<p>Before earth had water on it's surface, two main types of rocks formed to create continental plates.
basaltic rock
<a href="https://i.sstatic.net/5xsae.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/5xsae.jpg" alt="enter image description here" /></a></p>
<p>and Granitic rock
<a href="https://i.sstatic.net/oemws.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/oemws.jpg" alt="enter image description here" /></a></p>
<p>Continental crustal material is just sligtly less dense than basaltic rock because when heavier basaltic rock is subducted under the continents and melts once again, the lighter such elements like aluminum and silicon on the otherhand tend to move upward and the heavier elements and minerals like Iron migrate down. That makes for the iron-rich portions on the other hand are being recycled back into the mantle and the silicon rich rocks ascending to the surface. Volcanic eruptions that occur above the ocean may deposit new basaltic rock in turn weighing down continental plates but it's continuous activity however granite's higher melting point means more weight must be contributed
<a href="https://i.sstatic.net/YsOO0.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/YsOO0.jpg" alt="enter image description here" /></a></p>

CC BY-SA 4.0
2022-04-13T10:32:18.997Z
Suggest maybe a reread of some of the grammar, as it's pretty tough to understand.
JeopardyTempest
CC BY-SA 4.0
2022-04-13T12:06:57.717Z
"when heavier basaltic rock is subducted under the continents and melts once again": while there are some occurrences of oceanic plate melting (see adakites), it is generally not how subduction works. It's the mantle wedge above the subducting plate (the slab) that melts due to the release of fluids from the slab, which lowers the melting point of peridotite (a process called metasomatism).
Jean-Marie Prival
CC BY-SA 4.0
2022-04-13T07:11:24.333Z
2022-04-14T02:51:56.930Z
2022-04-14T02:51:56.930Z
LazyReader
<p>First they do get slowly mixed at the plate boundaries but the mixing is minimal compared to the amount of crust there is. They do not mix easily since they have different densities, they tend to separate out when melted so most mixing is done in their solid states. </p>

<p>An easy way to think of it is there is not enough continental crust to cover the entire planet so oceanic crust is filling in the gaps. A better analogy is to think of continental crust as scum on pond (or foam in a tub). The uppermost portion of the mantle and oceanic crust are basically the same thing, oceanic crust is just colder, while continental crust is different in composition it is lighter and floats on top but as I said there is not enough of it to cover the entire planet so like scum on a pond it forms clumps and gets pushed around by the underlying currents. And just like pond scum they separate out because they have different densities than the "water" so they collect on the surface.</p>

CC BY-SA 4.0
2022-04-13T22:20:11.783Z
I'm sorry, but this is a terrible answer! As a geo-enthusiast, I'm also slightly embarrassed by our community's inability to flag such answers as outright wrong.
Knob Scratcher
CC BY-SA 4.0
2022-04-14T20:05:44.603Z
@KnobScratcher we do, its called voting and comments.
John
CC BY-SA 3.0
2018-03-26T21:55:28.167Z
2018-03-27T01:36:37.117Z
2018-03-27T01:36:37.117Z
John
<p>Why does Earth have two clearly differentiated kinds of crusts, namely continental and oceanic? How did they differentiate initially and why don't they get mixed up over time? </p>

CC BY-SA 3.0
2018-03-27T05:55:05.917Z
Such a simple question, with such a hard and complicated answer. It is still one of the heavily debated topics in geology.
Gimelist
CC BY-SA 3.0
2018-03-27T11:32:53.683Z
I think an intrusive vs extrusive answer could be made, but as geology isn't my subject, I'd prefer that someone not be me.    http://www2.nau.edu/~gaud/RiodeFlag/ignsrck.htm
userLTK
<crust>
Why does Earth have two kinds of crusts?
Stephane Bersier
# Why does Earth have two kinds of crusts?

(Asked by: username_0 on 2018-03-26)

Why does Earth have two clearly differentiated kinds of crusts, namely continental and oceanic? How did they differentiate initially and why don't they get mixed up over time?

**Tags:** crust

> **Comments:**
> **username_1**: Such a simple question, with such a hard and complicated answer. It is still one of the heavily debated topics in geology.  
> **username_2**: I think an intrusive vs extrusive answer could be made, but as geology isn't my subject, I'd prefer that someone not be me.    http://www2.nau.edu/~gaud/RiodeFlag/ignsrck.htm  

---

## 2 answers

### Answer by username_6 on 2018-03-26. Score: 6 | Accepted answer

First they do get slowly mixed at the plate boundaries but the mixing is minimal compared to the amount of crust there is. They do not mix easily since they have different densities, they tend to separate out when melted so most mixing is done in their solid states.

An easy way to think of it is there is not enough continental crust to cover the entire planet so oceanic crust is filling in the gaps. A better analogy is to think of continental crust as scum on pond (or foam in a tub). The uppermost portion of the mantle and oceanic crust are basically the same thing, oceanic crust is just colder, while continental crust is different in composition it is lighter and floats on top but as I said there is not enough of it to cover the entire planet so like scum on a pond it forms clumps and gets pushed around by the underlying currents. And just like pond scum they separate out because they have different densities than the "water" so they collect on the surface.

> **Comments:**
> **username_7**: I'm sorry, but this is a terrible answer! As a geo-enthusiast, I'm also slightly embarrassed by our community's inability to flag such answers as outright wrong.  
> **username_6**: @username_7 we do, its called voting and comments.  

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### Answer by username_3 on 2022-04-13. Score: 0

Before earth had water on it's surface, two main types of rocks formed to create continental plates.
basaltic rock
[![enter image description here](https://i.sstatic.net/5xsae.jpg)](https://i.sstatic.net/5xsae.jpg)

and Granitic rock
[![enter image description here](https://i.sstatic.net/oemws.jpg)](https://i.sstatic.net/oemws.jpg)

Continental crustal material is just sligtly less dense than basaltic rock because when heavier basaltic rock is subducted under the continents and melts once again, the lighter such elements like aluminum and silicon on the otherhand tend to move upward and the heavier elements and minerals like Iron migrate down. That makes for the iron-rich portions on the other hand are being recycled back into the mantle and the silicon rich rocks ascending to the surface. Volcanic eruptions that occur above the ocean may deposit new basaltic rock in turn weighing down continental plates but it's continuous activity however granite's higher melting point means more weight must be contributed
[![enter image description here](https://i.sstatic.net/YsOO0.jpg)](https://i.sstatic.net/YsOO0.jpg)

> **Comments:**
> **username_4**: Suggest maybe a reread of some of the grammar, as it's pretty tough to understand.  
> **username_5**: "when heavier basaltic rock is subducted under the continents and melts once again": while there are some occurrences of oceanic plate melting (see adakites), it is generally not how subduction works. It's the mantle wedge above the subducting plate (the slab) that melts due to the release of fluids from the slab, which lowers the melting point of peridotite (a process called metasomatism).  

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