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Whenever I see fictional biospheres with massive or otherwise extreme megafauna capable of significant short-term erosion (such as toppling large rock formations, leveling mountains, digging huge burrows, things that would otherwise take "natural" geological processes hundreds, thousands, or even millions of years to accomplish) I always wonder what the actual long-term effects something like that would have on an environment. If you had a biosphere on Earth or an Earth-like planet containing huge, possibly kaiju-sized animals that regularly level or otherwise alter geological formations that take geological time scales to form in the first place, what would that planet look like after hundreds, thousands, or millions of years? Would they simply level everything into a flat, evenly eroded plain? How would that affect the biosphere in turn?

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    $\begingroup$ Hello and welcome! I can't answer your question but I do know that these things can be notoriously hard to predict. See this for example: scientificamerican.com/article/…. When even relatively small aquatic animals can have such an outsize effect on the overall landscape, any effect you come up with will probably be plausible enough for most readers. $\endgroup$
    – biziclop
    May 29, 2023 at 21:06

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Fauna and megafauna tend to live within the landscape. For serious erosion, look to rain. The steep slopes on the north side of the Hawaiian Islands are due to rain. Ice in the form of glaciers carve canyons. Rain cuts everything down to size. Floods do outsized erosion as the flood water move a lot of grit. In essence, floods can be like sandpaper on a huge scale. (See the scablands of eastern Washington state to see what repeated floods can do to an area.)

Fauna make paths, eat vegetation so that the rain can erode faster, build piles of calcium from their shells and bones (future limestone), but don't have impact much more than their bodies. Once the vegetation is eaten, they move on and don't come back till the plants grow back.

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Hard to predict.

Weak set of observations

How many results are the cause of species activities over geologic time? This is unknown and hard to know.

The primary action, is often is not recorded in geological record. Only the secondary and beyond affects are preserved. Teasing out causation vs correlation is difficult in present day events, never mind with only sketchy snapshots provided in the geologic record.

Some known erosion alterations by flora/fauna:

  • Great oxidation event that is believed to be leading cause of snowball earth.
  • Appearance of animals that could burrow into sea sediments below the microbial mats forever changed how sediments were preserved.
  • Beavers are known to have significant impact on environments where they live, slowing water flow rates which would tend to decrease erosion rates.

Megafauna

For megafauna on land, they only can exist in areas with large amounts of flora to support large fauna biomass. That is, places that tend have mega fauna are going to be places with lots of erosion resisting flora.

So activities that change the flora will be the ones that have highest odds of causing changes to erosion patterns.

But lack of data will make it hard to find which is cause which is correlation. Such as in South America there were large burrows excavated by mega-fauna how much did these effect erosion? I don't believe that has been considered. So how much erosion did or did not happen is not known.

Kaiju sized

Implies ocean based and unable to live on land. For Kaiju to be on land implies magic.

Magic combined with lack of knowledge of erosion caused by fauna means unknowns compounding on unknowns.

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My Answer:

Much the same as it does now. Why?

Toppling rock formations and pushing over trees is one thing, but actually levelling even a small hill is a major engineering feat that requires a lot of time, effort and energy.

You might get some distortion in the ground like natural trails forming, but otherwise the general landscape would be much the same.

The second reason is 'what's the point'? Why would a megafauna creature knock something over? Most of the time there's no need for them - they would just move around or over it.

This notion that it's going to be like Godzilla levelling Tokyo just doesn't make any sense - you might have specific instances like during hunting or during a mating season where fights and/or mating displays might result in a bit of damage to the landscape - but otherwise - they would just go about their business.

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