of the following:

  1. This all happens in a mountain island that fits in between the branches of a river that forks because of the island before emptying into the ocean

  2. The initial eruption instantly buries Gloriena with stone and ash

  3. Does not necessarily destroy the city-state of Harek-Set, either in the initial blast or the aftermath - they were told to wear earplugs and stuff

  4. The smoke released taints the sky, changing the climate to the point of severely impacting, but not necessarily exterminating all peoples - collapse of most but not all civilizations preferred

  5. The Dark Age would last on a scale of centuries, bringing humanity (et al) to its lowest point a few centuries or even a thousand years later, then petering out after that as things slowly get better.

The last two points are the most important. A quick web search names Tambora as the worst eruption in history, having caused "The Year without a Summer" but that's not extreme enough. I don't want a year without a summer, I want centuries. But still with people and some megafauna around to see. I'm okay with lots of things going extinct, but not all human(oid)s.

Here's some things I think might happen as a result, but I'm not sure:

  • The ocean to the west and its people would be affected the worst, as the desert winds go out to sea, taking the initial dust cloud as well as the toxic fumes that slowly seep afterwards from the Broken Mountain there first
  • The northern branch of the river's mouth would become a bog, as it would be dammed by the stone (and now-constant lava flow?)
  • Agrarian cultures everywhere would fail due to weakened sunlight (and possibly acidic rain?) so people and animals would generally try to migrate to warmer regions with lots of groundwater
  • Rain may tend to be acidic, severely impacting the Ocean People in the west and the Forest People in the east, but possibly not affecting the Great River since much of its water comes through the mountains which may sufficiently filter out the contaminants - I don't know if a volcano that produces good soil would make for poison rain if too much of it got in the air
  • Meltwater would dry up as the years get colder, threatening the near-polar and snowcap mountain cultures
  • Forests would turn to rancid swamps as the dying trees rot, creating more food for fungus and grubs which then overproduce themselves, maybe completely restarting the ecology

After doing some cursory research on volcanic eruptions, I'm realizing that a big blast that destroys a city across the river without destroying the city right behind it is kinda lowballing if I want the aftereffects to be obvious for the entire world for centuries to come. And if I make the blast too big then it would destroy Harek-Set. And if it's just a slow burn then then it would only be a threat to Gloriena, not its swift and unstoppable destruction. I'm thinking now this situation would make sense if Moram set off the volcano, but couldn't stop it, so it just keeps on billowing smoke for years after the Black Day, like Laki which erupted for 8 months, making things worse and worse as the years continue to get colder.

For further context, the world is roughly Earthlike with the main focus, the Riverlands, being approximately where the Mediterranean is on Earth. The Ocean to the west may be the only ocean, and is presumed to be landlocked, although it or another ocean must exist in the east to create the moist landbreeze that waters the origins of the Great River as well as the forest beyond the mountains. I've sketched a couple maps on paper but can't figure out how to get the image onto my computer; will most likely update later

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    $\begingroup$ Toba catastrophe. $\endgroup$ – AlexP May 9 at 1:22
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    $\begingroup$ @AlexP - Ooh, there’s a lot of interesting stuff there. After looking into it, it seems that the Toba eruption didn’t actually cause the catastrophic dark age that’s often attributed to it, but rather happened coincidentally before a mild cold era that may or may not be related to the great human genetic bottleneck. Now I’m wondering just how powerful an eruption does need to be to cause a century-spanning dark age. But the notion that the cold era was already happening before the eruption is quite narratively interesting… $\endgroup$ – Maddock Emerson May 9 at 2:37
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    $\begingroup$ You're going to need a series of catastrophies. Volcanos cause a temperature drop thanks to ash, sulfuric acid and water vapor. The ash settles over just a few years(1 to 3) and the water joins the water cycle. The only thing that remains is the sulfur which turns into aerosols which even then have lifespans. You need a series of catastrophies. Perhaps your volcano is the first one. Note:There are volcanoes that constantly erupt, but these dont explosively throw ash into the stratosphere. They just ooze lava. $\endgroup$ – Hippeus_Lancer May 9 at 6:58
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If you want something that messes up with the climate for centuries, you are looking for something akin to the Deccan traps

The Deccan Traps is a large igneous province of west-central India (17–24°N, 73–74°E). They are one of the largest volcanic features on Earth. They consist of multiple layers of solidified flood basalt that together are more than 2,000 m (6,600 ft) thick, cover an area of c. 500,000 $km^2$ (200,000 sq mi), and have a volume of c. 1,000,000 $km^3$ (200,000 cu mi).

The Deccan Traps began forming 66.25 million years ago, at the end of the Cretaceous period. The bulk of the volcanic eruption occurred at the Western Ghats some 66 million years ago. This series of eruptions may have lasted fewer than 30,000 years.

The original area covered by the lava flows is estimated to have been as large as 1.5 million $km^2$ (0.58 million sq mi), approximately half the size of modern India.

The release of volcanic gases, particularly sulfur dioxide, during the formation of the traps may have contributed to climate change. Data points to an average drop in temperature of about 2 °C (3.6 °F) in this period.

About the formation of such feature

It is postulated that the Deccan Traps eruption was associated with a deep mantle plume. The area of long-term eruption (the hotspot), known as the Réunion hotspot, is suspected of both causing the Deccan Traps eruption and opening the rift that once separated the Seychelles plateau from India. Seafloor spreading at the boundary between the Indian and African Plates subsequently pushed India north over the plume, which now lies under Réunion island in the Indian Ocean, southwest of India. The mantle plume model has, however, been challenged.

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You might want to look into 536 ad. A volcanic eruption caused crop failures, plagues, and societal collapses from Mexico to Mongolia.

Some people attribute the onset of the dark age to this.

https://www.history.com/news/536-volcanic-eruption-fog-eclipse-worst-year

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