Naked Science Forum
Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: PhirePhly on 15/04/2005 01:06:08
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I suppose this is more a paleogeology question than a geology question, but I'm curious as to the configuration of land mass of the early Earth.
I've seen half a dozen representations of Gondwanaland or Pangea. It appears to me that the globe of the Earth would have been pretty unbalanced with that much mass that far off center on only one side of the world.
Also, I can't imagine what sort of process would have produced such an imbalanced sphere.
I've seen at least one representation that made it look like a ring - an incomplete ring, but a ring nevertheless.
Is there any common wisdom on what caused the formation of the first above-sea-level land mass in the first place?
Thanks,
L. Lisov
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Pangea existed about 225million years ago and it split into Gondwanaland and Laurasia about 190 million years ago (http://geology.com/pangea.htm) the earth is about 4 billion years old so these are nowhere near original.
The continents are made up of less dense rock than the ocean floor and everything is floating on the semi-liquid mantle. The ocean floor is denser than the continents so it floats lower down, and is so covered in water.
If you pile a lot of weight somewhere the crust will sink down until it displaces enough mantle to balance the extra weight (like a boat does when you sit in it) this means that if you add up the weight from the surface down deep enough it will all come to the same sort of number (except where you are having really active tectonic things happening when you get tempory imbalances). This means that you don't get much of an imbalance anywhere, and anyway mount everest is only about 1/800th of the earth radius so it is a tiny amount of weight compared to the whole earth anyway.
How did the continents form in the first place?
Possibly the best way to think of the continents is as scum of lighter rocks on a pond of mantle.
Even if you start off with a perfectly distributed layer of scum all over the pond (earth) the wind blows, and you get currents moving the scum around*. If an even layer of scum is to move, it must get thicker somewhere to make clear water to move into (think about trying to play one of those sliding block games with no gap). This means the the scum will have to pile up thicker in some places and form continents. Because the scum is less dense than the pond water it's surface is higher than the water, similarly the continents are higher than the oceanic crust and get high enough to poke out of the water.
* Instead of winds moving the continents, the ocean floor is created and destroyed as part of a giant convection system, and the continents are dragged around in this process.
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Okay, I buy that it just sort of floated to the top, but it's evidently been trying to spread itself out all over the place - hasn't it? Hasn't the general movement of the continents been in the direction of relatively more even distribution on the globe?
What process would allow concentrated formation of such an elevation in land mass but then work to split it up and distribute it more evenly?
I saw something recently about the origin of the moon having something to do with a collision between the Earth and a near-Mars-sized object and I was kind of wondering if the "bump" of Pangea may have been a remnant of or a result of that collision.
Wouldn't such a collision also have added a great deal of energy to the core, enabling later tectonics (or keeping tectonics active longer, at any rate)?
Groo only knows why this question is even floating around in my head, but it is.
Thanks,
L. Lisov
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Yes the continents are the light rock that fell back down after the impact of a mars sized object and didn't end up on the moon. So they would have started off spread out evenly over the earth.
To explain how they coallessed I will go back to my scum on a pond analogy
If the pond starts off evenly spread and convection moves it all towards the middle:
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--> <--
you will get a gaps at the side and a continent in the middle...........===============..................
We have just produced a continent with sea around it
Now the convection could act to split this continent apart
...........=======--========..................
<-- -->
Which leads to 2 continents
.......-======-.......-======-..............
If these two continents collide violently (like india and asia are doing now) they will get even shorter and thicker forming mountains. (Mountains are like icebergs in that they have roots that are 7-8 times deeper than what you see on the surface, so you need to shorten a continent a lot to form a mountain range)
.........-===mmMMmmm====-...............
--> <--
so you have two competing effects collisions making the contients smaller and thicker, and errosion off the edges making them wider and thinner.
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what about the ring of fire [?]
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The pacific ocean is shrinking on several sides (partly because other oceals like the Atlantic are growing or vice versa), this means that the oceanic plate is being subducted under the continents (or in some cases other oceanic plates) when this happens some of the lighter continental rocks that got deposited on the top of the oceanic plate get scraped off to start with forming mountains, but others get dragged down with the plate.
These lighter rocks now melt (as contitnetal rocks tend to have a lower melting point than oceanic rocks) and because they are now hot and therefore less dense than the continent on the top they float to the surface, and form volcanos.
~~
/\
...... MMMMmmm=====----......
\ o
\ o
\
These volcanos form a ring around the pacific, hence the ring of fire.
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I love your diagrams Dave! nicely done!
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quote:
Originally posted by daveshorts
If the pond starts off evenly spread and convection moves it all towards the middle:
Not to be difficult (really), but there is no "middle" on a sphere as you describe it, an essentially liquid ball with drek floating on the surface. I can see, however, that if the moon were significantly closer to Earth than it is now, the tidal effect might have pulled all the "scum" towards one point (much as certain parts of London do now <hehehe>). I heard somewhere that the moon is drawing away from the Earth at some predictable rate.
Thanks,
L. Lisov
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Yeah I was ignoring the whole spherical thing as it made drawing diagrams more difficult ;)
It doesn't affect the argument much though, as you say.