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Mirage
04-18-2001, 10:55 PM
This is something that I always wondered about when it came to plate tectonics. Where does the energy come from to force the plates to collide so powerfully?

MEBuckner
04-18-2001, 11:03 PM
Basically, the plates are floating on a sea of constantly churning molten rock.
The movements of the mantle result from the need to transfer to the Earth's surface the heat generated within it by radioactive decay, and hence convective patterns vary with time. -- Plate Tectonics (http://www.britannica.com/eb/article?eu=61873&tocid=0), from the Encyclopedia Britannica (http://www.britannica.com)

Mirage
04-18-2001, 11:36 PM
Thanks for the suprisingly informative link.

But it brings up another question, is the heat within the earth really the result of radioactive decay? I've never heard this before. I always assumed that it was left over from when the Earth was a giant glob of molten solar system leftovers.

friedo
04-19-2001, 12:59 AM
Originally posted by Mirage
Thanks for the suprisingly informative link.

But it brings up another question, is the heat within the earth really the result of radioactive decay? I've never heard this before. I always assumed that it was left over from when the Earth was a giant glob of molten solar system leftovers.

Yep. The following is from 1001 Things Everyone Should Know About Science, by James Trefil

Thing #742

Where does the heat come from? There are two sources of heat in the interior of the earth. One is the original heat that was generated when the planet was formed, and the other is radioactivity. Geophysicists disagree over how much of the heat in the earth comes from each of these sources.


And here's an interesting tidbit about how much energy is released via radioactive decay over a long enough time.

Thing #599

If you took a cube of granite that you could hold in your hand and kept the heat generated by radioactive decay of nuclei normally found in the rock from escaping, within a million years enough heat would be generated to melt the rock completely.


This is a really great book, by the way. My gramma got it for me ages ago and I didn't start looking through it until recently. I bet it could answer 10% of the questions on this board.

MEBuckner
04-19-2001, 01:13 AM
MEBuckner
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Registered: Aug 2000
Posts: 1000
friedo
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Registered: May 2000
Posts: 2000
Wow, this is like the Big Even-Numbered Posts Thread.

Enola Straight
04-19-2001, 10:25 AM
I always figured that the main engine powering the churning in our magma comes from the moon...lunar tides.

After all, MAGMA is Earth's most abundant liquid, not sea-water.

The only other object in our solar system with active surface tectonics and vulcanism is IO, One of Jupiter's inner moons...BTW, an object with even more tidal stress, and therefore has more geological activity.

Podkayne
04-19-2001, 11:27 AM
Originally posted by enolancooper
After all, MAGMA is Earth's most abundant liquid, not sea-water.

Ah, but sea water is a lot easier to slosh around. Thus, most of the lunar tidal energy goes into the ocean, and tidal heating of the interior is negligable.

The only other object in our solar system with active surface tectonics and vulcanism is IO, One of Jupiter's inner moons...BTW, an object with even more tidal stress, and therefore has more geological activity.

As a side note, for comparison, the tidal deformation of the solid surface of Io is more than 100 meters, wheras the solid tides on Earth are less than 1 meter. Also, a major factor in Io's tidal heating is the eccentricity of its orbit. As it gets closer and farther from the planet in the course of each orbit, it gets more and less squished, leading to even greater friction.

Ringo
04-19-2001, 01:32 PM
Convection (the rising of hotter material and the sinking of cooler material) is thought to drive tectonic movement. Where a convection current forces hot magma up and it breaches the lithoshpere, new lithosphere is formed from the now cooling magma. This happens at mid-oceanic ridges and is the source of sea floor spreading, that, along with the thereby forced subduction of the growing oceanic plate at its meeting with continental plates gives us our evidence for convection.

RM Mentock
04-19-2001, 07:06 PM
beatle

Convection most probably does drive the plates, but the OP asks for the energy sourse--what drives the convection?

MEBuckner

That Britannica article notwithstanding, I think most of the radioactive heat production is close to the surface, and radioactivity is not a large factor in producing the mantle convection.

Mirage

If the heat were just left over heat, the gradient might have equilibrated by now. There has been a lot of discussion over the past thirty years about what actually drives mantle convection. One theory that was rejected, but seems to have come back, is the heat generated by the solidification of the inner core. Of course, that is just "left over heat," too, but the mechanism is more complex.

enolancooper

Lunar tides aren't that large. They've calculated the amount of energy being lost due to tidal friction, and it is not enough. But, Podkayne, I'm pretty sure that they have not yet absolutely determined where the tidal friction sink is--whether it's in the oceans or not.

Ringo
04-19-2001, 08:47 PM
And yeah, that's spot on, RM Mentock; I dashed off a quick lunch time reply focussing more on enolancooper's post about tidal forces than the energy question asked in the OP. I should have thrown in "mechanism" somewhere. And I'm dashing off another quickie now.

I haven't spent much time on the heat engine of the interior earth, and it sounds like an interesting subject. Crunch schedule the next several days, but if nobody comes along with a definitive essay in the next week, I'll study up and try to get back to this.

Amedeus
04-19-2001, 10:10 PM
Originally posted by friedo


Where does the heat come from? There are two sources of heat in the interior of the earth. One is the original heat that was generated when the planet was formed, and the other is radioactivity. Geophysicists disagree over how much of the heat in the earth comes from each of these sources.




I cannot seem to find any reference to it, but doesn't the immense pressures that the core is always under have something to do with the heat?

Bobort
04-20-2001, 03:41 AM
According to this (http://www.aip.org/enews/physnews/1998/split/pnu354-3.htm):
Up to 40% of the 40 terawatts of energy leaving the Earth's surface is believed to come ultimately from the radioactive decay of uranium-238 and thorium-232. Simple geological models predict where these isotopes are to be found: half in the crust beneath continents and half in the mantle.

Podkayne
04-20-2001, 08:52 AM
Originally posted by RM Mentock
That Britannica article notwithstanding, I think most of the radioactive heat production is close to the surface, and radioactivity is not a large factor in producing the mantle convection.


Why would it only be near the surface? Radioactive elements are heavy. They'll sink to the core.


But, Podkayne, I'm pretty sure that they have not yet absolutely determined where the tidal friction sink is--whether it's in the oceans or not.

Uh, "they" have been confident for a long time that it's in the oceans, specifically in friction between the ocean and the sea floor. (See http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=3773 ).

I was unable to find a handy resource (damn Nature for not being fully online) that estimates the energy dissipation due to solid tides, but I gather that this is because solid tides are so hard to measure (See e.g. http://bowie.gsfc.nasa.gov/926/staff/lemoine/GJI2001.html ). If you have any sources that say differently, I'd love to see them. Conversely, if anyone has some good links about solid tides, that'd be great, too.

jrepka
04-20-2001, 01:05 PM
Originally posted by Podkayne
Why would it only be near the surface? Radioactive elements are heavy. They'll sink to the core.

uranium is quite heavy, and logically I would expect that there is quite a bit in the core.

Potassium-40 is relatively light however, and is much more abundant. I recall from my undergrad days that Turcotte-Schubert attributed ~75-80% of surface heat flow to radiodecay but that edition is 20 years old now.

Don't know the heat of fusion for iron at terapascal pressures, but I expect that growth of the solid core generates quite a bit of heat.

Yossarian
04-20-2001, 01:41 PM
MEBuckner: Basically, the plates are floating on a sea of constantly churning molten rock.

Not quite. The mantle isn't molten--it's a plastic solid. At the base of the lithospheric plates (or, where they ride over the convecting asthenosphere) is the Low Velocity Zone, which is interpreted to represent a partially molten zone in the mantle. But most of the mantle is solid rock.

jrepka: uranium is quite heavy, and logically I would expect that there is quite a bit in the core.

and

Podkayne: Why would it only be near the surface? Radioactive elements are heavy. They'll sink to the core.

Uranium is considered a lithophile element (according to the Goldschmidt classification scheme). Heaviness has nothing to do with it; because of its high field strength, it's incompatible in most elements and will join the melt (magma) phase when the mantle partially melts, ascend to the crust, and concentrate there. Nothing to do with mass, everything to do with radius and charge.

enolancooper: After all, MAGMA is Earth's most abundant liquid

Magma is a complex mixture of several phases, including volatiles (which make it explode) and solids. There's a lot of liquid in a magma, but magma itself is not.

beatle, I've heard several in the geophysical community talk about slab-pull/ridge-push (in other words, gravity) as a possibly more viable mechanism--or at least a complimentary one--for plate motion. You have any opinion on this?

Chronos
04-20-2001, 05:09 PM
Quoth Podkayne:
Also, a major factor in Io's tidal heating is the eccentricity of its orbit. As it gets closer and farther from the planet in the course of each orbit, it gets more and less squished, leading to even greater friction.In fact, were it not for the eccentricity of Io's orbit, it wouldn't be tidally heated at all. It's already tidally locked (the same side always faces Jupiter, just like Luna/Earth), so any tidal distortions to its shape would be constant, if its orbit were circular. It's only when the shape changes somehow that you get tidal heating.

jrepka
04-20-2001, 05:15 PM
Originally posted by Pantellerite
Uranium is considered a lithophile element (according to the Goldschmidt classification scheme). Heaviness has nothing to do with it; because of its high field strength, it's incompatible in most elements and will join the melt (magma) phase when the mantle partially melts, ascend to the crust, and concentrate there. Nothing to do with mass, everything to do with radius and charge.

Pantellerite, I stand corrected, thanks for clarifying...

Podkayne
04-21-2001, 11:25 AM
So, Pantellerite, is it true then that most radioisotope heating occurs near the surface, and not in the mantle or core?

Yossarian
04-21-2001, 07:05 PM
Podkayne: No, because although Uranium is concentrating into the crust over time, it's not completely devoid in the mantle. Considering the mantle is the overwhelmingly dominant volume of the planet--as opposed to the crust, the volume of which is nearly negligible--it still has most of everything--including radioisotopes. Also, heating due to radioactive decay isn't just from U. K-40 and undoubtedly several others may contribute as much or more

Sorry I can't provide more quantitative figures for you; I'm posting at home instead of work.