the supercontinents

[fillindablank]

Every time I see a representation of a supercontinent like Pangea I immediately wonder what's on the other side of the planet. It seems unbalanced to me. Literally too. I find it hard to believe that there would be nothing but open ocean on the other side of the planet. I would think that there would be another continent on the other side too. Do we think that the somewhat evenly distributed landmasses as they are now is just co-incidence?

[matthyaouw]

Why do you find it hard to believe? Based upon what?

This website might me of interst to you: http://www.scotese.com/

[cpooley]

Generally, no unbalance from a continent all on one side. The continents are floating on a denser layer, so the mass of the continents disoplaces an equal mass of the denser rock. Geologist call the condition "isostacy".

Besides, if there was a local concentration of mass on one side (like with a freshly impacted asteroid), the Earth would just continue to rotate around the new center of gravity location.

The Earth is very large and very round. It is nor rigid enough to support large shape anomalies.

Interesting thing about the Super-continent is that it existed a few hundres million years ago, but as the Earth is 46oo million years old there must have been many arrangements of continents that cannot be reconstructed (without a time machine).

[Skyhunter]

Just because there is land on one side, does not mean that the Earth is not balanced. In fact, consistent length of days (24 hour rotation) suggests it is quite well balanced.

[cpooley]

for Skyhunter: The Earth is always "balanced" It has some center of gravity around which it rotates. The rotation rate is a function of the angular momentum it happens to have, and is nearly constant, but is slowly being transferred to the Moon's orbital momentum, so that is being driven outward, but very slowly.

As said earlier, the presence of a continent on one side has almost no effect on CG because it it "floating" on denser rock.

[DaveC426913]

Frankly, I have always had similar misgivings as the OP.

Not that I don't think it can't physically happen, I just think it is more likely that our view of the earlier geography is incomplete.

In the same sense that our original portraits of T.Rex as a slothful creature that stood upright and dragged his tail are now obsolete, I suspect that we will one day see a map of the ancient continents that seems less lopsided.

[ideasrule]

Frankly, I have always had similar misgivings as the OP.

Not that I don't think it can't physically happen, I just think it is more likely that our view of the earlier geography is incomplete.


If Pangaea were to be discredited, the problem that led to the idea of continental drift in the first place would pop up: how do you explain why fossils of the same species pop up all over the world, in areas like Antarctica, Africa, and South America? Supposing that has another explanation, we know the Atlantic ocean formed only recently because the Atlantic seabed gets progressively older at greater distances from the mid-Atlantic ridge, so at one time the Americas and Eurasia were joined. That evidence has to be explained away as well; the Americas and Eurasia/Africa make up pretty much the entire world's landmass. Recently, GPS measurements have become accurate enough to measure continental drift, and the measured movements correspond to what was known already from geological evidence. That has to be explained away too. Of course, I haven't even mentioned geological evidence like similarities in rock compositions across continents, mountain ranges nowhere near fault lines, the orientation of glacial striations, and the similarities in till composition. I really don't think Pangaea is going away.

The landmasses have a negligible effect on Earth's mass distribution. The entire crust, oceanic and continental combined, make up for less than 1% of the planet's volume. The landmasses comprise perhaps several percent of that 1%. Putting all the continents on one side won't throw the planet off-center enough to "destabilize" it, whatever that means.

[DaveC426913]

If Pangaea were to be discredited, the problem that led to the idea of continental drift in the first place would pop up: how do you explain why fossils of the same species pop up all over the world, in areas like Antarctica, Africa, and South America?

Well, I'm ont for a moment suggesting that they were not joined. It's just that, it seems to me, that the shape of Pangaea is only indirectly derived from the species data. Is there supporting evidence from geology or another field?

[cpooley]

To DaveC4... there is much direct geological evidence of continental drift--I used to work at Lamont Geological Observatory--there is the famous fit of South America and Africa, with gelogical structures torn apart with part in each continent. The growth along the Mid Atlantic Rift, the progressively older and thicker sediments as distance from the rift grows.
A volcano that has periodically erupted, leaving a trail of remnents leading to S America and Africa. Continental drift is a fact, evidence overwhelming, and before Pangaea there were earlier patterns of contenents, other "Pangaeas"

[DrClapeyron]

I am confused by the conversation but I think the original post was examining the absence of continental crust outside of the Pangea landmass.

It could be possible that other continental crust smashed continental crust from Pangea called accreted terranes, or may have been subducted. Both of which seem unlikely. Destruction (and things being hidden) makes the geologic history of the Earth incomplete.

[cpooley]

Pangaea is the continental crust of that time. It moves around, makes various patterns. Subduction is a part of this--a margin of some contental mass is pushed under the edge of another one, like is happening now with the Pacific northwest, the Cascade Range.

The continental material is lighter rocks floating on the denser mantle and being moved around by the slow convection currents (though solid, the rock is not absolutely rigid), and we can only trace and figure the various positions, configurations back a few hundred million years. No way to see what the map of Earth was like 3 billion years ago.

[Andre]

The landmasses have a negligible effect on Earth's mass distribution. The entire crust, oceanic and continental combined, make up for less than 1% of the planet's volume. The landmasses comprise perhaps several percent of that 1%. Putting all the continents on one side won't throw the planet off-center enough to "destabilize" it, whatever that means.

Maybe time for some basic physics about inertia tensors (http://baba.astro.cornell.edu/inertiatensor.pdf) and stable rotation etc? Also that unbalance could cause true polar wander (http://geology.about.com/od/tectonicsdeepearth/a/truepolarwander.htm) to realign the spin axis with a main inertia tensor, also recently discussed here (http://docs.google.com/gview?a=v&q=cache%3AwiQh9G4sPtYJ%3Awww.nac7.nl%2Fdoc%2Fprogr amma%2Fvermeersen.pdf+true+polar+wander+vermeersen&hl=nl&gl=nl&sig=AFQjCNH4nR1NwSSxC-U9Sziy87yiFooPqw&pli=1).

[DaveC426913]

To DaveC4... there is much direct geological evidence of continental drift--I used to work at Lamont Geological Observatory--there is the famous fit of South America and Africa, with gelogical structures torn apart with part in each continent. The growth along the Mid Atlantic Rift, the progressively older and thicker sediments as distance from the rift grows.
A volcano that has periodically erupted, leaving a trail of remnents leading to S America and Africa. Continental drift is a fact, evidence overwhelming, and before Pangaea there were earlier patterns of contenents, other "Pangaeas"

I am perfectly aware of continental drift and the Mid-Atlantic ridge. I think I'm not getting my point across.

[Xnn]

The continents have come together to form a supercontinent numerous times.
Each time, they subsequently broke up.
The last 3 times the continents came together are as follows:

Rodina between 1100 to 750 Ma
Pannotia between 600 to 540 Ma
Pangaea between 250 to 175 Ma

It amazes me that geologist have been able to piece together this as it is.
Also, when the earth was first formed the continents are thought to have
drifted faster than they do now. So, it's probably impossible to tell
exactly how many times they have come together and split up.

[Evo]

Why do you find it hard to believe? Based upon what?

This website might me of interst to you: http://www.scotese.com/
This is a great site. I see the OP is banned so no need to educate him.

[ideasrule]

Maybe time for some basic physics about inertia tensors (http://baba.astro.cornell.edu/inertiatensor.pdf) and stable rotation etc? Also that unbalance could cause true polar wander (http://geology.about.com/od/tectonicsdeepearth/a/truepolarwander.htm) to realign the spin axis with a main inertia tensor, also recently discussed here (http://docs.google.com/gview?a=v&q=cache%3AwiQh9G4sPtYJ%3Awww.nac7.nl%2Fdoc%2Fprogr amma%2Fvermeersen.pdf+true+polar+wander+vermeersen&hl=nl&gl=nl&sig=AFQjCNH4nR1NwSSxC-U9Sziy87yiFooPqw&pli=1).

I'm perfectly aware that a body's axis of rotation changes when the body's distribution of mass changes. That doesn't destabilize the planet, first of all. Second, as I was saying throughout the post you quoted, Earth's continents make up a tiny percentage of the planet's total mass and therefore have only a tiny effect on its rotation.