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What would happen to the land under a fantasy-style floating city?

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One day, a rare and powerful magic lifts a chunk of land off of the ground and into the air. You can think of the magic as a sort of flat bottomed bowl or circular cake-pan.

It is about 50 meters deep, has a flat bottom (relative to gravity), and a radius of about 2.5km. The magic is invisible and impenetrable (things can anchor to the top edges, but water and soil cannot fall through it).

The magic keeps it in "geosynchronous orbit" about 300 meters above the previous ground level. The city is in European latitudes (~48deg N) and is in a mixed forest biome like southern Germany or central France. They have an early renaissance level technology. Magic is extremely rare and limited to once-in-a-century miraculous events like this floating city. It is extremely durable though. Almost everyone is certain that the city will remain afloat for millennia at least.

I expect that initially, the edges of the hole will collapse into something a little more crater shaped. Water is likely to rush in (either groundwater or from water sources that flowed into the now airborne city), forming a shallow lake.

After that, it's hard for me to guess at things like the erosion pattern, how the shadow of the city would affect things even before the human element gets involved.

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    $\begingroup$ 50 meters means effectively no water aquifer so they better invest heavily in minimizing water usage. $\endgroup$
    – John
    Commented Oct 10, 2023 at 23:08
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    $\begingroup$ was their a river running through the city like most cities at the time? if so yes you get a lake very quickly. $\endgroup$
    – John
    Commented Oct 10, 2023 at 23:12
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    $\begingroup$ You say that a piece of land was lifted up. Wouldn’t it be subject to erosion, or does the magic protect against that? $\endgroup$
    – Neil Iyer
    Commented Oct 11, 2023 at 2:31
  • $\begingroup$ The help center prohibits giving your own answer and expecting more. What is your goal for asking this quesiton? What's wrong with the answer you already have? Do you really need to know the specifics behind the erosive pattern (assuming anyone could give you that without knowing the specific environmental conditions)? $\endgroup$
    – JBH
    Commented Oct 11, 2023 at 3:17
  • $\begingroup$ In fact the lake won't be shallow, if there was a river going through the city, the lake would at least fill to the river's former level, which would make it almost 50 meters deep. And medieval cities were all built on water sources. I guess its southern part would quickly become the bathing area for any locals that lived nearby. $\endgroup$
    – Vesper
    Commented Oct 11, 2023 at 8:40

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At the beginning, soon after the island floated above ground I agree that the edges would indeed collapse into a crater.

Later, as you mentioned, water from underground aquifers, rivers, or other nearby water sources will rush into the depression, forming a shallow lake. This will depend on the local hydrogeology and the proximity of water sources to the lifted land. The lake could vary in size and depth, depending on the amount of water available and the rate of the water's flow.

Over time (many years), erosion will play a significant role in shaping the landscape. Rainwater would likely from the floating city's edges, carrying sediments with it. The sediments will be deposited in the lake, gradually filling it in. Erosion will also affect the sides of the crater, smoothing and reshaping them over time. Additionally, the shadow cast by the floating city will have a significant impact on the local climate. It will block sunlight and create a microclimate with cooler temperatures and reduced evaporation in the area under the shadow. This cooler, more heavily shadowed area may lead to some unique types of flora and fauna that make their homes there.

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  • $\begingroup$ Since the initial area is forested, and its borders are assumed even, the lake should form at about 0.5m below the former surface level, over time. The time depends on whether an aquifer has been tapped etc, but the rainwater would definitely fill it within several years. 50 meters is enough to drain half the forest, and if there was a nearby swamp, it would also get drained into that crater, making an interesting area full of natural fuel to dig easily. $\endgroup$
    – Vesper
    Commented Oct 11, 2023 at 8:38
  • $\begingroup$ With the floating city being 300m high and 2.5km in radius most of the area underneath will just never see any sunlight. So I would think that most of the unique flora will just be no flora at all. Which then raises the question of why animals should go there. SoI would assume most of the area underneath would be just dead, similar to a cave once you are away from the entrance. $\endgroup$
    – quarague
    Commented Oct 11, 2023 at 13:25
  • $\begingroup$ @quarague except unlike a cave there would be constant airflow and a lot of temperatures driven circulation. Like some of the largest caves on earth you will have an entire complex ecosystem. $\endgroup$
    – John
    Commented Oct 11, 2023 at 20:25

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