# Thermal expansion of Earth

Matter tends to change its volume in response to changes in temperature--or, hot things tend to be expanded. Is the thermal expansion of Earth significant?

That is, is it significant when compared against a cooler or perhaps internally-inert Earth?

• It's complicated. Here is one paper. – Keith McClary Nov 22 '19 at 23:39
• @KeithMcClary I'll read into it, thanks! – BMF Nov 22 '19 at 23:39
• Matter also tends to change its volume in response to pressure. So as you go deeper into the Earth, increase in temperature would cause expansion while increase in pressure would cause compression. I'm not saying the later process would completely counterbalance the former (silicate compressibility is pretty low), but that would be another parameter to take into account... – Jean-Marie Prival Nov 23 '19 at 12:14
• @jmprival and an increase in pressure results in an increase in temperature which works to counteract compression... Yikes. – BMF Nov 23 '19 at 12:43

Here's a take on calculating earth's thermal expansion and shrinking, under certain assumptions mentioned in the text. It seems to me, from a short overview, that the core is assumed to be constant over the time. It focuses on the mantle part of the earth, its mineral phases and convetion. Take it with a grain of salt:

https://www.sciencedirect.com/science/article/pii/S167498711200148X

tl,dr: after consolidation, earth's radius was 120km larger than today. It shrank by 110km in a few million years, then expanded a bit (11km), then contracted by 8km, and subsequently contracted by around 12km to the present day.

If they say so :-)

• This is pretty much just what I was looking for, thanks! I was expecting the difference to be much less than 120 km, even perhaps insignificant. Fascinating ... – BMF Dec 23 '19 at 17:28

There is no thermal expansion of the Earth. On the contrary, the Earth is very gradually cooling, and therefore shrinking, but so gradually that the effects are imperceptible and perhaps in the short term unmeasurable. You can see the process at work on Mars, which has over the course of 4.5 billion years lost much of its internal heat owing to its smaller size and greater distance from the sun. The result is that volcanism has ceased and the planet has a much thicker crust, but the core is still molten. Therefore Mars must have shrunk a little, but we don't know exactly how much because we have no precise measurements from four billion years ago.

Gravitational flexion ie. tides in the Earths outer regions caused by the moon, as well as decay of radioactive elements in the core and mantle, generate heat which reduces the rate at which the Earth cools. The Earth's loss of heat is so gradual that its interior still won't be completely inert when the sun expands to become a red giant in 5 billion years time. The Earth will then stop cooling and the surface will be scorched and probably melted, but it's nothing to worry about because 5 billion years is an almost unimaginably long time. Humans will have gone to join the dinosaurs billions of years earlier.

• I guess what I'm really interested in is how much of Earth's radius can be attributed to its internal heat. I suppose this would be very difficult to know without, say, putting Earth somewhere else for several billion years and closely watching it. But, the Earth is made of rock and metal inside predictable conditions. Is it impossible to know this? – BMF Nov 22 '19 at 23:31
• Earth's internal heat budget (radiogenic/primordial) and recent Marsian volcanism could be further researched in the answer. – user18411 Dec 23 '19 at 9:08
• What recent volcanism? The Martian volcanoes have been extinct for many millions of years. – Michael Walsby Dec 23 '19 at 10:17
• Example: sci.esa.int/web/mars-express/-/… and others, like polar subsurface heating. If you care to search. – user18411 Dec 23 '19 at 10:51
• @John Why a new question? This question asks for just that. Well, maybe my wording is a bit funny. – BMF Dec 23 '19 at 16:42