<p>The idea that the heat of the earth is from the time of its formation dates back to the late 1800s before radioactivity was known.  Unfortunately for that theory it was shown that an initial blob of molten rock would cool to a solid chunk, radiating its heat into space, in a few tens of millions of years.</p>
<p>When radioactivity was discovered it was seen as a way out of the problem but the known radioactive elements were sufficiently rare in the earth's makeup that they couldn't provide enough heat.  Finally in the fifties the bulk composition  of the earth and the occurrence of radioactive isotopes were well enough known that it was shown that there is enough radioactive potassium, with a long enough half life, to provide the observed heat flux from the earth's interior.  Most of the heat is generated in the mantle, not the core since, as you point out, that is where the potassium is.  The mantle is also the layer that has convection currents in it to release that heat and drive plate tectonics.</p>
<p>The outer core is liquid because, at the pressure it is under,  being a liquid is the thermodynamically stable state of the heavy core minerals.  The ice under a skater's blade turns into a thin layer of water under the passing skater's pressure, lubricating the passage of the skate.  The water does not warm up much, it is still below freezing, but it is not stable as a solid under the pressure of the skate. As soon as the pressure is released the water returns to ice which is the stable state of H2O at atmospheric pressure and sub-freezing temperatures.  Similarly the outer core is liquid because that is its stable state given the pressure and temperature at that depth.</p>
<p>Increase the pressure more, as happens as you go deeper, and the stable state of the core material is a solid.  This explains the reason the inner core is solid.</p>

CC BY-SA 4.0
2020-09-23T15:17:03.353Z
This all seems very plausible, a reference supporting this would be an excellent addition.
Fred
CC BY-SA 4.0
2020-09-24T01:51:30.387Z
I think the links provided in the linked questions provided by (Jean-Marie Prival)[https://earthscience.stackexchange.com/users/18081/jean-marie-prival] and the links in the "Related Questions" section have lots of references. Wikipedia's Earth Science topics deal with this in much more detail and with references.  This is just the "elevator pitch".
verisimilidude
CC BY-SA 4.0
2020-09-23T01:49:37.120Z
2020-09-23T01:49:37.120Z
verisimilidude
<p>A hot molten core is important for making the Earth geologically active. I believe most of the heat is from the time of the planet formation, and the high pressures due to everything pushing down to the core. How much of this can be attributed to radioactivity? How can we know it for sure, considering the pressure and temperature make it in accessible? Research show that radioactive potassium plays a role in it, but if it is true the how can a light element such as potassium sink to the core where it can generate heat by radioactive decay?</p>

CC BY-SA 4.0
2020-09-23T07:20:01.977Z
Possible duplicate of https://earthscience.stackexchange.com/q/19588/18081 & https://earthscience.stackexchange.com/q/2623/18081
Jean-Marie Prival
CC BY-SA 4.0
2020-09-23T09:47:05.393Z
And possibly https://earthscience.stackexchange.com/q/9005/18081 This question has already been asked (and answered) many times.
Jean-Marie Prival
<geology><core><radioactivity>
How much role does radioactivity play in making Earth geologically active
schizoid_man
# How much role does radioactivity play in making Earth geologically active

(Asked by: username_0 on 2020-09-22)

A hot molten core is important for making the Earth geologically active. I believe most of the heat is from the time of the planet formation, and the high pressures due to everything pushing down to the core. How much of this can be attributed to radioactivity? How can we know it for sure, considering the pressure and temperature make it in accessible? Research show that radioactive potassium plays a role in it, but if it is true the how can a light element such as potassium sink to the core where it can generate heat by radioactive decay?

**Tags:** geology, core, radioactivity

> **Comments:**
> **username_1**: Possible duplicate of https://earthscience.stackexchange.com/q/19588/18081 & https://earthscience.stackexchange.com/q/2623/18081  
> **username_1**: And possibly https://earthscience.stackexchange.com/q/9005/18081 This question has already been asked (and answered) many times.  

---

## 1 answers

### Answer by username_2 on 2020-09-23. Score: 1

The idea that the heat of the earth is from the time of its formation dates back to the late 1800s before radioactivity was known. Unfortunately for that theory it was shown that an initial blob of molten rock would cool to a solid chunk, radiating its heat into space, in a few tens of millions of years.

When radioactivity was discovered it was seen as a way out of the problem but the known radioactive elements were sufficiently rare in the earth's makeup that they couldn't provide enough heat. Finally in the fifties the bulk composition of the earth and the occurrence of radioactive isotopes were well enough known that it was shown that there is enough radioactive potassium, with a long enough half life, to provide the observed heat flux from the earth's interior. Most of the heat is generated in the mantle, not the core since, as you point out, that is where the potassium is. The mantle is also the layer that has convection currents in it to release that heat and drive plate tectonics.

The outer core is liquid because, at the pressure it is under, being a liquid is the thermodynamically stable state of the heavy core minerals. The ice under a skater's blade turns into a thin layer of water under the passing skater's pressure, lubricating the passage of the skate. The water does not warm up much, it is still below freezing, but it is not stable as a solid under the pressure of the skate. As soon as the pressure is released the water returns to ice which is the stable state of H2O at atmospheric pressure and sub-freezing temperatures. Similarly the outer core is liquid because that is its stable state given the pressure and temperature at that depth.

Increase the pressure more, as happens as you go deeper, and the stable state of the core material is a solid. This explains the reason the inner core is solid.

> **Comments:**
> **username_3**: This all seems very plausible, a reference supporting this would be an excellent addition.  
> **username_2**: I think the links provided in the linked questions provided by (username_1)[https://earthscience.stackexchange.com/users/18081/username_1-username_1] and the links in the "Related Questions" section have lots of references. Wikipedia's Earth Science topics deal with this in much more detail and with references.  This is just the "elevator pitch".  

---

