Would this fire-based magical catastrophe turn a forest region into a desert?

Question

A massive magic ritual involving the nation's greatest spellcasters utilizing an entire city as their focus, either through miscalculation or sabotage, inadvertently open a massive portal into the center of the plane of fire! Oh no!

This incredibly intense gout of arcane flame erupts from the portal like a geyser. The city and its inhabitants, at least those closer to the epicenter, are vaporized. The flames don't stop pouring out of the portal for days (weeks?).

Would this sustained source of fire be enough to turn a sizeable region from moderately forested to a desert an arid, sandy region? Bonus points if you can say whether it is possible for the heat to be so intense at the epicenter that it could become "glassed" (for lack of a better word).

EDIT: To make my question a little more specific, could magically-created, high intensity heat and flames do enough damage to the earth around it to turn a temperate/forested area into a sandy/rocky, arid region comparable to something like the Gobi desert or some of the deserts in Australia or would nature eventually do its best to revert the area to what it was before?

Answer 1

Yes, the epicentre is glassed, probably yes, you have a desert

Any fire hot enough to vaporise a city is hot enough to vaporise the inevitable stone or concrete components of that city. That implies horrendously hot fire, probably 3000 degrees Celsius or so. That will melt whatever rock substance is there, even if it's pure silica or pure alumina. With days or weeks of heat, the particles of the soil will be sintered into large agglomerates. The land may not be glossy, but it will be hard.

The reason it's probably now desertlike, at least for a few decades is that a) water is likely to run off and not be available for anything in the soil, b) the soil is completely inert fused rock, c) all the precious alkalis and so on that returning life will need are gone, having volatilised. EDIT: Even calcium is gone at 3000 degrees, let alone sodium and potassium. All you have is mullite, quartz and alumina, which are extremely inhospitable to life. Maybe a tiny bit of serpentinite at best.

Of course, the further out from the epicentre you are, the less damage there is. Merely burning down a forest probably will not result in desert, unless it was a region on the verge of desertification anyway; to make a desert, you need to really ruin the inorganic components of the soil, and that takes extreme heat, not a forest fire. So you need to decide how big the portal is compared to the forest.

PS It sounds like the elites of your world are as incompetent as ours.

Answer 2

Technically No

A desert has more to do with rainfall than living conditions. If it gets more than 250 mm (10 in) of rain per year, it is not a desert! The underlying assumptions are that this place still receives the rainfall it previously did and it is not a desert in the first place.

Ground Zero is Still A Horrible Place

Metal, glass, and rock are melted down and everything that is not a resolidified puddle has been vaporized. This is practically a desert with frequent flash floods. This water may be a danger to anyone and anything in the area as well as a major force of erosion as time goes on.

Local pioneering species will have a field day. These are plants and lichens that go in first after a disaster wipes away all the other life in the area. With ample rain, thermal cycling, and time, these will make the area habitable once again by other flora and fauna. At some point in the future, this area will be indistinguishable from the surrounding habitat.

By another other measure, ground zero is an uninhabitable wasteland for a long time after the accident. Chernobyl and similarly abandoned cities show how quickly nature can reclaim and transform places.

Answer 3

Mount St. Helens:

Heat and fire are funny things. The closest analogy to your event is a volcanic eruption like Pompeii, Krakatoa, or Mount St. Helens.

I had the privilege of visiting Mount St. Helens very shortly after it was re-opened to the public. Superheated pyroclastic flows, raining volcanic pumice, massive volcanic ash clouds and a huge explosion leveled a big part of the mountain. For many, many miles around, it was a scene of devastation - and this was a number of years after. The nearby lake was still apparently at a boiling temperature with the heat trapped under a massive cap of trees and landslide.

But the Pacific northwest is a wet area, and rains were already starting to trigger small plants to grow amongst the miles and miles of knocked-down and partially charred trees.

Short-term, the local environment will look like a moonscape. The temperature of your plane of fire will determine if the area is "glassed" over, but Pompeii has some gruesome scenes where (if I recall right) the fats from someone cooked out and then glassified (is that a word?) just from the heat of the pyroclastic flows. So yes, glass (but not clean glass like you'd imagine that's easily recognizable as glass).

But this is still a fairly local event. If ANYONE in a 20 mile radius lived, then the fundamental environment in that area will recover surprisingly fast. Seeds buried in the ground could easily survive, and farmers will actually burn fields to free up minerals in plants, so the remaining soil could certainly be enriched. Soils will likely retain significant amounts of water from rain if there is rain (although erosion will certainly be an issue). Birds will fly over and poop seeds into the virgin landscape.

Here's a great time-lapse film of the area of Mount St. Helens before, during, and after. In ten years, you'll have ground cover and small trees/bushes sprouting. In 20, you'll have small trees and the landscape will look mostly normal if you don't look too closely. And the completely denuded remains of Krakatoa are today a tropical rain forest with no easy evidence of the eruption 139 years later.