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The continental plates are more or less loose from the oceanic plates which are heavier and of other material (basalt) and because continental plates are lighter they 'float' on top of the oceanic plates. But how did they get there?

I don't know if I'm right that islands like Iceland is made of the oceanicp plate because it is in middle of the oceanic plates? But did all 'countries' start like that? It doesn't seem so as most countries/continents are made of lighter material. So how did that lighter material get on top of the oceanic plate?

The explanation of an other answer was: ' This is because the magma that solidifies to form each of these crusts travels, by partially melting its way up, through different materials and different thicknesses. The consequence of this is that the crust making up the continents is less dense than that making up the bottom of the oceans.'

Does this mean that the magma comming from beneath (hotspot etc.) is getting mixed with different materials? Or is this magma already a mix of different material where the heavier ones would stay more or sink at the bottom of the solidifieing two plates? But what makes the upper one not tight joined or gradually to the one beneath?

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Too long for a comment, so I'll post here, but it's far from a complete answer.

Basalt can't become continental crust because the elements are different. Iceland is made up of primarily basalt rock. (80% per this article). So it's made up of a different material than continental crusts. Basalt rock is also less sturdy and less enduring than granite, so it's unlikely that a continental crust could be formed out of Icelandic eruptions or any similar eruptions. My best guess is that Iceland will break apart over time, preventing the formation of a permanent land-mass.

Granite, the primary material in continental crust, has more Aluminum and Potassium in it. Basalt, the heavier stuff, has more Magnesium. Source. They are made from distinctly separate processes and material, so one can't simply be turned into the other.

While I have no doubt that some mixing goes on within the earth's mantle, I don't believe a significant amount of new continental crust is regularly being formed because to form new crust, you would need free aluminum ready to bind with Oxygen and Silicates and I believe most of the aluminum has already done that. That's my layman's answer anyway. I'm open to being corrected.

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    $\begingroup$ There is no free aluminium anywhere in the Earth, it exists mainly as a huge variety of silicates, or rarely as oxides. But my main comment is that it is inappropriate to think of ancient continental crust differentiating from modern oceanic crust. Both are end products of more than 3 billion years of fractionation, whilst the vast majority of the Earths crust was formed from an undifferentiated mix soon after planetary condensation. $\endgroup$ – Gordon Stanger Oct 14 '16 at 7:19
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If I had to guess, I'd attribute the difference to gravitational separation - as you said, continental crust is less dense than oceanic crust.

I suppose chemically different magmas would seperate in a similar way.... Your question is a little difficult to understand sorry..!

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