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As the title suggests, what happens to a planet geologically and geographically over time?

This question came to mind from the many sessions of Civilization I have played where when you set up a custom game, "World Age", measured in I want to say millions of years, is an option.

But this got me thinking what are the impacts of age on a planet? For this exercise lets assume we are talking about an earth-like world that was created and stable (supporting human life) from year 0.

The world I am working on is roughly 5000 years old (that is how much timeline I have developed) and in my head it appears much the same that the earth does today.

Does my world that has 5000 years of developed timeline need to change to fit the realities of a younger world? What would that look like?

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    $\begingroup$ In geological time scales, 5000 years are nothing. $\endgroup$ – celtschk Oct 1 '14 at 19:52
  • $\begingroup$ @celtschk hence the question! :) $\endgroup$ – James Oct 1 '14 at 19:53
  • $\begingroup$ If your world looks like earth today, then 5000 years from now the main thing to take into account would be global warming. Is that what you are thinking of? Or are you thinking more elevation wearing? $\endgroup$ – Vulcronos Oct 1 '14 at 19:55
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    $\begingroup$ It should be noted that Civ 5 measures the world age in billions of years, not millions. Also, when you say your world is 5000 years old, do you mean the age of the world from its formation? Earth formed about 4.5 billion years ago, and took 1 billion years to even have an established planetary magnetic field (meaning it can now sustain an atmosphere and form life). If your world is roughly similar to Earth, then your age needs to be much more than 5000 years old. Is it 5000 years from the birth of intelligent life? $\endgroup$ – ajp15243 Oct 1 '14 at 20:40
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    $\begingroup$ Well in that case, you can have a 5000 year old world :). If that's also the case, they can presumably make the world as "young-like" or "old-like" as they want, and the 5000 years, as mentioned by others, wouldn't make any natural difference. $\endgroup$ – ajp15243 Oct 1 '14 at 20:47
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The concepts that the Civilization games use when modifying the world for "World Age" are based on the mechanical wear and tear that a world undergoes due to weather, seasonal change, and simple erosion. Hence why the Civ games measure age in the range of millions (erosion takes a really long time).

Civ World Age is taken into consideration before humans even show up, i.e. generating a world for humans to appear on at the basic level of civilization.

In Civ this is represented as an older world having smoother/flatter terrain and straighter rivers; a younger world hasn't been 'worn down by time' and has more jagged peaks and mountains, twisting rivers that haven't carved an 'easy path' yet, and more contrast between environments since there hasn't been an averaging out of terrian or geography yet.

However in the 5000 year range most of the change you'll see to a world would be from things done by the inhabitants and if you said earth like I'll assume a civilization that is also earth like: 5000 years won't see much change from weather or continental drift but humans can have a pretty drastic impact in that time.

Clearing forests, expanding cities, digging canals, man made lakes, and Civ "wonders" like the pyramids which would fit within your entire 5000 year time line from starting construction all the way to really old rock pile.

Edit: If the TOTAL age of your world is 5000 and people have been there since day 1, the other thing to consider would be that it may have a more Pangea (supercontinent) like appearance.

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  • $\begingroup$ so in general my world should, at the 5000 year mark be more...spiky and difficult to maneuver? (Those are of course the "technical" terms). $\endgroup$ – James Oct 1 '14 at 20:37
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    $\begingroup$ yes in technical terms at least according to the Civ model of age, but bear in mind that different things can "age" a planet much faster like meteor impacts, etc. $\endgroup$ – Culyx Oct 1 '14 at 21:06
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    $\begingroup$ I suppose I should have asked, is the model discussed from civ a legitimate way to expect a world to evolve geologically? $\endgroup$ – James Oct 1 '14 at 21:08
  • $\begingroup$ In an earth-like way yes but bear in mind that plate tectonics and volcanoes put all the mountains and high/rough points there in the first place, erosion is just going to smooth them out (slowly). Rivers will wind through the rough areas at first and slowly carve out a path of least resistance. $\endgroup$ – Culyx Oct 1 '14 at 21:24
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Hopefully an actual geologist can weigh in here, but I'd say no, Civilization's assumption that elevations and steepness of topology trend downward over geologic time is not correct.

Of course erosion does wear down individual mountain ranges - compare the Appalachians and Rocky Mountains. However, continental crust is constantly being pushed about by tectonic drift. Every few million years a new mountain range starts building. Is there anything we can use to suggest that today's topology is flatter now than billions of years ago?

The tallest mountains currently in existence on earth are near the physical maximum for earth's crustal material and gravity (https://skeptics.stackexchange.com/a/5866/3150). If there was a long-term trend toward flatter terrain on a global basis, surely the highest mountains today, after several billion years of following this hypothetical trend, would be well below the maximum.

Of course, the calculations behind the highest possible earth mountain could be wrong. And as a world builder you could decree that the rock on your world is lighter, or stronger.

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Three things happen to a planet over time (a very long time actually):

  • Erosion: wind, rain and even terrain vibrations will "flatten" the terrain over time. A very old planet would be very smooth and flat.

  • Core temperature drops: over time the planet cools down, which means less volcanoes, lower average temperature. If the core actually begins to harden completely, certain geological things would change as well, such as slower landmass movement, different compass directions and a change of rotation of the entire planet.

  • The planetary rotation slows down: which results in obviously longer days, a change of climate, and also gravity would increase, as the opposite force is dropping.

But please note that we are talking about millions of years for these changes. In 5000 years you would barely notice any of these.

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