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First off, a confession: I'm asking this question because of The Lord of the Rings. If you're not aware already, in the story, a magical ring can only be destroyed in a specific volcano. The reason for this is presumably "because magic".

But the issue got me wondering if this volcano is any different from every other volcano on the planet. Is there a significant difference between the magma in different volcanoes? If so, why?

Doesn't all magma come from the same place if you go down deep enough?

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    $\begingroup$ It's just a matter of heat, no magic: "It has been said that dragon-fire could melt and consume the Rings of Power, but there is not now any dragon left on earth in which the old fire is hot enough; nor was there ever any dragon, not even Ancalagon the Black, who could have harmed the One Ring, the Ruling Ring, for that was made by Sauron himself. There is only one way: to find the Cracks of Doom in the depths of Orodruin, the Fire-mountain, and cast the Ring in there." $\endgroup$ – algiogia Jul 28 '15 at 9:15
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    $\begingroup$ @Fred answers that there are 3 types of magma. But to answer your question, I think that volcanic layers can be traced to individual volcanoes even of the same magma type, since there may be minute differences in isotopic concentrations, or other unique signatures of provenance beyond just magma type. In short, "yes", unless two volcanoes are very closeby and their surface connection travels through identical material. This may not meat your criteria for "significant difference". $\endgroup$ – mankoff Jul 28 '15 at 13:57
  • $\begingroup$ In LOTR, there IS only one volcano anywhere in yhe part of Middle Earth that is known to the characters (which IMHO is why Tolkien never explicitly calls Orodruin a volcano: scifi.stackexchange.com/questions/96722/… ) $\endgroup$ – jamesqf Jul 28 '15 at 18:02
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In keeping with your Lord of the Rings inspiration, the first reference is from New Zealand, where the Lord of the Rings movies were made.

There are three main types of magma that volcanoes currently produce: basalt, andesite and rhyolite.

Basalt magma has a high temperature, around 1200ºC, it is poor in silica, has a low viscosity and a low gas content. When basalt erupts it produces a flowing magma.

Andesite magma has a mid range temperature, between 800ºC and 1000ºC, it also has a mid range silica content and moderate viscosity and gas content. When andesite erupts it flows but it is also moderately explosive.

Rhyolite magma has a low temperature, between 750ºC and 850ºC, it is silica rich and it has a very high viscosity and high gas content. When it erupts it is very explosive.

Other forms of magma which are much rarer are carbonatite melts and komatiite melts. Carbonatite melts may be cool, as low as 600ºC, whereas komatiite may have been as hot as 1600ºC.

Unlike the other forms of magma that are composed of silicate minerals, carbonate melts are composed of carbonates and can be confused with marble and may have to be geochemically analysed.

Most komatiites are very old, up to 4 billion years old. It is speculated that komatiite magma formation was possible when the Mantle was at least 500ºC hotter.

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    $\begingroup$ So to bring this back around to LoTR... perhaps the magma of Mount Doom was Basalt, while most other volcanoes in Middle Earth may have been Andesite or Rhyolite based. Given that the ring was forged in the fires of Mount Doom, perhaps it was made of an alloy whose melting temperature was a good 1100°C or so -- too high for Andesite magma to melt. Or magic. It could always just be magic :D $\endgroup$ – Doktor J Jul 28 '15 at 7:15
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    $\begingroup$ @DoktorJ You might be confusing cause and effect. It could be that Mount Doom has hotter magma because folks keep throwing magical rings into it. $\endgroup$ – David Richerby Jul 28 '15 at 11:40
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    $\begingroup$ @David Richerby: Indeed, the magma of Orodruin must have some special property, since the temperatures above are all at or below the working temperature of steel: en.wikipedia.org/wiki/Forging_temperature So if it was a matter of temperature alone, a good forge could melt the Ring as readily as a volcano. $\endgroup$ – jamesqf Jul 28 '15 at 18:10
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Because of variations in temperature, depth of emplacement, thickness of the crust, heterogeneity in the mantle, and variations in the magma distillation process, each volcano has a unique geochemical signature. However, volcanoes in similar tectonic settings have similar geochemistry.

Geochemistry of magma starts out as ultramafic when sourced from the mantle. However, the mantle is not entirely homogeneous, which is evident from the presence of hot spots and other thermal and gravitational variations. Ultramafic magma rising from the solid mantle can be generated by decompression (thinning of the crust) or by the introduction of a contaminant (water from subducting plates).

Once the mantle melt reaches the base of the crust, it's geochemistry can change via three different processes:

  1. Crystal settling - various mineral have differing melting/solidifying temperatures, allowing for a process of distillation where magma can gradually be made more felsic (from basalt-like to rhyolite-like)
  2. Inclusion of country rock (aka assimilation) - where crustal material is incorporated and melted into the magma
  3. Partial melting - where only the part of the rock material is melted

Crystal settling and partial melting follow the Bowen's reaction series (pictured), which tells you which minerals melt at which temperatures. This is essentially the same process of distillation of alcohol by separating it from water, which has a lower temperature of vaporization.

Each of these processes allows for a unique rock chemistry, even within the same batholith.

While we categorize magmas into different groups based on their geochemistry, we must keep in mind that the geochemistry of any igneous rock falls along a sliding (not discrete) scale (i.e. not every granite is the same). Otherwise igneous petrology would be really easy.

See the wikipedia article on igneous differentiation for more information.

Bowen's Reaction Series

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Let's make this question more fun. What type of magma is required to destroy the One Ring?

For that we need to know what it is made of. According to "What material was the One Ring made of?", it is made of gold. But that's boring - gold is easy to melt. The melting point is about 1000 °C, which is less than some of erupted magmas (basalt for example).

The ring is somewhat yellow, so it has to have at least some gold in it. How about we make an alloy of gold and any other noble metal? An alloy of 50-50 gold and palladium will melt at around 1400 °C. A 50-50 alloy of gold and ruthenium or rhodium will melt above 2000 °C! That's definitely going to require a very hot volcano that doesn't exist on Earth, and probably never did.

So let's assume that it is a certain alloy of gold and something else and we have a really hot volcano that can melt it. That's still no good - we're just going to have a molten blob of ring in our magma. What we really want is to dissolve the ring in the magma, to destroy it completely. For example, if you take table salt you can heat it up to 800 °C and melt it. That's no good - you still have salt, just liquid. A better way would be to stick it in water, and it's gone forever (as long as the water is there, that is). Now this is where it gets hard. Noble metals are notoriously known for not being soluble in magmas. Let's think of some processes that can do it:

  1. Have an oxidising magma. Noble metal solubility in magmas increases with their oxidation state. Oxidising magmas, for example where you would find magnetite, can dissolve more metals in them. This could work, but I find this rather boring.
  2. Have a hydrous peralkaline (i.e. a magmas with loads of sodium in it) magma with chlorine and fluorine. This stuff can dissolve everything, and that's why we mine them for rare metals, for example in Greenland and Canada. The problem is that according to LoTR, this is also the volcano where the ring was forged (and for now, also where the metals came from). Peralkaline hardly have any noble metals in them to begin with, so that's not a good option.
  3. Our best bet is to throw it in a sulphide magma (in contrast to the "regular" silicate magmas such as basalt etc). Sulphide magmas are excellent in dissolving noble metals. Have a pool of that, throw in the ring, and it's gone for good. The problem is that we don't have these kinds of magmas in volcanos. They always form together with silicate magmas, and they are much denser. They sink down whereas the boring silicate magma floats to the volcano's lava lake. However, because our ring is also denser than the silicate magma, it's going to sink down. This will happen faster in basaltic magmas. Basaltic magmas are also more likely to have a sulphide melt associated with them! What's even better is that noble metals are usually mined from these exact sulphide ore deposits. It all fits in!

So to answer your question - there are loads of magma types. The one that has the highest chance of destroying the One Ring is a basaltic magma with a conjugate immiscible sulphide melt phase. Good luck!

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  • $\begingroup$ An alternative theory is that the Ring does not need to be melted by external heat. It must, logically, have some internal structure and energy source in order to do all the stuff it does - much like a CPU chip. But you can destroy a CPU at temperatures much lower than the melting point of silicon (1414°C). So if that destruction also "shorts out" the internal energy source, the Ring melts itself. $\endgroup$ – jamesqf Jul 29 '15 at 18:14
  • $\begingroup$ Yea - so that's why I was talking about dissolving it instead of melting. If you really want to get complicated about it, you can assume the ring has some structure (let's say fcc) and all you need to do is to convert it to another (e.g. bcc). But then you don't need a volcano for that. $\endgroup$ – Gimelist Jul 29 '15 at 21:27
  • $\begingroup$ Another point I've just thought of: destroying the Ring must cause some sort of massive energy release, since it sends Orodruin from a low-level eruption into something major... $\endgroup$ – jamesqf Jul 31 '15 at 17:16

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