# Could Earth be shrinking?

1. Aug 14, 2010

### brainstorm

Would Earth's volume shrink as it cools and settles? If so, how much would its diameter have decreased since, say, the first life appeared? Also, how much variation would this cause in gravity at sea level and the length of a day/night cycle? How much more could it shrink by the time the core is completely cooled?

2. Aug 14, 2010

### mgb_phys

The earth's core temperature hasn't gone down very much since the first life.
The average diameter of the earth is measured extremely accurately (0.1mm levels by VLBI) and doesn't change much.

The length of the day has been increasing as the Earth's rotation slows (due to the moon) the years was 450-500 days long in the days of dinosaurs. This has a slight change on the gravity at the equator

3. Aug 14, 2010

### Staff: Mentor

BS, did you take into consideration how much additional material from meteorites is added to the earth every day?

4. Aug 14, 2010

### brainstorm

So the added mass is responsible for the rotation speeding up? Is the Earth growing in diameter then?

Also, how do you know that the greater number of days wasn't due to the year being longer because Earth was further from the sun, for example? BTW, what evidence demonstrates how many days were in a dinosaur year anyway, Flintstones calendars?

5. Aug 14, 2010

### Jim1138

Tidal effects slow down the Earth's rotation and increase the orbital radius of the Moon. Eventually, the Earth will tidal lock with the Moon, the Moon's orbit will decrease over billions (and billions) of years and eventually re-merge with the Earth. http://en.wikipedia.org/wiki/Tidal_acceleration

6. Aug 14, 2010

### Staff: Mentor

Did you mean to ask MGB?

7. Aug 14, 2010

### brainstorm

What is "tidal lock?"

What is "MGB?"

8. Aug 14, 2010

### Staff: Mentor

The member that you should have been responding to instead of me.

BS, do you pay any attention to what you respond to?

9. Aug 14, 2010

### buzzjig

If Earths core cooled down completely what would be the effects on the Formation of Earth?

I think that eventually, over millions of years, the enitre surface of the Earth would be covered in water, a huge ocean. There would be no new mountains being formed, and eventually errosion from rain would erode every protruding part of the earths surface until the surface was so flat the entire earth would be covered by water. Similar to what Europa looks like, a big ball of ice.

Could that happen?

10. Aug 14, 2010

### brainstorm

You called "BS" and suggested that mass-increase due to meteorites would be responsible for the rotation speed increase - or did I misinterpret your post?

Do I pay attention to what I respond to? More to the content than to the poster, honestly. I treat these forum discussions as open critical exchanges. I figure anyone is free to respond to anything anyone else says because its all public.

11. Aug 14, 2010

### Staff: Mentor

BS=BrainStorm

And you did misinterpret my post. I asked if you had considered the mass from meteorites, nothing else.

12. Aug 14, 2010

### brainstorm

Now I'm utterly confused by miscommunication. I thought you were disagreeing with the other poster with reference to meteorite mass addition. To answer you question, no, I did not in the slightest consider meteorites when posting. I was just thinking that the core could be shrinking as it cooled and that matter settles through time due to erosion and other entropic mechanisms.

What is your point with the meteorites? That they would add to the mass of the planet, the volume, or both? And with what implications/effects?

13. Aug 14, 2010

### Staff: Mentor

Cosmic "dust" adds to the mass of the earth.

You realize that if the earth's core cooled, which I take to mean that it shuts down, that the fate of earth would be similar to that of Mars. Do we have evidence of Mars shrinking when it's core shut down?

14. Aug 14, 2010

### brainstorm

I think you're right that everything that falls to Earth adds to its mass. I have no idea how much this is, how much it would affect the volume and mass over very long periods of time.

What basis do you have for thinking that Mars was more similar to Earth at some point? Is there even any evidence that it had a molten core besides the assumption that it was formed in a similar way and around the same time as Earth? As for the shrinking, I tried googling molten metal density and nothing accessible to my feeble mind emerged. I assume for some reason that hotter molten liquid maintains lower density than if it cools. This is admittedly naive and based only on the hunch that heat results in pressure/motion. Water expands when it freezes but this is exceptional, I think.

15. Aug 14, 2010

### Staff: Mentor

Just going by what the current scientific belief is.
http://science.nasa.gov/science-news/science-at-nasa/2001/ast31jan_1/

16. Aug 14, 2010

### Janus

Staff Emeritus
Mars' surface is dotted with extinct volcanoes, lava plains and other evidence of being geologically active in the past.

17. Aug 15, 2010

### loseyourname

Staff Emeritus
If you want to isolate volume changes due to cooling effects, just figure out what the mantle is made of, find the coefficients of volumetric thermal expansion for each, and figure out the volume change expected from temperature decrease. I think the main materials down there are iron, magnesium, and silicon in various mineral configurations. The inner core is solid, so it wouldn't expand or contract, but I guess the outer core and the entire mantle would.

18. Aug 15, 2010

### airbourne

A few posts in here have raised a a question (simple curiosity).

Does anyone have information of the volume of matter entering earth annually (dust etc) compared to the volume of matter we've put into orbit/sent permanently into space? I would assume much more mass is leaving the earth than entering it at present.

19. Aug 15, 2010

### brainstorm

That seems counter-intuitive. Things fall down into a gravity well easier than they find their way out of it. Plus, it's not as though the satellites put into orbit are leaving Earth; they're just gaining a lot of altitude on it.

edit: Loseyourname, I tried googling the issue of molten metal density/expansion and there was nothing simple and straight-forward enough for my feeble non-engineer mind. I would think this would be standard knowledge, like the fact that water expands when it freezes, but no one has chimed in with it.

20. Aug 19, 2010

Here is some interesting info:

Excerpt:
.....the Earth's total mass increases by one tenth of one millionth, or one one-hundred-thousandth of a percent, over the entire 4.5 billion years]

http://www.talkorigins.org/faqs/moon-dust.html

21. Aug 19, 2010

### Ophiolite

This, regretably, is oversimplified to the point of uselessness. You have completely ignored the phase changes in which significant changes of mineral composition or structure occur in response to ambient temperature and pressure. The net result of a decrease in temperature may be to promote a change to a less dense mineral, or mineral suite. This could result in an increase, decrease, or zero change in overall volume. Only a very sophisticated finite element analysis, based upon several questionable suppositions could hope to approach any sort of a meaningful answer.

22. Aug 19, 2010

### Staff: Mentor

Dust/meteorites mean mass increase, at the same time Earth is losing some of the mass as the atmospheric gases escape.

See for example http://gsabulletin.gsapubs.org/content/82/4/1085.short

23. Aug 22, 2010

### airbourne

24. Aug 22, 2010

### Studiot

25. Oct 16, 2010

### sigma143

Whether or not the earth is growing or shrinking in diameter could be diagnosed with why many Dinosaurs structures were so immense. Interjecting with the physics of biology, I am pondering how the bone structure etc. of immense dinosaurs could be related to this topic. The relative mass of the Earth may have remained unaltered, yet the displacement of the density of elements it consisted of may have changed.

If the dinosaurs were larger because gravitation towards the earth was weaker, then we could assume either the crust of the earth was farther away from the center of the earth(larger diameter), or the earth was spinning much faster and lessened the effect of gravitation (i'm not too sure if increased rotation speed would lessen gravitation; info please).
Or the dinosaurs bones and other structures developed in such a way because they had to be able to repel greater gravitation, of which we could say the crust was closer to the core of the Earth bach then(smaller diameter). The latter would assume that the rotation speed of Earth was rotating Even faster due to conservation of momentum. (not sure again if the effects of rotation would equalize the increased gravitational force)

I'm quite compelled with whether or not the earth was further away from the sun 200 million or more years ago as well, because this also would have affected the length of year! Was the orbit of the earth longer, which in effect would compensate for a faster rotation and exposure to temperature and pressure from the sun? The different convective currents inside the Earth due to this would alter the maleability of mantle and crust.

If so, how might radiation pressure from the sun have affected the saturation balance between solid, liquid, and gas on the surface of the Earth as well as temperature(displacing the true diameter of solid Earth) and thus the distribution of masses of these phases (as Ophiolite states) through this timescale?

Last edited: Oct 16, 2010