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?
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4$\begingroup$ 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. $\endgroup$– KenshinApr 24, 2014 at 16:15
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2$\begingroup$ As an aside, it doesn't make sense to say "3-4 times the temperature" when you're not even using absolute temperature. $\endgroup$– David RicherbyApr 24, 2014 at 22:55
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3 Answers
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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 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) 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.
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7$\begingroup$ 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. $\endgroup$ Apr 25, 2014 at 11:22
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$\begingroup$ 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. $\endgroup$– EeveeFeb 22, 2018 at 22:37
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In addition to the answers below and my comment above, I believe the following phase diagram, from DavePhd's answer here, sourced from here, 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.
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2$\begingroup$ 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. $\endgroup$– kaberettApr 29, 2014 at 21:01
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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 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 for detailed information.
