Is the magma in one volcano different from the magma in every other volcano?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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!