Plausible matter cycle for rock-based life

Question

Organic matter follows a cycle in which decomposers such as fungi and bacteria break down waste and corpses, and the byproducts of decomposition become the building blocks for all living beings (directly or indirectly).

But what about lifeforms that are based on rocks?

A few examples of such beings:

• The trolls of Discworld and the Goron people of the Legend of Zelda series of games litrrally eat all kinds of rocks; the former are actually moving, living rocks themselves.

• The gem people from Steven Universe can eat anything, or refrain from eating if they so wish; but they are carved out of rocks, and each one's true form is also a small rock. Population growth for their civilization consumes entire planets for raw materials.

• The Xorn, Stonepeople and Earth elementals in general from Dungeons & Dragons also consume rocks as part of their diet.

My main concern is that rocks cannot be grown like wheat and corn. If a population of evolving, sentient creatures feed on small bits of a mountain, over geological eras the mountain will cease existing relatively quick. So what could be a plausible matter cycle for such lifeforms?

Answer 1

Things that are made of minerals need to eat minerals.

https://www.wideopenspaces.com/these-wild-animals-are-chewing-your-antler-sheds/

Like this squirrel. There are no calories in that ancient antler. The squirrel wants the calcium. It is actually going to have to expend metabolic energy to process the calcium. But it must because its skeleton is made of calcium phosphate salts.

We humans must eat calcium too; 30% of our dry weight is calcium phosphate from our skeletons. You can buy pulverized rock as a calcium supplement.

Dolomite is regularly added to animal feed for this reason and also it is cheap.

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Like the squirrel your rock things made of rock need to eat rock because that is where they get the minerals they are made of. They might process the rock into some other form, like our bodies process the calcium carbonate in dolomite into calcium phosphate to build bones. Or the rock beings might break down the rocks they eat into molecules then reconstruct the molecules into rocks of needed shape, like radiolarians use dissolved SiO2 to grow their silicon shells.

The takeaway is that eating serves two purposes: metabolic needs to supply energy and anabolic needs to build bodies. Those two paths can overlap but for rock creatures and squirrels (among others) there are things that are eaten which are chiefly useful for anabolic purposes.

The question:

So what could be a plausible matter cycle for such lifeforms?

This squirrel could be chewing on dolomite for its calcium. Instead it is chewing an antler recently left behind by a dead creature. The dolomite is a stone which ultimately exists for a similar reason: things made of calcium died. Over millenia, ocean creatures accumulated calcium to build their bodies then died, and the accumulated calcium turned into stones.

In a stone ecosystem it would be the same. Stone beasts could chew up the remains of recently dead stone beasts. Or stone beasts that die and accumulate over millenia would turn back to stone, and stone beasts in some later age would eat rocks that once were their ancestors.

Answer 2

Short version, no mountains don't grow where you want them but the material in them gets recycled just the same.

Long version, without a specific biochemistry in mind we can only discuss things in fairly general terms:

For "rock people" to make sense as an evolutionary outcome you need a whole rock ecology from which they have evolved. A complete ecology will include specialised primary consumers, parasites, predators, and crucially decomposers. To dispose of a rock creature, assuming a primarily silicon based metabolism, is probably not as easy as a carbon based equivalent (there's a lot of issues with silicon biochemistry, most compounds are either exceptionally unstable or problematically stable compared to their carbon analogues). It will almost certainly involve enzymes that break out the non-silicon components and oxidise the silicon. Predators and scavengers will primarily produce silica as a solid waste with a minimum of other elements as dictated by their digestive process and metabolic needs.

As a point of comparison modern grasses incorporate large amounts of silica and much of this ends up flushed into the oceans out of herbivore dung. It is then taken up by diatoms (specialised blue-green algae that use it to create rigid body structures) and eventually settles to the seafloor where it is eventually uplifted as new sedimentary rock along convergent plate margins or subducted and erupted as new igneous rock. There is a noticeable change in ocean floor sediment cores that marks the emergence of grasses and a corresponding increase in diatom populations and the deposition of their skeletons. Ultimately anything that comes out of the ground goes back in the end, unless we shoot it into space.

Depending on the biomass density of your rock ecology, (I would note that of the cases you have pointed to only the Gem People are unrestrained by a competing, and in fact extreme dominant, carbon-based ecology that holds most of the life elements that aren't carbon or silicon, the bio-accessible calcium, phosphorus etc...) their efforts are going to be insignificant against the background of "natural" uplift and erosion. A world filled to earthlike biomass densities with silicon based life would probably have to be more geologically active with accelerated mountain building for the cycle to be sustainable but biomass cannot exceed available resources for long without a collapse.