I want to make a fantasy world where iron, copper, silver, gold and other "common" metals are far less abundant in the planet's crust than they are IRL, forcing people to use stone and bone for most weapons and building materials. Is there any way I could reasonably explain this without invoking "a wizard did it", or is this just one of those things that wouldn't make sense without gods or magic being involved somehow?
It is believed that most of the metals in Earth's crust came in the late heavy bombardment, since the metals which were part of the original materials of Earth would have sunk to the core due to its higher density while the Earth was a ball of liquified magma. So you only have to say that there was no late heavy bombardment (or it wasn't that heavy) to explain why, even if your world has plenty of iron, gold, silver or copper inside, there's nearly nothing in the upper crust.
Your civilization can be the 2nd wave of people living on that world.
The first one depleted the surface of most of the metals (extensive mining, like we have done so far), leaving only rocks behind when they left the planet with a generation ship loaded (also) with all those metals.
Iron is not present as metallic iron on the Earth's crust, but mostly as iron oxides. Oxygen combines with many metals to form oxides, and these are the main sources of iron for us. These are basically rocks, and they're lighter than the pure metal, which when the Earth was still forming was molten and sank towards the center.
Now, as you see, the amount of iron left on the crust depends on the initial amount of oxygen available to oxidise it. Oxygen bonds first to other elements and then goes for the iron. If there had been a little less oxygen in the nebula from which the Solar System formed, there might be almost no iron left near the surface. Metallic, non-oxidised iron would just melt and sink to the core. This happened to nickel, by the way, and to gold and silver. It could very well happen to copper, too. The initial amount of oxygen in the protoplanetary nebula was a lucky number.
So that's how you can have a non-magical, natural explanation to a metal-depleted planet: it's there, but it's all in the core. I doubt you'd need to explain all that in your story.
From this paper on the web: https://www.ncbi.nlm.nih.gov/pubmed/17840628
The abundance and distribution of iron on the moon is derived from a near-global data set from Clementine. The determined iron content of the lunar highlands crust ( approximately 3 percent iron by weight) supports the hypothesis that much of the lunar crust was derived from a magma ocean. The iron content of lower crustal material exposed by the South Pole-Aitken impact basin on the lunar farside is higher ( approximately 7 to 8 percent by weight) and consistent with a basaltic composition. This composition supports earlier evidence that the lunar crust becomes more mafic with depth. The data also suggest that the bulk composition of the moon differs from that of the Earth's mantle. This difference excludes models for lunar origin that require the Earth and moon to have the same compositions, such as fission and coaccretion, and favors giant impact and capture.
The moon has much lower concentration of metals that does Earth. Something like this -- or perhaps even this more than once -- may have occurred in your world. To achieve appropriate gravity, you may need to make the world larger since the core would be less dense.
I was thinking that the planet's star could be one of those produced early in the life of the universe, but checking Wikipedia's abundance of chemical elements page shows that iron is particularly stable in supernova processes, and particular common. (And I assume you are aware of the astrophysicist's different definition of metal.)
So it would have to be especially early in the life of the universe, and there are question about whether there would be enough of the other elements which support life.
Speaking of which, iron is really important to life as we know it, so you might have to re-imagine the basis of life on that world. Copper is heavier than iron, so that option doesn't help. More examining the relative abundances chart on that page shows that replacing oxygen with fluorine is also going the wrong direction.
Worlds where blood is copper-based and people breathe fluorine were once interesting in the genre, which would have provided a bit richer source in which to do secondary research. Still, if you are interested in trying to imagine blood based on (say) lithium or magnesium and, uhm, hydrogen, you might want to start with that literature. I don't remember the names of the novels and they don't come up in a quick search, sorry.
That's going to be a lot of research into the chemistry of light elements to find substitutes for RNA and DNA and all sorts of stuff we take for granted. Doctoral thesis material.
So, it kind of does look like you might need some sort of event which involves processes we don't understand to actually suppress the production or iron and heavier metals (usual sense), or some series of events in which the elements are extracted and transported out of your world, one type of which has been suggested.
Excessive exploitation of resources would be a reasonable cause.
Another possibility might be simply age of the world. The metals are still there, but natural and intelligent-life processes have so thoroughly distributed them into the environment that re-capturing them for use in weapons, etc. is beyond the technology your intelligent species has.
That is another direction that has been interesting in SF literature from quite a ways back. Clarke's Against the Fall of Night was somewhat close in theme, and I think Herbert's Dune did a little more than nod in that direction. (So does Star Wars, come to think of it.)
Everything else looks supernatural to me.
But you can always redefine deity to being some super-advanced race experimenting with different parameters for life? Suppress the weaponizable materials and see if the members of this race can learn to live in peace?
Well, we could have iron in the core or even the mantle, as it’s only the composition of the crust that provides resources.
What are iron deposis now come from banded iron formations. Early acidic seas disolved much of the iron in the crust, then the presence of oxygen deposited it at the bottom of the sea.
So, we need to arrange for those iron sediments to be inaccessible. Perhaps they happened to be in techtonic areas that became subducted. Perhaps they are currently all under the oceans rather than on continents.
As others have said, our crustal metals are mostly from the late heavy bombardment as our original metal load would have sunk. I do not think that a planet could realistically be shielded from the late heavy bombardment period, though.
Instead, lets throw in one more impact at the end of the late heavy bombardment--something even heavier than the Theia event and straight in, not a glancing blow. Melt the planet again, the metals sink.
Wow, lots of answers about a (Ok, pretty straightforward) geological explanation.
How about a biological/chemical process faster than corrosion that slowly eats metals that are exposed to oxygen/salt/other materials? Simply get the metals to be more exposed to it and set the corrosion rate.
Asimov kinda did something similar. In one of his short stories humans land on some planet, arrogantly announcing their intentions of conquest to the primitive natives. The natives shrug, tell the humans that they have a day to leave. A day later the stupid humans are amazed to find that metals are slowly (but not too slowly) being destroyed by some unknown force. The leader tells his scientist to investigate and they have some ideas, but to actually find the reason they need technology, and their tech is obviously based on the now removed metals. Stranded, the scientists half-jokingly tell the leader that they must rebuild tech from rocks and wood.
Here we can have some more literary freedom. People's minds can do some crazy things. I'm thinking about:
- Fear of metals, based on religion/severe allergy to metals, at some point in the past society dictated destruction of previously found metals
- An underground civilization that took all the resources deep below the crust
What you likely need, is a world with iron ore not being abundant or not being within easy reach. There are several ways for this to happen: 1) very prehistoric civilization (more then several million years ago) depleted most easily recoverable iron ore deposits 2) due to peculiar geology, no young mountains on land, or the rest of land was recently (in geological terms) under shallow seas and to reach iron ore deposits you must go down more than a mile, etc. So if the only iron you can get is low quality bog iron, and even that is sparse you'll get the world you want without redesigning laws of physics.
You may want to look a the New Sun trilogy by Gene Wolfe. It's an extraordinary feat of worldbuilding (Wolfe is regarded as the "scifi writers' writer", a term of profound respect for his vision and execution.)
In this series, the world is so old, the ages of history are marked by the exhaustion of different resources, such as iron.
- Make your world so old that all the iron has been used up or rusted away, and all that remains is too deep in the bowels of the earth to be reasonably acquired.
Given that this is a fantasy world, here are some fantastical ideas as to why there's no metal ore.
Metal is a magical reagent and is consumed by spells and rituals. Over the centuries of sustained and intensive use of magic across the world, reserves of ore simply run out. It's still possible to get metals ores, but you have to mine deeper or in more dangerous places.
Metal suppresses magic and is therefore judged to be tainted and unnatural. After a coalition of magically aligned forces defeated the last metal users, a campaign of magically purifying the land and converting metals ores in natural rock took place. (Actually, this is basically "a wizard did it" now I come to think of it.)
Dragons in this world eat metal, and it doesn't come out the other end in any recoverable way. In ages past, dragons would enslave people, forcing them to produce metal for them to eat. After some centuries, metal ore was no longer common enough to sustain a large population of dragons and humanity was able to overthrow them.
When they were creating the world, the gods couldn't agree on what it should be like. Many eons of argument resulted in a world in constant flux, unable to support any long-lasting civilisation. After some time, the gods decided that the only way forward was to dismantle the world and each use part of it to create a new world. Our world with very little metal was created by a nature god who prioritised taking the water, dirt, flora and fauna for their world. Most of the metal is in the worlds created by the gods of war and industry.
You cold try to setup a religious belief requiring everyone to offer any silver, iron, gold he can find. Then you have mountains of metals, but no one can build anything with it, since they haven't tried to melt this holy material.
Without the help of metal tools, I'm pretty sure mining very hard. Making it rare quickly.
I think it's interesting because you have a lot of potential implications, for example :
- Different tribes fighting each other to break in their metal stocks and steal them
- Some kind of blacksmith conspiracy that set up this religion to build power staffs, relics and gain power
- Fights to control mines
Well, since you used the magic tag in the question... There is a saying that goes:
"Any sufficiently advanced technology is indistinguishable from magic."
- Arthur C. Clarke
Well, there is also a counter saying that goes:
"Any sufficiently analyzed magic is indistinguishable from science."
- Phil and Kaja Foglio
In a world where magic is a thing, iron may interact with it in a way that gives the metal properties that it will not have in our world. For example:
Greater density: this would have caused iron to sink further into the planet during planetary formation. So it would be less available in general on the surface.
Iron attracts iron: and not just magnetically. Could have the same effect as above during planetary formation.
Iron is "enchantoactive": a neologism I just coined. In Terry Pratchett's Discworld series of books, enchantment is the name given to thaumic (magic) radiation, and behaves somewhat like radiation does in real life (though its effects are more comical than tragic, such as turning coins into grubs). There is also a metal called octiron, which could serve as a source for further inspiration:
A rare metal naturally imbued with magic (making it extremely ill-behaved), it is extremely dark in colour and in its unrefined state produces high levels of thaumatic radiation. When used for common purposes it will often yield unexpected results such as the great bell of Unseen University which when struck produces negative sound. The hub of the Discworld is thought to be composed of Octiron giving rise to the strong magical field of the disc.
If iron in your world has such magic/enchantment/radioactivity properties, it could naturally decay into other elements over eons, just like Uranium eventually decays into lead.
I remember a hard sci-fi book where colonists in a spaceship end up having to choose in which period of the universe history they were going to choose a planet to colonize. They ended up in a much earlier period of the universe, at the time when too little stars have aged enough to die as supernovae. Since all heavy elements are fused on stars and ejected to the galaxies via supernovae, metal in that age of the universe was very scarce. The book in question is "Tau Zero", by Poul Anderson.