Geology Liveblog: Terminology, Melting, and the Chocolate Sauce of the Earth

[wolffyluna]

So, this is the first part of the 'Wolffy Liveblogs Geology To Get Her To Revise It'. These first few liveblogs are going to be focused on petrology and minerals.

So, what are those, exactly?

Petrology is the study of the origins of rocks. Why rocks form, what conditions did they form under. There is igneous (rocks that formed from volcanoes or the mantle) petrology, metamorphic petrology (rocks that formed by other rocks being altered under high heat and/or pressure) and sedimentary (rocks formed from small bits of stuff, usually other rocks.) petrology. I'm mostly looking at igneous and metamorphic petrology, because the lecturer for the class I am revising considered sedimentary rocks to be 'fundamentally disgusting.' So.

Rocks are... rocks. There's a technical definition, but it's not that different from the lay 'it's a rock' definition.

Rocks are made of minerals, which are inorganic chemical compounds with a certain structure that make up rocks. Most minerals form solid solutions, which more or less means that there is a range of particular chemical compositions it can be before it stops being that mineral, and that it is solid.

To give some examples: CO2 cannot form a solid solution, even if you freeze it, because you can't change any of those atoms and have it still be CO2. You can't have half the oxygens replaced with sulfur and still have CO2. But olivine, (Mg,Fe)2SiO4 can (and does) form solid solution. The Mg and Fe in the brackets mean that either Mg or Fe can fill up that slot/space in the mineral, and there are two slots for every SiO4 unit. You could have some olivine that only has Fe, or you could have some that had 50% Fe and 50% Mg, or 100% Mg, or anything in between. A lot of minerals can form these solid solutions, but they often has a more limited range of solid solutions that are possible.

Minerals have end members, which are (sometimes hypothetical) minerals that don't have a mixture of different elements in their 'slots'. For example, olivine that had no Fe and just Mg, for a formula of Mg2SiO2, is the olivine end member forsterite. (The iron equivalent is Fayalite.) End members get used because sometimes it's a useful way of expressing what the solid solution is (for example, you might say a sample of olivine is 70% forsterite and 30% fayalite), or because it can be helpful to put the elements into buckets for analysis purposes.

Minerals and chemical composition can be used to classify rocks. Humans love classifying things! One way (that sounds like it shouldn't work, but does) is classifying igneous rocks based on what proportions of dark coloured minerals they have, on a scale from ultramafic (90% dark), to mafic (mostly dark), to intermediate, to felsic (mostly light). Another common way is how much silica/SiO2 the rocks have, from ultrabasic (~40 wt% silica) to acidic (~80% silica, iirc? It's mostly an American system anyways.) The basicity and maficity scales actually line up pretty neatly.

So, most of the Earth's rock is in the mantle, which is the bit in between the crust and the core. Except us flesh creatures live on the crust, which is weird. The crust is mostly (except for all those dirty dirty sediments) of rocks from the mantle that have melted. Most mantle rocks actually aren't liquid, they're very slow flowing solids. Think like plasticine, except... more solid than that.

And because the crust rocks are often melted mantle rocks, the crust rocks are often quite different from the mantle rocks.

Because rocks don't melt all at once. (Or crystallise all at once, for that matter.)

Let's use our good friend olivine as an example. Magnesium doesn't like melting. It will, but it needs to be at a higher temperature, and generally if there's anything that would rather melt, it will melt after that. Iron is pretty okay about melting, compared to Magnesium. So when olivine melts, the iron melts first, and the magnesium stays behind. The melt gets enriched in fayalite, and the residue gets enriched forsterite. This assumes things stay in equilibrium. Often they aren't so these effects get more extreme. Also, this is a very simplified system, and once other minerals get involved it gets a bit more complicated.

But melting olivine isn't a bad model for melting peridotite (the main rock in the mantle, which is mostly olivine), which is how basalt forms.

These melting processes are an example of fractionation, which is... basically any process that makes rocks more different from each other than when they started, basically. Melting and crystallising are the common ones.

So, I mentioned basalt. Basalt is a mafic igneous rock. It's not all of what comes out of volcanoes, but often other volcanic things come from a basalt-y source.

Now, volcanoes tend to give geologist headaches. Volcanoes erupt liquid rock. That's fine.

...except for the fact that most of the mantle is too cold nowadays to melt rock. Unless something happens to it.

Which caused some consternation.

Now, why mid ocean ridge volcanoes erupt got worked out with-- not zero banging-heads-against-walls, but less than others. Mid ocean ridge volcanoes happen when the ocean crust pulls itself apart because of plate tectonics. This pulling apart leads to an area of low pressure, which draws up mantle material... and melts it. The lower pressure leads to a lower melting point, the maths and data from the furnaces agrees with that, everyone is happy. (Except for the ocean crust geologists, because even if that's an explanation its still hard to get samples, and that's so unfair.)

Then there were a bunch of volcanoes in the middle of tectonic plates that made people scratch their heads. But they did find an explanation-- hot spots, also known as mantle plumes. These are... hot... plumes of mantle that reach the crust and erupt. (Hawaii is an example of this.) Now, this is an explanation of why there are volcanoes there and why there is melted stuff-- but 'why are mantle plumes a thing? where do they come from' is still-- let's say, the subject of debate.

But this was of no help to the subduction zone volcanologists, who were just lost. Subduction zones are areas where an oceanic and continental plate meet, and the oceanic plate grinds underneath the continental plate. A bit past this zone you get lots of volcanoes. Think the of the Pacific Ring of Fire. Lots of volcanoes there. Confusing volcanoes.

Because there was no reason to believe the mantle there was unusually hot, like with hot spot volcanoes. And while there was some pressure difference from the top to the bottom of the 'mantle wedge' between the oceanic plate and the continental plate above it, it wasn't enough to melt anything.

Which brings us to chocolate sauce. Let's say you are trying to make a delicious chocolate sauce (or cheese sauce, it's a similar principle.) You can melt plain chocolate, but you can't use it as a sauce at room temperature, because it solidifies. But let's say you add some cream as you melt your chocolate over the stove. That's not what you want. Cream has a much lower melting point than chocolate. (Which does bring to mind the disturbing image of solidified cream.) Now, your sauce stays melted at room temperature. Hooray!

Subduction zone magma/lava is like this. It melts not because it's hot or at low pressure, but because there is something else mixed in that makes it melt at a lower temperature.

Water. (And some CO2, as well.) The oceanic plate is, for lack of a better description, wet. It also contains minerals that have reacted with water and incorporated it into their structure. But when you heat these minerals up, they decompose, and the water goes free. And this water gets into the mantle wedge, mixes with the rocks there, and lowers the melting point enough that subduction zone volcanoes happen.

Tune in next time for-- hopefully some more pretty pictures. Also rocks that aren't just olivine.

Comments

[chocochipbiscuit]

This is A+ nerdery and I love it.

[wolffyluna]

Thank you!

[yvannairie]

Oh, that chocolate sauce metaphor is very good.

[wolffyluna]

It is, isn't it? I first heard it 2-3 years ago, and it's still stuck in my head.

[brin_bellway]

>>the lecturer for the class I am revising considered sedimentary rocks to be 'fundamentally disgusting.'<<

??????


(to clarify: this is a great post, please continue, but that one bit is making me go WTF)

[wolffyluna]

From my understanding, some of his dislike of sedimentary rocks is just perfomative (because he is an igneous geologist, rahrah friendly rivalry with the sediment people.)

But I think some of it is because sediments are a bit of a pain to analyse through the sort of techniques that an igneous geologist would use? Because sedimentary rocks are made of stuff from elsewhere, and it's often unclear where that elsewhere was, how many elsewheres there are, etc. So, knowing that a particular sandstone contains plagioclase isn't... as helpful as it would be as knowing a particular gabbro has plagioclase. Like, 'Congrats, there's a weathered rock somewhere that contains plagioclase. Real helpful'. The minerals are coming from everywhere, who knows where, and to date them you have to pray you find a useful fossil.

(Metamorphosed sediments are allowed, but are on thin ice, according to this lecturer. Though you can actually radiometrically date a metamorphosed sediment to a certain extent, so...)

[brin_bellway]

Ah. Here I was wondering if it was something about them being made of dirt and scraps and such.

[wolffyluna]

I mean, kinda? But less because 'dirt is icky' and more 'dirt is hard to analyse.'

[contrarianarchon]

Fun! Olivine!

(And how is frozen cream a disturbing mental image)

[wolffyluna]

Intellectually, I'm aware frozen cream would be, like, ice cream or something similar, but my brain just pictures solid blocks of cream, a lá ice cubes.

Solid blocks is not how cream is meant to be.

[contrarianarchon]

Embrace the cream-cubes!

[chocochipbiscuit]

I could see it as a cute-bougie way of adding cream to hot coffee. Maybe at an upscale pretentious coffee shop. The kind of place where they make frozen coffee cubes so your iced coffee doesn't get diluted as you drink it. :P

[wolffyluna]

Maybe? (I actually had no idea coffee ice cubes were a thing. Then again 'Oh no, my coffee is more dilute!' is, like, the opposite of a problem for me :P.)