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What is the reason for having silicon in so large quantity on earth surface?

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  • $\begingroup$ The English Wikipedia article on silicon gives a good answer to your question: en.wikipedia.org/wiki/Silicon#Occurrence $\endgroup$ – daniel.neumann Mar 29 '16 at 8:31
  • $\begingroup$ @daniel.neumann actually it's a good answer to why silicon (oxide) is common in Earth as a whole, but it does not address why it's particularly concentrated in the crust. $\endgroup$ – Gimelist Mar 29 '16 at 9:01
  • $\begingroup$ @Michael . Silicon is more common in the mantle than in the core because of its low density. The mantle material is transported as lava, which has a large silicon content, to the surface (crust). Therefore, we find large quantities of silicon in the crust. $\endgroup$ – daniel.neumann Mar 29 '16 at 9:28
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    $\begingroup$ @daniel.neumann I know - I'll write a full answer about it soon $\endgroup$ – Gimelist Mar 29 '16 at 11:28
  • $\begingroup$ @daniel.neumann -- it's not quite correct that mantle material comes to the surface as lava. The mantle underlies the crust, and what you see coming out as lava is actually all molten crust, not mantle material. But it is true that we believe that the chemical composition of crust and mantle are relatively similar, compared to the differences to the core at least. $\endgroup$ – Wolfgang Bangerth Mar 30 '16 at 10:53
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Silicon, in the form of silica (oxidised silicon: Si4+ or SiO2), is indeed very common on Earth's surface.

This answer has two parts: why is silicon common in the Earth as a whole, and why it is concentrated particularly in the crust (therefore the surface).

Why is silicon common in the Earth

Silicon forms in nucleosynthesis processes in large stars (larger than our suns) and in supernovae. Turns out that quite a lot of silicon can be formed this way. When our solar system was still a hot molecular gas cloud, our region in place was rich in silicon (among other elements) so eventually Earth has lots of silicon in it. This is slightly beyond my field of expertise, so I will refer to the link daniel.neumann gave in his comment and to this good explanation at UCSB Science Line.

Why is silicon extremely rich in the Earth's crust?

Now we know there's quite a lot of silicon in the Earth to begin with. How do you end up with even more silicon in the crust (outermost layer) than in the mantle (middle layer, between the crust and the iron metallic core)?

The answer lies in what happens to rocks when they melt. You have a rocky mantle, with let's say 35% to 45% of SiO2. When this rock gets heated or decompressed (or wet, but nevermind this for now), it melts. But it doesn't melt completely, it melts only partially. This will always be richer in SiO2 than the original rock, and the residue of the melting now has even less SiO2. Once SiO2-rich melt moves upwards in the Earth (sometimes even erupting as volcanoes), it moves more SiO2-rich material to the crust. Over billions of year and large successive partial melting events, you end up having quite a lot of SiO2 in the crust.

This gets even better, because in those SiO2-rich rocks, you also have lots of quartz - which is a mineral composed of pure SiO2: 100% SiO2. This is a very resistant mineral to alteration and weathering. So if you have a granite that has only a third of quartz, with time the other two thirds (mostly feldspars) will decay and turn into clays (aka dust, mud). Only the quartz will be left behind, and the rest will wash away with water to the oceans (eventually). This is why you have impressive sand dunes which are "seas" of almost pure SiO2:

enter image description here

Some other questions on this website you might be interested in:

What was the likely composition of Earth's early crust (how did crustal composition evolve)?

Why do felsic materials have lower melting points than mafic?

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What is the reason for having silicon in so large quantity on earth surface?

I'm assuming this is asking about the Earth's crust versus its mantle, rather than the crust versus the core, or the crust versus the universe.

There's lots of silicon in the mantle. Ordered by mass, the six most abundant elements in the mantle are oxygen, magnesium, silicon, iron, calcium and aluminum. These same six elements are also the most abundant elements in the crust, but with a different ordering. Ranked by mass, the six most abundant elements in the crust are oxygen, silicon, aluminum, iron, calcium, and magnesium. The singular most marked difference between mantle rock and crustal rock is magnesium. It's not so much that the crust is enriched with silicon as it is that the crust is strongly depleted of magnesium compared to the mantle. Magnesium is the only common lithophilic element that displays this behavior.

The reason is the partial melt that occurs where volcanoes and magmatic intrusions form. The chemical nature of magnesium tends to makes magnesium compounds stay with the solidus. This in turn differentiates the crust from the mantle.

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  • $\begingroup$ Magnesium is the only common lithophilic element that displays... $\endgroup$ – Gimelist Mar 29 '16 at 22:19
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The top 5 elements most abundant in the earth are: Source with nice table wikipedia...

  • Oxygen
  • Silicon
  • Aluminium
  • Iron
  • Calcium

Silicon is a basic component in rock forming minerals: quartz, feldspar hornblende and other silicates. Silicon is common throughout the entire earth (probably less so in the core).

All of the inner planets Mercury, Venus, Earth ( plus Moon) and Mars) appear to have silicon in abundance. Rocky/stony meteorites also contain abundant silicon.

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  • $\begingroup$ I think those are the 5 most abundant elements in the Earth's lithosphere or crust. I don't think it holds for the entire Earth. $\endgroup$ – equant Apr 3 '16 at 2:43

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