Note that it needs to include coal and chert, therefore the common answer "a rock is made of minerals" is incomplete.
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1$\begingroup$ Chert is made of minerals: chalcedony for example. Instead of chert perhaps you would want to ask about obsidian, which is amorphous. Wikipedia suggests that a rock is an assemblage of minerals or mineraloids which includes more things because it is less strict (including opal and coal). $\endgroup$– equantApr 19, 2014 at 4:58
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1$\begingroup$ Usually I run to the Oxford Dictionary of Earth Sciences (Allaby & Allaby, 1999) for this kind of definition. In this case, though, it says ‘A consolidated or unconsolidated aggregate of minerals or organic matter’. This would seem to include, for example, meatballs. The broadness suggests to me that it's difficult to come up with a pithy definition that includes all rocks (as commonly understood) and excludes all non-rocks. $\endgroup$– PontApr 19, 2014 at 9:18
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$\begingroup$ @Pont: meatballs are not naturally formed over millions of years. Certainly some notion of time and natural processes must be components of the definition. $\endgroup$– PaulApr 19, 2014 at 17:39
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$\begingroup$ @Paul: Adding ‘naturally occurring’ to the Allaby & Allaby definition definitely improves it, but I don't agree about the time: volcanic bombs, for example, form more quickly than meatballs. The ‘organic matter’, however, would still allow things like peat. In fairness to A&A, I assume they're stating necessary rather than sufficient properties. I think Wikipedia's attempt is probably more useful, but to some extent it's just hiding the ambiguity under the term ‘mineraloid’ which has no strict definition as far as I know. $\endgroup$– PontApr 19, 2014 at 18:35
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3 Answers
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I usually tend to think that the shorter a definition is the more it introduces ambiguity and errors. So I would just think about your audience and choose an appropriate length (You didn't mention the level you are teaching for).
Similar to the definition of life you can offer to your audience characteristics, which some rocks will conform more or less. This is just a list I thought out:
Rocks are solids at the Earth's surface
- I would definitely include solid ice, which is the primary lithology of Greenland and Antarctica
- How solid? Most of them are incompressible to high pressures(Exception porous sediments)
- With lower temperatures more things become rocks (even our Sun).
- Density varies greatly (bring a lead-ore and a floating tuff)
Rocks are consolidated
- To differentiate them from sediments rocks have underwent a certain amount of compaction and cementation
- Rocks form through purely anorganic, organic, or a mixture of those processes
- This category probably needs the most explanation on how diverse the pathways to a rocks are
- Rocks form through natural processes
- Human-made rocks exist so instead of tossing that aside, talk about the interesting differences and similarities. Natural rocks fulfill the requirments of chemistry, physics, thermodynamics. Human-made rocks additionally fulfill requirements of pragmatism: a brick needs to be square in order to build a wall; holes make it light and insulate better; etc...
- Anorganic rocks fall into natural categories which obey the conditions set by their chemistry and the resulting eutectic points, reduce chemical oversaturations, and minimalization of the Gibbs free energy (There is a reason why granites in all their diversity, are actually pretty similar)
- Organic rocks obey the requirements by organisms of which nutrients exist in excess and can be used to synthesize a skeleton or shell.
- Rocks don't fulfill the requirements of life
- Talk about what makes living matter special. Talk about, that on planets devoid of life everything is a rock or sediment.
- Rocks have a scale
- One atom or chemical compound is not enough to make a rock. At what scale does language speak of a rock. What scale is so big that other words are introduced. Talk about how words are also just concepts defined by other words which are concepts themselves (This is what makes science fun)
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1$\begingroup$ When you say, "on planets devoid of life everything is a rock or sediment" are you saying that this statement is limited by the rules you've listed before it? Because this statement can't be correct on its own. $\endgroup$– equantApr 20, 2014 at 21:13
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$\begingroup$ @equant Do you mean because the statement excludes ocean and atmosphere which could exists independently of life? $\endgroup$ Apr 21, 2014 at 7:47
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1$\begingroup$ sort of, I meant that the original statement doesn't exclude gasses, liquids, amorphous solids and probably some other things I've forgotten that don't qualify as rock. $\endgroup$– equantApr 22, 2014 at 15:51
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$\begingroup$ @equant amorphous solids can still be rocks, amber, coal, lapis, obsidian, pumice, few would argue they are not rocks. $\endgroup$– JohnMay 17, 2021 at 3:48
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A rock is a rock. It is made of rocks.
A rock is the solid mineral material forming part of the surface of the earth and other similar planets, exposed on the surface or underlying the soil or oceans.
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The definition I use is class is.
A rock is a solid compound composed of one or more mineral or mineraloids.
its the broadest but also the easiest definition, there are biotic, abiotic, and human created rocks. To go to chemistry basically any solid is a rock. Sand is a pile of tiny rocks.
Some argue that only naturally occurring compounds should count but the usage in literature is pretty obvious. Man made materials are called anthropic rocks in the literature. this has become nessisary ever since since humans started introducing artificial minerals. Fordite, slag, concrete, bricks, glass, ceramics, there are formations composed mostly of anthropic rock now. Some even argue there should be four rock types; metamorphic, sedimentary, igneous, and anthropic.
The only real consideration I ever see is the exclusion of living matter (but not dead matter) This can lead you into a great discussion of how there are not real hard categories in nature. Any category humans come up with will get fuzzy around the edges because nature is a continuum not discreet.
Of course "mineral or mineraloids" just means a solid of known chemical compositional which case you could shorten it to say, "a rock is any nonliving solid compound"
