As the World Churns

Dec 28, 2009
By combining measurements of Earth's magnetic field from stations on land and ships at sea with satellite data, scientists were able to isolate six regularly occurring waves of motion taking place deep within Earth's liquid core, with varying timescales. Image credit: NASA/JPL

(PhysOrg.com) -- "Terra firma." It's Latin for "solid Earth." Most of the time, at least from our perspective here on the ground, Earth seems to be just that: solid. Yet the Earth beneath our feet is actually in constant motion. It moves through time and space, of course, along with the other objects in the universe, but it moves internally as well.

The powerful forces of wind, water and ice constantly erode its surface, redistributing Earth's mass in the process. Within Earth's solid crust, faulting literally creates and then moves mountains. Hydrological changes, such as the pumping of groundwater for use by humans, cause the ground beneath us to undulate. Volcanic processes deform our planet and create new land. Landslides morph and scar the terrain. Entire continents can even rise up, rebounding from the weight of massive that blanketed the land thousands of years ago.

Indeed, the outermost layers of the celestial blue onion that is Earth-its crust and upper mantle-aren't very solid at all. But what happens if we peel back the layers and examine what's going on deep within Earth, at its very core? Obviously, Earth's core is too deep for humans to observe directly. But scientists can use indirect methods to deduce what's going on down there. A new study in the journal , by Jean Dickey of NASA's Jet Propulsion Laboratory, Pasadena, Calif. and co-author Olivier deViron of the Institut de Physique du Globe de Paris, University Paris Diderot, Centre National de la Recherche Scientifique, Paris, has confirmed previous theoretical predictions that the churning cauldron of molten metals that make up Earth's liquid outer core is slowly being stirred by a very complex but predictable series of periodic oscillations. The findings give scientists unique insights into Earth's internal structure, the strength of the mechanisms responsible for generating Earth's and its geology.

Peeling Back the Onion

In order to better understand what's going on inside our planet, it helps to first get a lay of the land, so to speak.

Earth has several distinct layers, each with its own properties. At the outermost layer of our planet is the crust, which comprises the continents and ocean basins. Earth's crust varies in thickness from 35 to 70 kilometers (22 to 44 miles) in the continents and 5 to 10 kilometers (3 to 6 miles) in the ocean basins. The crust is mainly composed of alumino-silicates.

Next comes the mantle. The mantle is roughly solid, though very slow motion can be observed inside of it. It is about 2,900 kilometers (1,800 miles) thick, and is separated into an upper and lower mantle. It is here where most of Earth's internal heat is located. Large convective cells in the mantle circulate heat and drive the movements of Earth's tectonic plates, upon which our continents ride. The mantle is mainly composed of ferro-magnesium silicates.

Earth's innermost layer is the core, which is separated into a liquid outer core and a solid inner core. The outer core is 2,300 kilometers (1,429 miles) thick, while the inner core is 1,200 kilometers (746 miles) thick. The outer core is mainly composed of a nickel-iron alloy (liquid iron), while the inner core is almost entirely composed of a pure solid iron body.

Earth's "Magnetic" Personality

Scientists believe Earth's magnetic field results from movements of molten iron and nickel within its liquid outer core. These flows, which are caused by interactions between Earth's core and its mantle, are neither even, nor evenly distributed. The electrical currents generated by these flows result in a magnetic field, which is similarly uneven, moves around in location and varies in strength over time. Earth's magnetic field is also slightly tilted with respect to Earth's axis. This causes Earth's geographic north and south poles to not line up with its magnetic north and south poles--they currently differ by about 11 degrees.

In just the last 200 million years alone, Earth's magnetic poles have actually reversed hundreds of times, with the most recent reversal taking place about 790,000 years ago. Scientists are able to reconstruct the chronology of these magnetic pole reversals by studying data on the spreading of the seafloor at Earth's mid-oceanic ridges. Unlike the doomsday scenario popularized by Hollywood in the movie "2012," however, such reversals don't occur over days, but rather on geologic timescales spanning hundreds to thousands of years-very short in geologic time but comparatively long in human time. The time span between pole reversals is even longer, ranging from 100,000 to several million years.

Earth's magnetic field is essential for life on Earth. Extending thousands of kilometers into space, it serves as a shield, deflecting the constant bombardment of charged particles and radiation known as the solar wind away from Earth. These solar winds would otherwise be fatal to life on Earth. At Earth's poles, the perpendicular angle of the magnetic field to Earth there allows some of these particles to make it into our atmosphere. This results in the Northern Lights in the northern hemisphere and the Southern Lights in the southern hemisphere.

Here on the ground, Earth's magnetic field has many practical applications to our everyday lives. It allows people to successfully navigate on land and at sea, making it a critical tool for commerce. Hikers use it to find their way. Archaeologists use it to deduce the age of ancient artifacts such as pottery, which, when fired, assumes the magnetic field properties that were present at the time of its creation. Similarly, the field of paleomagnetism uses magnetism to give scientists glimpses into Earth's remote past. In addition, geophysicists and geologists use geomagnetism as a tool to investigate Earth's structure and changes taking place in the .

Getting to the Core of the Matter

Since Earth's liquid core is the primary source of Earth's magnetic field, scientists can use observations of the magnetic field at Earth's surface and its variability over time to mathematically calculate and isolate the approximate motions taking place within the core.

That's what Dickey and deViron did. They combined measurements of Earth's magnetic field taken by observatory stations on land and ships at sea dating back to 1840 with those of the Danish Oersted and German CHAMP geomagnetic satellite missions, both of which were supported by NASA investments. These measurements were then used as inputs for a complex model that employs statistical time series analyses to determine how fast liquid iron is flowing within Earth's core.

"Although we do not observe the core directly, it's amazing how much we can learn about Earth's interior using magnetic field observations," said Dickey.

In order to approximate the flow of liquid in the core, the scientists visualized its motion as a set of 20 rigid cylinders, each rotating about a common point that represents Earth's axis. "Imagine that each cylinder is slowly rotating at a different speed, and you'll get a sense of the complex churning that's taking place within Earth's core," Dickey said.

The scientists analyzed the data to identify common patterns of movement among the different cylinders. These patterns represent how momentum and energy are transferred from the liquid core-mantle interface inward through the liquid core toward the inner core with diminishing amplitudes.

Their analyses isolated six slow-moving oscillations, or waves of motion, occurring within the liquid core. The oscillations originated at the boundary between Earth's core and its mantle and traveled inward toward the inner core with decreasing strength. Four of these oscillations were robust, occurring at periods of 85, 50, 35 and 28 years. Since the scientist's data set goes back to 1840, the recurrence period of the longest oscillation (85 years) is less well determined than the other oscillations. The last two oscillations identified were weaker and will require further study.

The 85- and 50-year oscillations are consistent with a 1997 study by researchers Stephen Zatman and Jeremy Bloxham of Harvard University, Cambridge, Mass., who used a different analysis technique. A later purely theoretical study by Harvard researcher Jon Mound and Bruce Buffett of the University of Chicago in 2006 showed that there should be several oscillations of this type; their predicted periods agree with the first four modes identified in Dickey and deViron's study.

"Our satellite-based results are in excellent agreement with the previous theoretical and other studies in this field, providing a strong confirmation of the existence of these oscillations," said Dickey. "These results will give scientists confidence in using satellite measurements in the future to deduce long-term changes taking place deep within our restless planet."

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DGBEACH
3.5 / 5 (4) Dec 28, 2009
They certainly seem sure of themselves when talking about the earth's core. We of course really don't know what's down there. They base themselves upon reflected sound waves...like we tap on a honeydew melon to see if its ready.
One thing's for sure, there's plenty of thermal energy down there for us to use. We just need to figure out how to get at it!
StillWind
1.3 / 5 (15) Dec 28, 2009
While the Global warming hoax is the most conspicuous example of scientific chicanery, the current geological paradigm couldn't be more ridiculous. How can anyone support the idea of plate tectonics, when it is so clearly wrong? In fact, paleontology easily disproves plate tectonics, as if the idea of pushing granite cratons through solid mantle wasn't silly enough.
Likewise the idea that there is "liquid" iron at the Earth's core defies any logic, yet continues to be foisted on an ignorant public and legions of students, who are paying to be misinformed.
Nik_2213
4.6 / 5 (11) Dec 28, 2009
Uh, if granite cratons can't get through the mantle, how do volcanoes' magma diapirs manage it ??

Sorry, StillWind, your argument was destroyed almost fifty years ago by discovery of the magnetic reversal zebra 'stripes' and 'offsets' on bed of Atlantic, symmetric about the mid-line spreading axis...

Worse, the geology and paleontology split by eg Atlantic *require* plate tectonics. Only, for a very long time, there was no practicable mechanism to open and close oceans, spawn back-arc eruptions etc etc.

I remember my 'Traditional' Geography teachers' shock when they first saw the 'zebra': It was irrefutable. It overturned so much they'd believed and taught. They adjourned to local tavern and drowned their dismay...
A_Paradox
4.2 / 5 (6) Dec 28, 2009
Nik,
"Sillwind" has got to be a troll.
Nobody with a computer and that much English, could be so ignorant.
A_Paradox
4 / 5 (1) Dec 28, 2009
DGBEACH,
If you are serious, and not just trolling, then I agree with you, there is a vast amount of geothermal energy available for some lucky people to make use of. I live in Australia where there are some plans afoot to access hot rocks in the outback of Queensland, NSW, and maybe the Northern Territory.

If you are in the USA then maybe you should be questioning your federal politicians about why they are not supporting the exploitation of the volcanic plume underlying Yellowstone Nat. Park. Just remind them that the trick will be to go down BESIDE the plume, NOT into the top. Remote control mining in water filled shafts going into the side of the plume, about 10 km down I guess, will allow access to the equivalent of "TNT x gigatonnes" of thermal energy for the next 10,000 years or so!
NotAsleep
3 / 5 (3) Dec 28, 2009
Two questions about this article:

1) "Hydrological changes, such as the pumping of groundwater for use by humans, cause the ground beneath us to undulate."

Are they saying water wells cause earth quakes? I've heard of sinking cities but I've never seen mention of well pumping causing undulations

2) "Unlike the doomsday scenario popularized by Hollywood in the movie "2012," however, such [magnetic pole] reversals don't occur over days, but rather on geologic timescales spanning hundreds to thousands of years... The time span between pole reversals is even longer, ranging from 100,000 to several million years."

...which one is it? Hundreds of years or several million years? Both seem a bit extreme and I don't trust wikipedia (which says average 300,000 years)
AnonymousSquid
1 / 5 (1) Dec 28, 2009
Is there any insight as to why there is this activity in the core? Is it due to the gravitational pull of the Sun and/or Moon? I would believe if the Earth was standing in the middle of nowhere in the universe, all alone, it would be dead cold and solid inside.
Loodt
2 / 5 (3) Dec 28, 2009
If the modelling is done with 21 instead of 20 cylinders, can we then not deduct a more logical harmonic sequence of 28, 35, 49 and 84 years? The article did say the last two oscillations, currently given as 50 and 85 are not robust. Why pick 20 cylinders as the starting point for the modelling?
NotAsleep
3 / 5 (2) Dec 28, 2009
AnonymousSquid, it just so happens there's another Physorg article on the subject:

http://www.physor...904.html
LKD
1 / 5 (3) Dec 28, 2009
Dead cold inside? Likely.

I think that is a great question. What do gravitational pulls do to the core? There is some evidence Jupiter effects sun spots, the effect has to play out in the earth core too, wouldn't it?
NotAsleep
3 / 5 (2) Dec 28, 2009
Dead cold inside? Likely.


Actually, no, it isn't. Read the article I posted above and try again
LKD
3 / 5 (2) Dec 28, 2009
I did. And thank you for that, and my apologies for my mistaken understanding.
Phelankell
3 / 5 (2) Dec 28, 2009
Is there any insight as to why there is this activity in the core? Is it due to the gravitational pull of the Sun and/or Moon? I would believe if the Earth was standing in the middle of nowhere in the universe, all alone, it would be dead cold and solid inside.


One word, pressure.
El_Nose
4.2 / 5 (5) Dec 28, 2009
@ notasleep

I believe
such reversals don't occur over days
the movie makes a pole reversal happen very quickly and the article is stating it takes hundreds to thousands of years.

It then goe son to say
The time span between pole reversals is even longer, ranging from 100,000 to several million years.
which refers to the interval between the revesals. At least in this case the article is very consistant.

-- I hope I stated that well.
LKD
not rated yet Dec 28, 2009
NotASleep:

After reading through some articles about the core of various planets, to get a fuller understanding of the processes involved in the planet's mantle and core's, I think I will stick with my initial assertion. The radioactivity of the core seems to not be a source that NASA cites in their Worldbook, and I have come across a few articles that seem to believe that solar radiation, magnetism, gravitational compression, tidal energy, plate tectonics, and lingering formation energy are the source of the majority of the heat.

I really find this fascinating, and would love to read more on it.
AnonymousSquid
not rated yet Dec 28, 2009
@NotAsleep

Yeah, indeed, interesting article, I'm surprised though the pull of Moon and Sun has no impact at all ( by deforming constantly the Earth's crust as it revolves ). Thanks for the reference.
StillWind
1.6 / 5 (8) Dec 28, 2009
I am neither a troll, or ignorant. The points you made do nothing to support the fallacy of plate tectonics.
In fact, there are so many obvious holes in the "theory" one would have to surmise that those that support it are ignorant of even basic physics.
Just where is all the missing crust? How is it that spreading centers vastly surpass subduction zones? Please explain the physics that allows the Indian subcontinent to sail cross the Indian Ocean and drive up into Asia, and if it did manage this impossible feat, where are the resultant ridge lines of this amazing conveyor?
These are just a few examples of questions that can't be answered by the current paradigm.
Your arrogant responses only prove your own ignorance.
omatumr
1 / 5 (1) Dec 28, 2009
I am also puzzled by this report.

The Earth accreted in layers, beginning with the accretion of iron meteorites to form its iron core. See the landmark paper by Professor Karl K. Turekian and S. P. Clark, Jr. ["Inhomogeneous accumulation of the earth from the primitive solar nebula", Earth Planet. Sci. Lett. 6 (1969) 346-348]. This scenario is consistent with measured abundances of radiogenic Ar-40, Xe-129 and Xe-136 and primordial He-3 and heavier noble gases in the Earth's mantle and atmosphere today.

The Sun probably also accreted in layers.

With kind regards,
Oliver K. Manuel
hylozoic
not rated yet Dec 28, 2009
What bearing will any of these observations have on the hypothesis of crustal displacement? Any one?
StillWind
3 / 5 (2) Dec 29, 2009
If volcanoes are the result of a "mantle plume", then we can limit crustal displacement to the distances observed between the Hawaiian islands, Yellowstone and the Cascades, or other such related volcanic features.
Husky
not rated yet Dec 29, 2009
I think the liquid core mentioned in the article is perhaps a broad and confusing generalisation for the whole area thats under the mantle. Its well accepted that the very centre core is solid, but perhaps the article points to the liquid layer of metal surrounding this steel ball and call this "the core"
StillWind
1 / 5 (4) Dec 29, 2009
I understand the model, its the physics that doesn't fit. A liquid outer core was surmised to explain the geomagnetic field, but just because you can create a model doesn't mean that is what's happening.
StillWind
1 / 5 (4) Dec 29, 2009
Its the same with continental drift. they observed that North and south america fits with europe and africa, and they found connecting strata, but it completley ignores the fact that north america also fits with asia, and south america was also connected to australia, india, and madagascar.
All the continents ft nicely on a smaller globe.
croghan26
not rated yet Dec 30, 2009
I guess this is a 'if yer so smart, why ain't you rich' question, StillW.

Do you have an alternate explaination that covers all the ground that the ones you so blthly trash do?

Is it 'because God wants it?' Is it... 'there are things that man was never meant to know?'
A_Paradox
5 / 5 (1) Dec 30, 2009
Oliver K Manuel, what is your point? Why is it significant if the Earth, the other planets and or the Sun "accreted in layers"

During the period of collapse of the interstellar cloud, the newly forming Sun and planets became the focus of the kinetic energy of all the in-falling dust and gasses.

I have read that during the peak of the accretion, the temperature at the "surface" [the place where the particles from space crashed into the stuff that was already there :-] would have been as high as ten thousand degrees C. That kind of temperature allowed a great deal of mixing, and of course during the thousands of millions of years since then the elements, crystals and compounds which constitute the Earth's matter have stratified on the basis of relative densities but with constant churning wrought by convection.
dolson
3 / 5 (2) Dec 30, 2009
StillWind, your arrogant tone and lack of data have caused you to fail. Should you wish to actually dialog about your ideas, you would do best to first enroll in a finishing school for the socially handicapped. At present, few of us can distinguish between you and a filthy toilet seat.
A_Paradox
not rated yet Dec 31, 2009
Husky,
I think the article is clear enough in its references to the inner solid core and the outer liquid core. I think we could ask why the word "core" is used for both the solid and liquid iron regions. Maybe it is because the iron is so much denser than the crystalline mantel. I think this must be the case because otherwise why is the liquid iron so definitely separated from the mantel. The greater density of the core was discovered decades ago through analysis of seismic data.

I tend to think that latent heat energy released at the solid iron-liquid iron boundary as solidification occurs is carried away by electron flows as much as by convection of the liquid iron atoms.
RealScience
5 / 5 (1) Jan 01, 2010
StillWind - The Hawaiian islands you cite are another excellent piece of evidence supporting continental drift.
The cold crust descending into the mantle in subduction zones shows up in seismic data.
(But your point with the Hawaiian islands and crustal displacement is good.)

AnonymousSquid -
The heat within the earth must be replenished by something more potent than chemical reactions - Lord Kelvin showed over a century ago that the earth would cool with 100 million years otherwise.

However the tidal effects of the moon and other planets are far too small to contribute materially, leaving radioactivity as the accepted answer.
A_Paradox
5 / 5 (1) Jan 04, 2010
Yep, RealScience is on the money!
StillWind's
"missing crust" has been subducted;
volcanoes associated with subduction spew out ash and lava characteristically different from "hot spot" volcanoes like those of Hawaii and Galapagos;
sea floor crust with magnetic banding parallel to the mid-ocean spreading centres clearly supports and time-stamps the creation of new oceanic crust as well as recording the ever repeating reversals of the Earth's magnetic field which has been occurring for billions of years;
The hot spot volcanoes caused by the magma plume from deep in the Earth's mantel which created the Hawaiian Islands, has been creating such volcanoes for many hundreds of millions of years and the evidence for this is the very long chain of sea mounts to the north west of Hawaii;
no-one is disputing the theory that what we now call India, Africa, Madagascar, Australia, New Zealand, and South America were all at one stage connected together as the super continent of Gondwanaland.