Plate technonics (looking for geologist opinions)

[Bruce]

The recent tsunami brought up an interesting discussion at work about plate techtonics. Maybe our local geologists can shed some light.

When I was a wee lad in school, we learned that the shifting of the plates was caused by convection currents in the magma deep below the earth's surface. I remember that the science book used a coffee cup model to explain convection currents. Later on, after taking college physics courses, I had some problems with this theory.

Would convection currents in magma be strong enough to shift plates on the surface of the earth? Unlike a coffee cup, the earth is more or less a closed system. The crust of the earth encases most of the heat from the magma. If there was enough heat escaping from the core of the earth to cause convection currents in the magma, I would think it would be enough heat to melt any ice on the surface. Not only that, but I would think that the pressures involved deep within the earth would be far different from the coffee cup model. What are the fluid dynamics of molten magma at those depths? Is there any real data to support the notion that convection currents exist in the magma?

My theory is that the shifting of the plates is due primarily to the unbalanced and ever-shifting gravitational pulls of the sun and moon. I would guess that there is a slight bend in the surface of the earth in the direction of the sun and moon at all times that perturbs the crust. But then again, from what I remember, gravitational forces are primarily centered on the center of gravity between two objects. I don't know. Is there any theories that discuss distortions in the shapes of non-solid objects when gravitational forces from other bodies is involved, perhaps more so with the gas giants?

One final question: What makes scientist think that the core of the earth is made of nickel? I can understand why they think the rest is iron because of the earth's magnetic field, but why nickel at the core? Thanks.

 

[Eleatic Stranger]

My theory is that the shifting of the plates is due primarily to the unbalanced and ever-shifting gravitational pulls of the sun and moon

I'm not a geologist, but this sounds fishier to me than convection currents. How would you explain the mid atlantic ridge?

(More to the point - if it was just shifting gravitational pulls, why would the plates continually move in the same general direction and at generally non-varying rates? )

 

[Bruce]

I'm not a geologist, but this sounds fishier to me than convection currents. How would you explain the mid atlantic ridge?

(More to the point - if it was just shifting gravitational pulls, why would the plates continually move in the same general direction and at generally non-varying rates? )

Erm....because the earth rotates in the same general direction?

 

[nbenami]

My theory is that the shifting of the plates is due primarily to the unbalanced and ever-shifting gravitational pulls of the sun and moon.

Oh dear god.

I'm sorry, I just can't muster any response other than that.

I love how on the internet anyone can "have a theory."

 

[Bruce]

Oh dear god.

I'm sorry, I just can't muster any response other than that.

I love how on the internet anyone can "have a theory."

You know it! My very own theory. I'll bet nobody else has ever thought of it. I'm not going to check, though. I have faith that it is an original.

 

[Bruce]

Oh dear god.

I'm sorry, I just can't muster any response other than that.

I love how on the internet anyone can "have a theory."

And just wait until you hear about my laws!

 

[BillyJoe]

Bruce,

In any case you have only an hypothesis.
All you need now is some evidence to turn it into a theory.



BillyJoe

 

[Bruce]

I have a chunk of volcanic rock from Mt. St. Helens. Does that count?

 

[LucyR]

One final question: What makes scientist think that the core of the earth is made of nickel? I can understand why they think the rest is iron because of the earth's magnetic field, but why nickel at the core? Thanks.

I'm guessing your theory wasn't seriously intended, but this question is interesting. Actually even the inner core is mainly iron. I posted the beautiful paragraph below (originally in answer to Iamme) a number of times on this forum and each time it has sunk like a lead balloon:

Originally posted by Iamme


But how do geophysicists know what is deep inside the earth? Sound testing only goes so deep . They have gone pretty deep with core samples, but I don't think they've managed 4,000 miles. The only information I have gleaned about the inards of the earth is crust (earth , water, oil, coal, minerals, gems), mantle (rock and magma), core (nickle-iron) ...which I've wondered: how did they know THAT?

Our current picture of the physical and chemical nature of the Earth's deep interior is based on fairly compelling arguments. Consider the core for example. Yes, it's true that we cannot obtain samples directly. However, whatever is down there has to have properties that are at least consistent with the planet's mass and gravitational field. We also expect it to be in line with known cosmological abundance; we don't expect it to be for example, iridium or platinum. The Earth also has a measurable magnetic field; something in the planet must be responsible for this phenomenon. Iron meets all these requirements. As far as 'sound testing' is concerned we can indeed probe the core. All we need to do is monitor the arrival times of seismic waves (i.e. generated by earthquakes) that have passed through the region of interest (see the preliminary reference Earth model, or PREM, for more information). We can then determine their velocities. Again, the elastic properties of any candidate material under core conditions (up to ~ 3.6 million atm, and ~ 6000 K) must be consistent with these velocities. Although it's difficult to achieve these conditions in the lab, our current results obtained from dynamic shock wave measurements and also static diamond anvil cell measurements are not inconsistent with a composition that is mainly iron. More general seismic investigations also tell us that the core actually has at least two separate parts, a solid inner core, and a liquid outer core (it turns out that it's convection in the outer core that's responsible for our magnetic field). Again the phase diagram and equation-of-state (i.e. the relationship between pressure, volume and temperature) of any candidate material must be consistent.

Of course, it would be unreasonable to think that it is pure iron. Determining the precise composition is an active field of research. You may find many references that assume an iron inner core with some nickel - don't take this too seriously: a number of recent papers claim an inner core that is actually lighter than pure iron. We can however say with some confidence that the outer core is lighter, and probably contains a fair amount of something like silicon.

In conclusion, scientists don't claim to know in the way I think you mean it. They simply try to come up with a theory that is consistent with all the available evidence. As I said, in the case of core composition, there is still doubt, and new suggestions are always being made: e.g. the existence of a third core region, an 'inner' inner core. A recent first-principles calculation even suggests that the crystal symmetry of iron under core conditions is not what we have assumed it to be for many years. If correct, this will require a significant modification of our current picture.

 

[BillyJoe]

Bruce,

I have a chunk of volcanic rock from Mt. St. Helens. Does that count?

No, I don't think it does. Volcanic rock doesn't even have a brain as far a I know. Do you? (count, I mean)

BillyJoe

 

[BillyJoe]

In any case we have not yet answered your (presumably) serious question....

Would convection currents in magma be strong enough to shift plates on the surface of the earth?

 

[MRC_Hans]

Bruce,

No, I don't think it does. Volcanic rock doesn't even have a brain as far a I know. Do you? (count, I mean)

BillyJoe

Counting can be done without a brain. Tobacco can count, for instance.






At least the sort they put in Camel cigarettes.







On the flap of the Camel pack it says (or at least used to say): "It is the tobacco that counts"

Hans

 

[BillyJoe]

Well, now I'm wondering whether there's a serious question here at all.



Lucy,

I don't think it sunk like a lead balloon. Perhaps they had no reply at all to your excellent answer. Did you ever think of that?

BJ

 

[LucyR]

Lucy,

I don't think it sunk like a lead balloon. Perhaps they had no reply at all to your excellent answer. Did you ever think of that?

No, but I'm pathetically grateful for the suggestion.

 

[BillyJoe]

 

[Bruce]

Thank you, LucyR.

, BJ.

I'm still curious about how it was determined that convection in the outer core causes the earth's magnetic field.

 

[fishbob]

convection in the outer core causes the earth's magnetic field.

Bruce - you may have overlooked this key phrase from among the many excellent sentences posted by LucyR" "consistent with all the available evidence. "

My theory about your theory about the shifting of the plates is that your daily sensory input is unbalanced due to the ever-shifting daily media output - and too much beer.

 

[pupdog]

For a ludicrous account of plate tectonics, see Wingnut Daily . Picture a fat man jumping into a bathtub--that's like what happened when a mountain range fell into the ocean to form the mid-Atlantic Ridge and Noah's Flood...

There are "earth tides" in the solid Earth, but they are not responsible for plate tectonics. Spreading centers and subduction zones have nothing to do with earth tides (but a lot to do with convection cells).

 

[Bruce]

My theory about your theory about the shifting of the plates is that your daily sensory input is unbalanced due to the ever-shifting daily media output - and too much beer.

Yeah, well your theory is stupid. There's no such thing as too much beer.

 

[OldBlueMoon]

That old moon, it does make tides

The theory that the moon & sun affect plate tectonics is not new, in the respect that some geologists think that strong tidal days will have a likelier result of earthquakes. Although compelling, research into this has shown no significant effect of strong tidal days and an increase in seismic activity.

That being said, the moon certainly contorts the earth as it orbits around the earth. It is very slight in the core and mantle, but very noticable in the liquid oceans. Where the water is compressed, you get low tide. At the same time, the earth is slightly compressed also (instead of by meters, like the ocean, it is by centimeters) This could conceivably in the long run make earthquakes more likely in certain places, but it really has no bearing on which way the plates of the crust move. They are moving in all different directions.

So, the moon does smoosh the earth, but it doesn't move the plates.