\n\n"}\n\n\n\n\n\n\n\n\n\n\ntoo 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
\nSo,\nfirst things first is deciding what \u2018disordered\u2019 means in this\ncontext. After all, there\u2019s\none very\nprimordial sorting that any planetary body goes through-\na\ndensity\ngradient by depth.\nYou get your core, your mantle, your atmosphere, heavy\niron and nickel falling and light volatiles rising.\n So\nwe already have some order for free, although the various compounds\nwithin each layer are still a jumbled mess.\n
\n\n\nNow,\nnaturally, your interior heats up, partly because of that pressure\nand partly because it tends to be super radioactive. \u00a0And as we all\nknow from grade school, thermal expansion is a thing- hot substances\nget larger, and less dense. \u00a0But it was their high density that put\nthem that far down in the first place! \u00a0So in larger terrestrial\nbodies like Earth, this means that they float back up to the top\nagain before\ncooling and falling,\nlike a lava lamp (on smaller bodies like Mars heat\nconducts out\ntoo fast for any bulk overturn to happen). \u00a0In\nthe\nsofter areas like the atmosphere and the mantle, the Earth is\nconstantly being \u2018stirred\u2019, homogenizing those layers. \u00a0Like the\natmosphere, the mantle is mostly-but-not-entirely uniform because shit\u2019s\ncomplicated, but that\u2019s what your\nhigh-entropy baseline is as well as the lion\u2019s share of Earth\u2019s\nmass.
\nThat \u2018baseline\u2019 is a muddle of mineral types we\ncall peridotite. \u00a0It has a lot of silicon and oxygen as you might\nexpect, as well as a number of metals and other bits, particularly\nmagnesium, iron, and calcium. \u00a0It\nwouldn\u2019t survive long at the surface; water etches it away quite\nquickly, but of course it\u2019s protected from the nastier reactions\nthrough the expedient of being really far away from the surface where\nall the volatiles went.\n Still,\nit\u2019s been changing a bit over time. \u00a0A\nprimordial and molten planet\u2019s mantle wouldn\u2019t\nbe quite the same as the one we have now, because we\u2019ve spent\nseveral billion years drawing elements out of it, cooling them at the\nsurface, and then occasionally injecting new compounds back in.
\nAnd\nthis process doesn\u2019t quite happen randomly. \u00a0There\u2019s a particular\nclass of elements we call \u2018lithophiles\u2019 (no relation to the\nbacteria), mostly because they react well with oxygen, and\ncorrespondingly they\u2019re the first ones to jump ship during mantle\ncooling and float up to the surface, staying there more\nor less\npermanently. \u00a0That\u2019s your crust, and it\u2019s why continents and\noceans floors don\u2019t look much like the mantle proper- there\u2019s a\nself-selection going on among the elements. \u00a0Once\nyou run the high-entropy mulch through this selection process, what\ncools out is an old friend- the familiar igneous rocks. \u00a0When they\u2019re\nextruded in to the air or water by volcanoes, they look like basalt,\nwhen they just moosh up against the bottom of the existing crust\nwithout ever touching air, they look more like granite.\u00a0 So that\u2019s a further source of differentiation and order, but those\ndifferences are fairly minor in the grand scheme of things- mostly\nhaving to do with how many metals are mixed in, and the corresponding differences in density.
\nAnd to the\nfirst order, that\u2019s pretty much what the crust is. \u00a0The\nquestion was why you see so much order in the Earth\u2019s crust, but\nhonestly it\u2019s like 90%\ngranite and basalt, which are pretty close to being a random\nhomogeneous-if-you-squint\nmix of the lithophilic fraction of Earth\u2019s bulk mantle composition.\n What\u2019s tricking you is those volatiles again, because even though\nthe Earth\u2019s crust is almost entirely igneous, the visible land\nsurface is almost entirely not. \u00a0Sedimentary\nrocks are only IIRC 5% of the volume of the crust, but they\u2019re a\nsolid majority of what you see when\nyou\u2019re clambering\naround in the air.\n
\n\n\nThe\nocean floor is maybe a good way to start thinking about this. \u00a0At the\nmid-ocean ridges, which are giant lines of volcanoes injecting new\ncrust all the time, it\u2019s basically just pure basalt. \u00a0As you walk\nin a straight line along the ocean floor from those volcanoes towards\na continent, you\u2019ll notice little bits of debris start\naccumulating, mostly dead organisms and excrement and so on. \u00a0The\nfarther you walk, the more you see, because the basalt is acting like\na conveyor belt moving between your two landmarks, and the farther\nyou get from the volcano the older it is and the more time it\u2019s had\nto pick up random bits of detritus. \u00a0Eventually you\u2019re wading\nthrough it, and by the time you get to the edge of the ocean it\u2019s\nhundreds or thousands of meters thick, with nary a hint of exposed\nbasalt. \u00a0But it\u2019s still under there, much thicker than the layer of\ngoo on top of it. \u00a0So there\u2019s this patina of order laid across the\nigneous crust, with linearly increasing mud thickness. \u00a0One of the\nmore reliable geological gradients in the solar system, as it\nhappens. \u00a0\n\n
\n\n\nThe\ncontinents are trickier because they don\u2019t die\nof old age.\n That granite is much less dense than the basalt you get in oceans,\nso it floats for basically forever without getting injected back\ninside the planet. \u00a0Sediments\naccumulate and\nget remixed\nover billions of years instead of millions, and a diversity of forms\nproliferates because you can have second-order, third-order,\nfourth-order weathering, weathering of metamorphic rocks, biological\nchemistry, on and on and on. \u00a0Around the edges, you get the scraping\nweirdness of plate subduction, every now and then you even get weird\nthings happening when some vast object bumps the continent from\nunderneath. \u00a0But because a plate itself is so large, most of the\ninteresting and dynamic activity is all happening at the edges,\nleaving the bulk granite more or less inert for billions and\nbillions of years. \u00a0Everything that you\u2019re calling ordered happens\nin a narrow film on the outer edge of a narrow film.
\nBut\nthat surface environment, narrow as it may be, is quite intense and\ndestructive. \u00a0So for any given patch of continent, at any given time,\nthe surface is in flux- if it\u2019s not actively being buried, it\u2019s\nactively being eroded.\n So\nany sedimentary rocks that you see come from these areas where the\nsurface was preserved through rapid accumulation, shattered\nfragments of the erosional areas\nfinally\nbeing blasted to a place where they\u2019re buried\ntoo quickly to be destroyed before finding protective sequestration\naway from the surface. \u00a0We call them \u2018basins\u2019. \u00a0Often but not\nalways underwater- \u00a0like river deltas at shorelines, that kind of\nthing. \u00a0But there\u2019s plenty of examples of preserved deserts and\nrivers as well, anywhere that wind and water could bring a lot of\nrandom bits of stuff in and leave them there.
\nThe\nconditions in any given basin are going to depend on a lot of\nenvironmental factors- biological activity, atmospheric and\nenvironmental conditions, the power of the force that brings\nsediments in, how\nold the rocks are, an\nendless list really. \u00a0Basins themselves can be quite large, many\nmiles across, and the depositional conditions within any given moment\nwill usually be pretty similar because entropy. So you\u2019ll see\nsimilar \u2018packets\u2019 of debris fragments landing all over the basin\nat about the same period of time. \u00a0But as we all know, the atmosphere\nis a fickle bitch. \u00a0So as time passes, so do those conditions, and so\nthese basins produce distinct layers that vary in fragment size,\ncolor, chemistry, and so on. \u00a0And there are so many different options\nfor basin conditions that you get a rich taxonomy of different\nsedimentary rock types. \u00a0Then they\u2019re all buried, later exhumed,\nand outcroups have taken on that \u2018order\u2019 that you\nnoticed.
\nAnother\nmajor source of order comes from the fact that the crust is so\nbrittle. \u00a0At the planetary scale, the crust has a tensile strength of\nbasically zero, so every time something happens tectonically it\nshatters. \u00a0Uplift, load\ndeposition, torque, you name it and there\u2019s probably a patch of\nEarth\u2019s crust losing its shit about it. \u00a0These cracks, which you\nknow as fault lines, are therefore ubiquitous, most of them not\nreally representing a whole lot of motion, most of them again near\nthe surface because that\u2019s where force imbalances have to happen.\n
\n\n\nAnd\nwhen water is flowing through the near-surface, it will tend\npreferentially to flow along these fault lines, because they\u2019re the\nweak points and pre-drilled tunnels. \u00a0And when that flow takes the\nwater from one place to another, with a different temperature and\npressure and lithological environment, the chemical equilibrium of\nimpurities in that water changes. \u00a0It leeches certain ions from the\nsurrounding rock, and precipitates others out. \u00a0And this is a great\nway to collect large masses of very specific elements in one place-\ngold, for instance. \u00a0Most mining for precious metals is about finding\nsuch places. \u00a0That\u2019s why\nyou tend to find precious ores in \u2018veins\u2019; they fill the original\nfault lines that the water was flowing through, long and thin and\ntwisty. \u00a0(Iron is an exception, it\u2019s a whole other thing.)
\nAnyway\nI uh, seem to have written a fairly long essay. \u00a0But yeah! \u00a0Rocks.fascinating, thanks!
\n\u201cevery now and then you even get weird things happening when some vast object bumps the continent from underneath\u201d
\nwhat
\nLook, sometimes an unimaginably large cthonic entity rises from deep in the earth, causes a small mountain range by brushing too close the surface, and passes away as mysteriously as it came.\u00a0 It\u2019s 2K18, get with the program already.\u00a0 Sigh
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