<p>In addition to the answers below and my comment above, I believe the following phase diagram, from DavePhd's answer <a href="https://earthscience.stackexchange.com/questions/530/why-is-earths-outer-core-liquid/535#535">here</a>, sourced from <a href="http://www.geo.arizona.edu/xtal/geos306/ironphasediagram360.jpg" rel="nofollow noreferrer">here</a>, is more appropriate for the pressure levels near the Earth's core of about 330 to 360GPa.</p>

<p>We can see from the image that for pressures between 330 and 360GPa, the melting temperature ranges from about 6200 K to 6600 K, which is much higher than Earth's inner core temperature of about 5700 K.</p>

<p><img src="https://i.sstatic.net/iIt52.jpg" alt=""></p>

CC BY-SA 3.0
2014-04-29T21:01:59.613Z
For interest, I note that the surface temperature of the Sun is about 5800K.

As an aside, in web-accessibility terms it's good practice to use [meaningful hypertext for links](http://webaim.org/techniques/hypertext/).
kaberett
CC BY-SA 3.0
2014-04-25T04:53:49.493Z
2014-04-25T04:53:49.493Z
2017-04-13T12:50:30.540Z
Kenshin
<p>You are only considering the melting point at atmospheric pressure.  Melting point depends upon pressure.  The pressure in Earth's core is about 350 GigaPascals.  It is important to study the <a href="http://en.wikipedia.org/wiki/Phase_diagram">phase diagram</a> of the substance being considered.  A phase diagram explains what phase (solid, liquid, gas) a substance is in at various temperatures and pressures.  </p>

<p>See <a href="http://www.gps.caltech.edu/~sue/TJA_LindhurstLabWebsite/ListPublications/Papers_pdf/Seismo_2069.pdf">Phase diagram of iron, revised-core temperatures</a> for detailed information.</p>

CC BY-SA 3.0
2014-04-24T16:16:03.610Z
2014-04-24T16:16:03.610Z
DavePhD
<p>Earth's inner core is solid even though the temperature is so high because the pressure is also very high.  According to <a href="http://en.wikipedia.org/wiki/Inner_core" rel="noreferrer">the Wikipedia article</a> on the Earth's inner core, the temperature at the center is $5,700\ \text{K}$ and the pressure is estimated to be $330$ to $360\ \text{GPa}$ ($\sim3\cdot10^{6}\ \text{atm}$).  </p>

<p>The phase diagram shown below (taken from <a href="http://www.agu.org/books/gd/v031/GD031p0083/GD031p0083.pdf" rel="noreferrer">this paper</a>) shows the liquid/solid transition, where fcc and hcp are two different crystalline forms of solid iron.  You can see clearly from the slope of the line going off toward the upper right that iron should be solid at this temperature and pressure.</p>

<p><img src="https://i.sstatic.net/w82eb.png" alt="enter image description here"></p>

CC BY-SA 3.0
2014-04-25T11:22:49.460Z
The phase diagram would be more useful for this answer if it went above 100 GPa. Also, I suggest giving the actual citation and/or the DOI for references, in case of link rot. This one is [10.1029/31GD07](http://dx.doi.org/10.1029/31GD07).
Matt Hall
CC BY-SA 3.0
2018-02-22T22:37:56.503Z
This is a great answer. But I think it would be useful if another source was used besides Wikipedia, because Wikipedia is edited by the community, and you have no idea what people add to those posts sometimes.
Eevee
CC BY-SA 3.0
2014-04-24T16:18:32.650Z
2014-04-24T16:25:01.167Z
2014-04-24T16:25:01.167Z
Chris Mueller
<p>I have never understood why earth's inner core is solid.  Considering that the inner core is made of an iron-nickel alloy (melting point around 1350 C to 1600 C) and the temperature of the inner core is approximately 5430 C (about the temperature of the surface of the sun). Since Earth's core is nearly 3-4 times the melting point of iron-nickel alloys how can it possibly be solid?</p>

CC BY-SA 3.0
2014-04-24T16:15:27.153Z
the temperature at which a phase change takes place depends on pressure.  For instance because there is more pressure at low altitudes than at high altitudes, water boils at less than 100 degrees Celsius at high altitudes like mount Everest.  In general, the higher the pressure, the higher the melting and boiling points are.  Therefore potentially the melting point of iron-nickel alloys is significantly greater in the Earth's core than on the Earth's surface due to an enormous pressure within the Eaerth's core.
Kenshin
CC BY-SA 3.0
2014-04-24T22:55:07.033Z
As an aside, it doesn't make sense to say "3-4 times the temperature" when you're not even using absolute temperature.
David Richerby
<geophysics><core>
Why is Earth's inner core solid?
tux
# Why is Earth's inner core solid?

(Asked by: username_0 on 2014-04-24)

I have never understood why earth's inner core is solid. Considering that the inner core is made of an iron-nickel alloy (melting point around 1350 C to 1600 C) and the temperature of the inner core is approximately 5430 C (about the temperature of the surface of the sun). Since Earth's core is nearly 3-4 times the melting point of iron-nickel alloys how can it possibly be solid?

**Tags:** geophysics, core

> **Comments:**
> **username_1**: the temperature at which a phase change takes place depends on pressure.  For instance because there is more pressure at low altitudes than at high altitudes, water boils at less than 100 degrees Celsius at high altitudes like mount Everest.  In general, the higher the pressure, the higher the melting and boiling points are.  Therefore potentially the melting point of iron-nickel alloys is significantly greater in the Earth's core than on the Earth's surface due to an enormous pressure within the Eaerth's core.  
> **username_2**: As an aside, it doesn't make sense to say "3-4 times the temperature" when you're not even using absolute temperature.  

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## 3 answers

### Answer by username_5 on 2014-04-24. Score: 31 | Accepted answer

Earth's inner core is solid even though the temperature is so high because the pressure is also very high. According to [the Wikipedia article](http://en.wikipedia.org/wiki/Inner_core) on the Earth's inner core, the temperature at the center is $5,700\ \text{K}$ and the pressure is estimated to be $330$ to $360\ \text{GPa}$ ($\sim3\cdot10^{6}\ \text{atm}$).

The phase diagram shown below (taken from [this paper](http://www.agu.org/books/gd/v031/GD031p0083/GD031p0083.pdf)) shows the liquid/solid transition, where fcc and hcp are two different crystalline forms of solid iron. You can see clearly from the slope of the line going off toward the upper right that iron should be solid at this temperature and pressure.

![enter image description here](https://i.sstatic.net/w82eb.png)

> **Comments:**
> **username_6**: The phase diagram would be more useful for this answer if it went above 100 GPa. Also, I suggest giving the actual citation and/or the DOI for references, in case of link rot. This one is [10.1029/31GD07](http://dx.doi.org/10.1029/31GD07).  
> **username_7**: This is a great answer. But I think it would be useful if another source was used besides Wikipedia, because Wikipedia is edited by the community, and you have no idea what people add to those posts sometimes.  

---

### Answer by username_1 on 2014-04-25. Score: 22

In addition to the answers below and my comment above, I believe the following phase diagram, from DavePhd's answer [here](https://earthscience.stackexchange.com/questions/530/why-is-earths-outer-core-liquid/535#535), sourced from [here](http://www.geo.arizona.edu/xtal/geos306/ironphasediagram360.jpg), is more appropriate for the pressure levels near the Earth's core of about 330 to 360GPa.

We can see from the image that for pressures between 330 and 360GPa, the melting temperature ranges from about 6200 K to 6600 K, which is much higher than Earth's inner core temperature of about 5700 K.

![](https://i.sstatic.net/iIt52.jpg)

> **Comments:**
> **username_3**: For interest, I note that the surface temperature of the Sun is about 5800K.  As an aside, in web-accessibility terms it's good practice to use [meaningful hypertext for links](http://webaim.org/techniques/hypertext/).  

---

### Answer by username_4 on 2014-04-24. Score: 12

You are only considering the melting point at atmospheric pressure. Melting point depends upon pressure. The pressure in Earth's core is about 350 GigaPascals. It is important to study the [phase diagram](http://en.wikipedia.org/wiki/Phase_diagram) of the substance being considered. A phase diagram explains what phase (solid, liquid, gas) a substance is in at various temperatures and pressures.

See [Phase diagram of iron, revised-core temperatures](http://www.gps.caltech.edu/~sue/TJA_LindhurstLabWebsite/ListPublications/Papers_pdf/Seismo_2069.pdf) for detailed information.


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