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Anyone who has a well-versed background in geology will know that the rock of the continents is granite.

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It forms as lava cools down very slowly, creating large minerals and crystals that shielded the rock from cracking and thus the brunt of erosion. Granite also "floats", if you will, because despite its toughness, it's not dense like basalt, the rock of the oceans.

Now the question for continental makeup in an alternate Earth is this--is there a tough but still buoyant alternative to granite, preferably a rock tougher to resist against erosion than granite itself?

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  • $\begingroup$ If there was, I think geologists would be aware if it. I only have 1 year uni geology but am pretty sure something like that would have been mentioned, at least in passing. You might have to lose the hard science tag... Maybe scienced -based would still be appropriate as that will still limit answers to scientifically 'theoretically' feasible without actually asking for formulas, references and citations! Good luck... $\endgroup$ – EveryBitHelps Sep 5 '16 at 3:43
  • $\begingroup$ adamantium .. :p $\endgroup$ – Julius Sep 5 '16 at 3:57
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    $\begingroup$ I have a degree in geology, technically Granite is not formed from lava cooling, it is an intrusive igneous rock meaning it was formed when magma cooled down slowly within the earths crust. An acidic lava would form the extrusive igneous rock rhyolite and would have fine grains from rapid cooling. $\endgroup$ – Sarriesfan Sep 5 '16 at 9:31
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    $\begingroup$ Can you give a hint why you NEED a rock which is more resistant to erosion than granite? And are you talking erosion on a human timescale? 10 million years? 100 million years? A billion years? Also, are you talking physical erosion (dominates in cold climates), chemical erosion (dominates in hot climates) or both? $\endgroup$ – DrBob Sep 5 '16 at 17:05
  • $\begingroup$ Does geology have an equivalent to the Anthropic Principle? Because I feel like you could make a very scientific case for it's application here. $\endgroup$ – Lord Dust Sep 5 '16 at 22:13
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Granite ‘floats’ because it is a ‘sial’ rock. That means its composition is predominantly Si (silicate minerals) and Al (aluminium minerals). It is lighter than the ‘sima’ rocks of oceanic crust (mainly magnesium silicate minerals). Sial is less dense than sima, since it contains less iron and less magnesium, and more aluminium.

The bulk of the continents are going to be igneous rocks (like granite and basalt), since they form first, and are what the proto-continents are made of. You can’t have a sedimentary rock unless there is a pre-existing rock to break down. You can’t have a metamorphic rock unless there is a pre-existing rock to subject to a new heat and/or temperature regime.

So if you identify an igneous rock which is tougher than granite for your planet, you need to (a) find out its main chemical composition and density, then (b) alter the chemical composition of your sima so that your new rock still ‘floats’ on it. This is what Sarriesfan’s answer proposes, I think. I don’t know enough about the toughness of basalt to say how it compares to granite.

Meanwhile... erosion and weathering (IIRC ‘weathering’ is breaking the rock up, and ‘erosion’ is moving the broken bits to somewhere else).

Granite consists of the minerals quartz, feldspar and mica (biotite, muscovite and/or amphibole). Here’s what happens to those minerals when they are weathered. Quartz and muscovite mica are (usually) most resistant to weathering because they are physically tougher. Here’s some pictures of the stages of weathering in granite.

In warmer climates like the tropics, chemical weathering (chemical breakdown) predominates. This is because the speed of chemical reactions roughly doubles for every 10 degrees C increase in temperature. This paper lists how various types of rocks respond to chemical weathering. Chemical weathering includes being dissolved in water, so a wet climate will have faster chemical weathering than a dry climate.

  1. Toughest = granite. Comes equal first with gneiss & mica schist
  2. Gabbro and sandstone
  3. Volcanic rocks
  4. Shales
  5. Serpentine and amphibolite
  6. Carbonate rocks (12 times more prone to chemical erosion than granite). Chalk, limestone, marble.
  7. Gypsum
  8. Rock salt (80 times more prone to chemical erosion than granite)

In temperate and arctic climates physical weathering (frost shattering and so on) become more important in destroying rocks. Basically, things which spring to mind when a layman thinks of erosion – battering by waves, being fractured by ice crystals or scoured away by the wind – are the main way that rocks are broken down in cold and temperate places. Chemical weathering still happens, but at a much slower pace.

In very cold climates, quartz isn’t so tough! Here is a paper on cryogenic weathering of granite in Antarctica. The quartz is being fractured preferentially over other minerals because of the low temperature.

So if your planet is amazingly cold, other rocks may last longer than granite, since its quartz is taking a hammering and the chemical reactions which destroy carbonates (like the marble which nigel222 suggested) will be very slow. It won't be very hospitable to human life, however!

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Alter the relative abundances of elements within the solar system the planet formed and have most of the planet made from an ultra basic rock such as olivine, with these taking the place of basalt. Basalt then becomes the less dense rock, which would take the place of granite in forming the continents.

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  • $\begingroup$ can you please edit my answer to add more information about the relative toughness of basalt and olivine, if you have data on this? Thanks. $\endgroup$ – DrBob Sep 7 '16 at 15:27
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Marble?

I will immediately admit that it is not harder than granite. However it has a different advantage.

Granite is made of lots of grains of different minerals. Some of thsee decompose when exposed to water and air. Granite crumbles away, given time.

Marble is highly homogeneous. Also it can not oxidize. It's weakness is that it dissolves in even slightly acidic water. So on Earth water flows along faults in the marble, gradually widening them. The flow increases. Caves form. Eventually a huge collapse happens.

So Granite hills and mountains last longer, but there are some pretty massive marble outcrops.

Marble might do better on a world where atmospheric carbon dioxide ( and therefore rain acidity) was very low. Perhaps a world where plankton convert CO2 to carbonate and dump it on the ocean floor when they die, with a greater efficiency than on Earth? Marble is of course metamorphic limestone, created in the first instance by such plankton.

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http://www.space.com/26335-coldest-white-dwarf-star-diamond.html

Scientists have detected an ex white dwarf of the size of the earth that seems to be made of diamond.

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    $\begingroup$ You beat me to it by seconds! $\endgroup$ – Nolo Sep 5 '16 at 4:00
  • $\begingroup$ Sorry, but diamond's a mineral, not a rock. $\endgroup$ – JohnWDailey Sep 5 '16 at 4:06
  • $\begingroup$ geek.com/wp-content/uploads/2015/10/hank-minerals-590x350.jpg $\endgroup$ – Julius Sep 5 '16 at 4:07
  • $\begingroup$ @JohnWDailey Granite is a mineral and whatever is stronger than granite will be a mineral too. $\endgroup$ – a4android Sep 5 '16 at 13:42
  • $\begingroup$ @a4android Granite is a ROCK, not a mineral. $\endgroup$ – JohnWDailey Sep 5 '16 at 13:59

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