Anonymous asked: damn, your comment about where did the carbon come from has got me wondering why the earth’s crust is so… ordered, if you see what I mean. like, you don’t have just tiny particles of elements that happened to react with each other, in a random mostly-homogeneous mix–you have large areas of the same type of rock, large veins of iron or whatnot, and so on. like going with like, to an extent.

togglesbloggle:

argumate:

too lumpy and solid to combine and homogenise? or it combined and then separated due to different densities and varying levels of heat? I have no idea where planets came from, I only live on one.

!!!!!!!!!! is excite

So, first things first is deciding what ‘disordered’ means in this context. After all, there’s one very primordial sorting that any planetary body goes through- a density gradient by depth. You get your core, your mantle, your atmosphere, heavy iron and nickel falling and light volatiles rising. So we already have some order for free, although the various compounds within each layer are still a jumbled mess. 

Now, naturally, your interior heats up, partly because of that pressure and partly because it tends to be super radioactive.  And as we all know from grade school, thermal expansion is a thing- hot substances get larger, and less dense.  But it was their high density that put them that far down in the first place!  So in larger terrestrial bodies like Earth, this means that they float back up to the top again before cooling and falling, like a lava lamp (on smaller bodies like Mars heat conducts out too fast for any bulk overturn to happen).  In the softer areas like the atmosphere and the mantle, the Earth is constantly being ‘stirred’, homogenizing those layers.  Like the atmosphere, the mantle is mostly-but-not-entirely uniform because shit’s complicated, but that’s what your high-entropy baseline is as well as the lion’s share of Earth’s mass.

That ‘baseline’ is a muddle of mineral types we call peridotite.  It has a lot of silicon and oxygen as you might expect, as well as a number of metals and other bits, particularly magnesium, iron, and calcium.  It wouldn’t survive long at the surface; water etches it away quite quickly, but of course it’s protected from the nastier reactions through the expedient of being really far away from the surface where all the volatiles went. Still, it’s been changing a bit over time.  A primordial and molten planet’s mantle wouldn’t be quite the same as the one we have now, because we’ve spent several billion years drawing elements out of it, cooling them at the surface, and then occasionally injecting new compounds back in.

And this process doesn’t quite happen randomly.  There’s a particular class of elements we call ‘lithophiles’ (no relation to the bacteria), mostly because they react well with oxygen, and correspondingly they’re the first ones to jump ship during mantle cooling and float up to the surface, staying there more or less permanently.  That’s your crust, and it’s why continents and oceans floors don’t look much like the mantle proper- there’s a self-selection going on among the elements.  Once you run the high-entropy mulch through this selection process, what cools out is an old friend- the familiar igneous rocks.  When they’re extruded in to the air or water by volcanoes, they look like basalt, when they just moosh up against the bottom of the existing crust without ever touching air, they look more like granite.  So that’s a further source of differentiation and order, but those differences are fairly minor in the grand scheme of things- mostly having to do with how many metals are mixed in, and the corresponding differences in density.

And to the first order, that’s pretty much what the crust is.  The question was why you see so much order in the Earth’s crust, but honestly it’s like 90% granite and basalt, which are pretty close to being a random homogeneous-if-you-squint mix of the lithophilic fraction of Earth’s bulk mantle composition. What’s tricking you is those volatiles again, because even though the Earth’s crust is almost entirely igneous, the visible land surface is almost entirely not.  Sedimentary rocks are only IIRC 5% of the volume of the crust, but they’re a solid majority of what you see when you’re clambering around in the air. 

The ocean floor is maybe a good way to start thinking about this.  At the mid-ocean ridges, which are giant lines of volcanoes injecting new crust all the time, it’s basically just pure basalt.  As you walk in a straight line along the ocean floor from those volcanoes towards a continent, you’ll notice little bits of debris start accumulating, mostly dead organisms and excrement and so on.  The farther you walk, the more you see, because the basalt is acting like a conveyor belt moving between your two landmarks, and the farther you get from the volcano the older it is and the more time it’s had to pick up random bits of detritus.  Eventually you’re wading through it, and by the time you get to the edge of the ocean it’s hundreds or thousands of meters thick, with nary a hint of exposed basalt.  But it’s still under there, much thicker than the layer of goo on top of it.  So there’s this patina of order laid across the igneous crust, with linearly increasing mud thickness.  One of the more reliable geological gradients in the solar system, as it happens.   

The continents are trickier because they don’t die of old age. That granite is much less dense than the basalt you get in oceans, so it floats for basically forever without getting injected back inside the planet.  Sediments accumulate and get remixed over billions of years instead of millions, and a diversity of forms proliferates because you can have second-order, third-order, fourth-order weathering, weathering of metamorphic rocks, biological chemistry, on and on and on.  Around the edges, you get the scraping weirdness of plate subduction, every now and then you even get weird things happening when some vast object bumps the continent from underneath.  But because a plate itself is so large, most of the interesting and dynamic activity is all happening at the edges, leaving the bulk granite more or less inert for billions and billions of years.  Everything that you’re calling ordered happens in a narrow film on the outer edge of a narrow film.

But that surface environment, narrow as it may be, is quite intense and destructive.  So for any given patch of continent, at any given time, the surface is in flux- if it’s not actively being buried, it’s actively being eroded. So any sedimentary rocks that you see come from these areas where the surface was preserved through rapid accumulation, shattered fragments of the erosional areas finally being blasted to a place where they’re buried too quickly to be destroyed before finding protective sequestration away from the surface.  We call them ‘basins’.  Often but not always underwater-  like river deltas at shorelines, that kind of thing.  But there’s plenty of examples of preserved deserts and rivers as well, anywhere that wind and water could bring a lot of random bits of stuff in and leave them there.

The conditions in any given basin are going to depend on a lot of environmental factors- biological activity, atmospheric and environmental conditions, the power of the force that brings sediments in, how old the rocks are, an endless list really.  Basins themselves can be quite large, many miles across, and the depositional conditions within any given moment will usually be pretty similar because entropy. So you’ll see similar ‘packets’ of debris fragments landing all over the basin at about the same period of time.  But as we all know, the atmosphere is a fickle bitch.  So as time passes, so do those conditions, and so these basins produce distinct layers that vary in fragment size, color, chemistry, and so on.  And there are so many different options for basin conditions that you get a rich taxonomy of different sedimentary rock types.  Then they’re all buried, later exhumed, and outcroups have taken on that ‘order’ that you noticed.

Another major source of order comes from the fact that the crust is so brittle.  At the planetary scale, the crust has a tensile strength of basically zero, so every time something happens tectonically it shatters.  Uplift, load deposition, torque, you name it and there’s probably a patch of Earth’s crust losing its shit about it.  These cracks, which you know as fault lines, are therefore ubiquitous, most of them not really representing a whole lot of motion, most of them again near the surface because that’s where force imbalances have to happen. 

And when water is flowing through the near-surface, it will tend preferentially to flow along these fault lines, because they’re the weak points and pre-drilled tunnels.  And when that flow takes the water from one place to another, with a different temperature and pressure and lithological environment, the chemical equilibrium of impurities in that water changes.  It leeches certain ions from the surrounding rock, and precipitates others out.  And this is a great way to collect large masses of very specific elements in one place- gold, for instance.  Most mining for precious metals is about finding such places.  That’s why you tend to find precious ores in ‘veins’; they fill the original fault lines that the water was flowing through, long and thin and twisty.  (Iron is an exception, it’s a whole other thing.)

Anyway I uh, seem to have written a fairly long essay.  But yeah!  Rocks.


Tags:

#geology #interesting #long post

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