Tuesday, April 10, 2012

Why Is The Earth's Crust Mostly Silicon?

A couple posts ago, I mused that it was a pretty goddamn convenient coincidence that most of the crust of the planet we live on was made of the one element that's absolutely essential to all modern technology.  Being a generally lazy person, I was ready to just shrug and say "eh god did it" until I remembered that I'm, at best, an agnostic and not supposed to be doing that.  So I went with my backup plan-- shrugging and saying "eh, astronomy/geology did it."  It wasn't really germane to the earlier post's topic anyway, but the whole point of this blog is to actually try to find out the answers to all the things in life I usually just shrug and accept.  Plus thinking about it got me curious about two things I know next to nothing about: how heavy elements are formed and how the earth was formed.

The answer to this one goes all the way back to the beginning of the universe, when all the matter in existence (your desk, my computer, Andy Reid, etc) was created in the first couple of hundred thousand years after the Big Bang.  Problematically, that matter at the time consisted almost entirely of hydrogen and helium, since a rapidly cooling quark-gluon-lepton plasma (the mess left by the Big Bang) is going to relax into the least energetic state possible. In this case, that means lots of individual or double protons that were eventually able to capture an equivalent number of electrons as the universe continued to cool.

Hydrogen is great for making water and explosions and everyone loves balloons, but as you've probably noticed almost everything solid in the universe is made up of heavier elements like carbon, silicon, and iron.  So how did we go from "shit-tons of hydrogen, helium, and not much else" to the clusterfuck of 100+ elements that makes up the periodic table?

Short answer: explosions, and lots of 'em.  All those clouds of hydrogen and helium in the early universe would eventually (~1 billion years) coalesce into discrete masses, aided by gravity.  Eventually these masses got dense enough that the hydrogen and helium at the cores was under enough pressure to undergo fusion.  The result was lots and lots of gigantic primordial stars. 
 
These primordial stars, being much purer hydrogen-helium blobs than most of our current crop, were able to burn a lot hotter and, as a result, could get quite a bit bigger.  More mass means more core pressure means way more fusion than we see in most "modern" stable stars, which mostly just make helium; large numbers of protons could be fused into heavy elements. Every element from carbon through iron is/was formed via extreme stellar fusion this way.

Conveniently, the stellar mass that's necessary for this kind of higher-order fusion to occur also tends to make a giant star (superstar?) extremely unstable, so after creating heavier elements in its core for awhile it generally goes boom in a supernova/hypernova event, spreading those elements out through the universe.  As a result, the universe's supply of heavy elements consists overwhelmingly of the stuff between carbon and iron on the periodic table.  The elements heavier than iron, created from less-common non-fusion processes in large stars (physical limits on stellar mass mean iron is about the heaviest thing you can make with pure stellar fusion), are quite a bit rarer and get even more so as their atomic number goes up.

Relative abundance of the elements in our solar system (and, by extension, the galaxy/universe).  The weird sawtooth pattern is due to the fact that elements with even atomic numbers have a higher binding energy than odd-numbered ones.  Note that the y-axis is log scale, so differences are bigger than they look.  (thx Wikipedia)

So a lot of the early history of the universe was just giant stars forming and exploding, making lots of heavy elements in the process (it's worth mentioning that this is still going on, although less frequently).  At the same time this supernova-fest was happening, more reasonably-sized stars that didn't explode all the damn time were also getting formed and coalesced into clusters, galaxies, etc, eventually giving us approximately the universe we know and love today.  Once there were stable stars, the whole process of gravitational capture of heavy elements and planetary accretion started creating solar systems, including ours. 

So at the end of the day (or couple billion years or whatever), the top ten most common heavy elements in the galaxy (in order of abundance) are oxygen, carbon, neon, iron, nitrogen, silicon, magnesium, sulfur, argon, and calcium.  It's a pretty safe bet that most of these are going to have a lot to do with Earth's composition.  We can rule neon and argon out almost immediately though; they're noble gasess and aren't going to form anything solid without lots of coercion.  Of the others, oxygen has a tremendous advantage: it can form stable, solid compounds with everything else on the list except the carbon and nitrogen, and lots of other elements too.  More importantly, it's the only top-ten element that's capable of doing this.  So it's pretty much a given that the crust is going to be made up of mostly "rock-like" (solid at planetary temperatures) oxides of abundant elements.  Oxygen, ergo, is pretty much a lock for most common crustal element, and indeed wins by more than a factor of two over the first runner-up.

So now that we're battling for second place, the question now becomes "which oxides?"  You can roughly work this out by looking at all the rock-like oxides, rating them by the galactic abundance (or lack thereof) of the other element involved, and then accounting for each oxide's molecular weight.  The weight matters because Earth was basically a liquid during its formation; heavier elements/compounds had a tendency to sink down toward the core, while the lighter ones floated around in what would become the crust.  So while you'd expect iron oxide to be the most common compound in the crust, its relatively high molecular weight causes it to place a distant fifth, after the silicon, aluminum, calcium, and magnesium oxides.  Same deal with magnesium, to a lesser extent; the less common, but much lighter aluminum and calcium oxides end up beating it out even though aluminum isn't even in the top ten of galactic abundance.

Relative abundance of elements in the Earth's crust.  Note that the green blob (elements that form rocky oxides) is kicking everything else's ass. (thx Wikipedia)
Silicon, though, is the best of all worlds: not only is it the second most common rock-like-oxide forming element in the galaxy (after iron), but the oxide it forms is also pretty light as these things go. Result: lots of silicon oxide in the overall composition of the earth, and nearly all of it floating at the top in what would eventually cool down and become the crust.  The only other oxide that even comes close is aluminum, and even it still lags more than a factor of three behind silicon oxide in crustal abundance.

So as usual, there's a perfectly reasonable, if somewhat long and complicated, explanation for why the most common element in the crust of our home planet is also one of the most useful.  Yes, it's a complete coincidence that silicon happens to also be a semiconductor as far as I can tell, but at least now we know why there's so much of it around.  Still, if silicon didn't semiconduct we'd be pretty SOL; the next most common Si-like elemental semiconductor is germanium, which is about six orders of magnitude less abundant than silicon.  (Slight caveat for the pedantic: carbon, in diamond form, will semiconduct, but not in ways that are very conducive to the low-power digital electronics we like so much.  Still, we might've made it work if we had to, we're clever like that.) 

An interesting, largely unrelated fact I learned while looking all this up is that the galaxy (and by extension probably the universe) is, even now, still more than 99% composed of hydrogen and helium.  All the rest of the other elements put together barely comprise enough matter to even rate as a contaminant.  Even weirder, that contaminated field of hydrogen-helium only comprises about 5% of the universe; the rest is apparently dark matter and dark energy.  And that's where I'll stop, because I really don't want to have to go there.  

Thanks to Wikipedia for most of the basics of this one, and the blog's astrophysicist pal for some fact-checking of the parts with stars in them.

2 comments:

  1. This is an awesome explanation, giving both really broad background, but relevant, observations, as well as the actual answer, in conceptual terms, which I much prefer over factually correction explanations that still leave me scratching my head as to why and what is really going on. It is also a fun read - uncommon with this sort of thing. Thank you.

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  2. You are a life saver. This is the best explanation I ever saw why oxygen is more in earth's crust. Thank you.

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