Why are there alternating polarity reversals ?

[dougettinger]

Alternating polarity reversals of the magnetic dipole fields of Mars, Earth and to some extent on the Moon have occurred. Why ? Do the spinning solid cores inside a liquid core (existing or previously existing) actually stop spinning and reverse direction causing this global reversal of polarity ?

Doug Ettinger
Pittsburgh, Pa

[neufer]

Alternating polarity reversals of the magnetic dipole fields of Mars, Earth and to some extent on the Moon have occurred. Why ? Do the spinning solid cores inside a liquid core (existing or previously existing) actually stop spinning and reverse direction causing this global reversal of polarity ?

<<Scientific opinion is divided on what causes geomagnetic reversals. Many scientists believe that reversals are an inherent aspect of the dynamo theory of how the geomagnetic field is generated. In computer simulations, it is observed that magnetic field lines can sometimes become tangled and disorganized through the chaotic motions of liquid metal in the Earth's core. In some simulations, this leads to an instability in which the magnetic field spontaneously flips over into the opposite orientation. This scenario is supported by observations of the solar magnetic field, which undergoes spontaneous reversals every 7–15 years. However, with the sun it is observed that the solar magnetic intensity greatly increases during a reversal, whereas all reversals on Earth seem to occur during periods of low field strength. Present computational methods have used very strong simplifications in order to produce models that run to acceptable time scales for research programs.

The rate of reversals in the Earth's magnetic field has varied widely over time. 72 million years ago (Ma), the field reversed 5 times in a million years. In a 4-million-year period centered on 54 Ma, there were 10 reversals; at around 42 Ma, 17 reversals took place in the span of 3 million years. In a period of 3 million years centering on 24 Ma, 13 reversals occurred. No fewer than 51 reversals occurred in a 12-million-year period, centering on 15 million years ago. These eras of frequent reversals have been counterbalanced by a few "superchrons" – long periods when no reversals took place.>>

[bystander]

Earth's Inconstant Magnetic Field
NASA News - 2003 Dec 29

Our planet's magnetic field is in a constant state of change, say researchers who are beginning to understand how it behaves and why.
...
To understand what's happening, we have to take a trip ... to the center of the Earth where the magnetic field is produced.

At the heart of our planet lies a solid iron ball, about as hot as the surface of the sun. Researchers call it "the inner core." It's really a world within a world. The inner core is 70% as wide as the moon. It spins at its own rate, as much as 0.2o of longitude per year faster than the Earth above it, and it has its own ocean: a very deep layer of liquid iron known as "the outer core."

Earth's magnetic field comes from this ocean of iron, which is an electrically conducting fluid in constant motion. Sitting atop the hot inner core, the liquid outer core seethes and roils like water in a pan on a hot stove. The outer core also has "hurricanes"--whirlpools powered by the Coriolis forces of Earth's rotation. These complex motions generate our planet's magnetism through a process called the dynamo effect.

Using the equations of magnetohydrodynamics, a branch of physics dealing with conducting fluids and magnetic fields, Glatzmaier and colleague Paul Roberts have created a supercomputer model of Earth's interior. Their software heats the inner core, stirs the metallic ocean above it, then calculates the resulting magnetic field. They run their code for hundreds of thousands of simulated years and watch what happens.

What they see mimics the real Earth: The magnetic field waxes and wanes, poles drift and, occasionally, flip. Change is normal, they've learned. And no wonder. The source of the field, the outer core, is itself seething, swirling, turbulent. ... The changes we detect on our planet's surface are a sign of that inner chaos.

They've also learned what happens during a magnetic flip. Reversals take a few thousand years to complete, and during that time--contrary to popular belief--the magnetic field does not vanish, It just gets more complicated. Magnetic lines of force near Earth's surface become twisted and tangled, and magnetic poles pop up in unaccustomed places. A south magnetic pole might emerge over Africa, for instance, or a north pole over Tahiti. Weird. But it's still a planetary magnetic field, and it still protects us from space radiation and solar storms.

Supercomputer models of Earth's magnetic field. On the left is a normal dipolar magnetic
field, typical of the long years between polarity reversals. On the right is the sort of
complicated magnetic field Earth has during the upheaval of a reversal.


Reversals of Earth's Magnetic Field Explained by Small Core Fluctuations
PhysOrg General Physics - 2009 Apr 23

According to a new model, small fluctuations in convective flow in Earth’s core can explain how the Earth’s magnetic field reverses.

Simple Mechanism for Reversals of Earth’s Magnetic Field

• Physical Review Letters, 102, 144503 (2009 Apr 07), DOI: 10.1103/PhysRevLett.102.144503
  arXiv.org > physics > arXiv:0806.3756 > (Submitted: 23 Jun 2008 (v1), Last Revised: 09 Feb 2009 (v3))

We show that a model, recently used to describe all the dynamical regimes of the magnetic field generated by the dynamo effect in the VKS experiment [1], also provides a simple explanation of the reversals of Earth's magnetic field, despite strong differences between both systems.

[dougettinger]

Do you know whether this model as shown was ever continued to the point where it completely stabilized with the dipole completed reversed ?

Doug Ettinger
Pittsburgh, PA

[dougettinger]

<<Scientific opinion is divided on what causes geomagnetic reversals. Many scientists believe that reversals are an inherent aspect of the dynamo theory of how the geomagnetic field is generated. In computer simulations, it is observed that magnetic field lines can sometimes become tangled and disorganized through the chaotic motions of liquid metal in the Earth's core. In some simulations, this leads to an instability in which the magnetic field spontaneously flips over into the opposite orientation. This scenario is supported by observations of the solar magnetic field, which undergoes spontaneous reversals every 7–15 years. However, with the sun it is observed that the solar magnetic intensity greatly increases during a reversal, whereas all reversals on Earth seem to occur during periods of low field strength. Present computational methods have used very strong simplifications in order to produce models that run to acceptable time scales for research programs.

The rate of reversals in the Earth's magnetic field has varied widely over time. 72 million years ago (Ma), the field reversed 5 times in a million years. In a 4-million-year period centered on 54 Ma, there were 10 reversals; at around 42 Ma, 17 reversals took place in the span of 3 million years. In a period of 3 million years centering on 24 Ma, 13 reversals occurred. No fewer than 51 reversals occurred in a 12-million-year period, centering on 15 million years ago. These eras of frequent reversals have been counterbalanced by a few "superchrons" – long periods when no reversals took place.>>

Neufer, I had not realized that as many as 51 reversals have been catalogued for the Earth. In checking with Extinction Events, I found no correlations with the times for magnectic reversals. Obviously, geomagnetic reversals are not influenced by factors outside the Earth's surface.

Doug Ettinger
Pittsburgh, PA

[neufer]

Earth's Inconstant Magnetic Field NASA News - 2003 Dec 29

<< [Magnetic] reversals take a few thousand years to complete, and during that time--contrary to popular belief--the magnetic field does not vanish, It just gets more complicated. Magnetic lines of force near Earth's surface become twisted and tangled, and magnetic poles pop up in unaccustomed places. A south magnetic pole might emerge over Africa, for instance, or a north pole over Tahiti.

But it's still a planetary magnetic field, and it still protects us from space radiation and solar storms.

Supercomputer models of Earth's magnetic field. On the left is a normal dipolar magnetic
field, typical of the long years between polarity reversals. On the right is the sort of
complicated magnetic field Earth has during the upheaval of a reversal.

[neufer]

Neufer, I had not realized that as many as 51 reversals have been cataloged for the Earth. In checking with Extinction Events, I found no correlations with the times for magnetic reversals. Obviously, geomagnetic reversals are not influenced by factors outside the Earth's surface.

It is all too complex to state that anything is "obvious" here.

"External events" cover a lot of different situations:

<<Others believe that geomagnetic reversals are triggered by external events
which directly disrupt the flow in the Earth's core. Such processes may include

1) the arrival of continental slabs carried down into the mantle by the action of plate tectonics at subduction zones,

2) the initiation of new mantle plumes from the core-mantle boundary, and

3) possibly mantle-core shear forces resulting from very large impact events.

Supporters of this theory hold that any of these events could lead to a large scale disruption of the dynamo, effectively turning off the geomagnetic field. Because the magnetic field is stable in either the present North-South orientation or a reversed orientation, they propose that when the field recovers from such a disruption it spontaneously chooses one state or the other, such that a recovery is seen as a reversal in about half of all cases.>>

[dougettinger]

I was not aware of these proposed external events. Thanks.

Doug Ettinger

[bystander]

"External events" cover a lot of different situations:

<<Others believe that geomagnetic reversals are triggered by external events
which directly disrupt the flow in the Earth's core. Such processes may include

1) the arrival of continental slabs carried down into the mantle by the action of plate tectonics at subduction zones,

2) the initiation of new mantle plumes from the core-mantle boundary, >>

Gravity lows mark burial sites of ancient tectonic plates
Science News; 05 June 2010; Vol.177 #12 (p. 13)

Dips in Earth's gravitational field tied to 'slab graveyards'

Scientists have unearthed a new explanation for several low-gravity spots detected around the world. They’re blaming the anomalies on vast “slab graveyards” that lie buried deep near the planet’s core.

When these slabs of rock were buried long ago, they released water that reduced the density of overlying rock, Caltech geophysicist Michael Gurnis and his colleagues reported online May 9 in Nature Geoscience. Low-density rock has less mass, and so less gravitational pull.

Scientists had previously noticed that gravity’s tug is smaller where tectonic plates, or large sections of Earth’s crust, once plunged below the surface, Gurnis says. The team’s new findings, he notes, provide insight into the causes of super-low values measured in four regions — in particular, areas south of Asia, along the coast of Antarctica south of New Zealand, in the northeastern Pacific and in the western Atlantic.

Beneath those four areas, seismic waves travel slower than normal at shallow depths —1,000 kilometers or less — but faster than normal deeper down, says Gurnis. Because sound waves travel faster through dense material (for example, water compared with air) the difference in speed with depth told the team that lighter, less dense material is lying over a layer of very dense material. Most likely, the top layer’s relative buoyancy stems from its water-rich composition, the researchers contend.

The water could have been injected in the mantle as a result of slow-motion tectonic collisions between lighter and heavier plates. As dense slabs slipped downward through the mantle, water in those rocks — as well as water in the seafloor sediments that had accumulated above those slabs — was released. “When that water was added to the surrounding rocks, it lowered their melting point and reduced their density,” buoying them upward, Gurnis explains.

The dehydrated remains of the dense tectonic plates now lie deep in the mantle, where they boost the speed of seismic waves, says Gurnis.

The notion that upwelling deep in the mantle could be linked to water carried downward in long-buried tectonic slabs is new, says Carolina Lithgow-Bertelloni, a geophysicist at University College London. A similar upwelling of water-rich material near subduction zones has long been associated with volcanism at Earth’s surface, she notes.

Previously, studies suggested that hot spots along the core-mantle boundary create buoyant plumes of rock that rise within Earth’s mantle (SN: 7/9/05, p. 24). In the new study, however, lowered density arises from presumed changes in the composition of rock, not its temperature.

[dougettinger]

I find it very hard to believe that bouyant plumes rise through the stiff, dense mantle from the liquid core and cause hot spots such as Yellowstone and the Icelandic volcanoes. The mantle is not like a lava lamp that demonstrates convection in action. I also find it hard to believe that subduction zones with diving plates has any affect on the much deeper liquid and solid cores. Subduction zones definitely produce earthquakes and subduction zone type volcanoes - never volcanoes with hot spot characteristics.

Thanks anyway for the reference.

Doug Ettinger
Pittsburgh, PA

[neufer]

Dips in Earth's gravitational field tied to 'slab graveyards'
Science News; 05 June 2010; Vol.177 #12 (p. 13)

<<Scientists have unearthed a new explanation for several low-gravity spots detected around the world. They’re blaming the anomalies on vast “slab graveyards” that lie buried deep near the planet’s core. When these slabs of rock were buried long ago, they released water that reduced the density of overlying rock, Caltech geophysicist Michael Gurnis and his colleagues reported in Nature Geoscience. Low-density rock has less mass, and so less gravitational pull. The notion that upwelling deep in the mantle could be linked to water carried downward in long-buried tectonic slabs is new, says geophysicist Carolina Lithgow-Bertelloni. A similar upwelling of water-rich material near subduction zones has long been associated with volcanism at Earth’s surface, she notes.>>

Clare: So is it fun humiliating me?

Jerry: No, it’s not you. It’s your stomach, he’s taking with this funny, booming, jovial voice.

Clare: So you think I’m fat?

Jerry: Nobody said you’re fat. He’s a loving character, like the Kool-ade guy.

Clare: He is fat!

Jerry: No, he’s just a little bloated.

Clare: Good-bye!

Jerry: It’s mostly water weight.

[dougettinger]

I never mean to humuiliate anybody. But, I certainly read Science News articles and the journals that are referenced with a critical eye. Any water driven downward under another plate in a subduction zone should quickly differentiate and be driven to the surface as is witnessed by subduction zone volcanoes.
The result, of course, in the intervening time before differentiation is completed is low density rock and, hence, gravitational anomalies. I am criticizing the statement that low density slabs would sink very deeply and come close to the Earth's core. This is simply not believable.

Doug Ettinger
Pittsburgh, PA

But like Clare, a subduction zone volcano can become very pissed off and explode without warning. Hot spot volcanoes are like the Koolade guy. They only get a little bloated. Most of the water in a hot spot volcano has already been squeezed outward.