We are past due for our magnetic field to 'reverse' itself.

ObbiQuiet · #1 3rd December 2003, 12:44 AM

(a program I saw on Nova a while ago)

They had said that once every 200,000 years or so the earth's magnetic field reverses itself. They also said that when that happens the earth will be less-protected from sunlight, causing a general heat-increase (this last part is what my dad told me, I'm not sure if this is accurate).

Is it possible that a particularily large change in the earth's magnetic field (Assuming the change isn't always constant) could have killed off the Dinosaurs, or at least made it impossible for creatures of their incredible mass to survive?

Chi_Cygni · #2 3rd December 2003, 12:56 AM

No.

The effects wouldn't be that severe.

Also, historically, the field reversals have been very stochastic. Sometimes separated by millions of years, sometimes far less.

It certainly isn't periodic.

ObbiQuiet · #3 3rd December 2003, 12:58 AM

I blame my father for the bad information!

*storms off*

Arikay · #4 3rd December 2003, 01:05 AM

Dont worry, most people seem to get stuff about the magnetic field wrong, it doesn't help that a recent movie decided to play on these claims. "The Core" I believe said that if the magnetic field stops people would die from the radiation.

A reversal wouldn't let too much more in than it does now.

its thought that we are past due because on average, the time between reversals is much shorter than the time between now and our last one. Matter of fact, I believe that this is our largest gap. However, "past due" doesnt have to mean within the next year, or 100 years, it could mean that in 10,000 years it will reverse, since time is much different to the earth compared to us.

Mechanical Bliss · #5 3rd December 2003, 01:11 AM

Although, looking at the geomagnetic time scale I have on hand, it appears as though there are a couple of polarity reversals right at the Cretaceous/Tertiary boundary, I don't think the consequences of a magnetic reversal would be so significant. Yes there would be exposure to cosmic radiation, but I am not aware of any evidence in the geoloic record indicating a link between mass extinctions and geomagnetic reversals. Also, the periodicity of reversals is not that strictly defined. Periods between reversals have been on the order of millions of years as well.

Taffsadar · #6 3rd December 2003, 03:18 AM

Originally Posted by Arikay

Dont worry, most people seem to get stuff about the magnetic field wrong, it doesn't help that a recent movie decided to play on these claims. "The Core" I believe said that if the magnetic field stops people would die from the radiation.

.

It said alot of things. I was starting to wonder if Hovind was the script writer. It was really that horrible.

notto · #7 3rd December 2003, 07:30 AM

Originally Posted by Arikay

Dont worry, most people seem to get stuff about the magnetic field wrong, it doesn't help that a recent movie decided to play on these claims. "The Core" I believe said that if the magnetic field stops people would die from the radiation.

Are you saying the Core wasn't scientifically accurate?

My hopes of being a terranaut have been crushed.

Taffsadar · #8 3rd December 2003, 09:33 AM

Originally Posted by notto

My hopes of being a terranaut have been crushed.

Well it's better to have your dreams crushed than it is to be crushed by your dreams...

PhantomLlama · #9 3rd December 2003, 02:05 PM

Originally Posted by Taffsadar

It said alot of things. I was starting to wonder if Hovind was the script writer. It was really that horrible.

It must have been very hard to get recruits for the 'terranauts'.

Applicant: So, what does the job entail exactly?

Official: We're going to send you down so far into the earth that the temperature is hot enough to melt iron, and the pressure is crushing enough to make it solid again.

Applicant: I think my expertise could be better used in NASA.

TrueCreation · #10 3rd December 2003, 05:38 PM

Originally Posted by Arikay

Dont worry, most people seem to get stuff about the magnetic field wrong, it doesn't help that a recent movie decided to play on these claims. "The Core" I believe said that if the magnetic field stops people would die from the radiation.

A reversal wouldn't let too much more in than it does now.

--Is there anything detailed in the scientific literature in this regard? Recently I have had a moderate interest in looking into this issue but can't seem to find the resources.

its thought that we are past due because on average, the time between reversals is much shorter than the time between now and our last one. Matter of fact, I believe that this is our largest gap.

--Far from it, the geomagnetic plarity time scale shows that there has been significant change in the frequency of geomagnetic reversals over at least the last ~180 My. Here is some data I compiled from Harland, et al. [1990], also available in Turcotte and Schubert [2002]:

--And another illustration of the duration of normal and reverse polarity chrons on this time scale:

--The general trend is as we move forward in time from 170 Ma, geomagnetic reversals occur often and gradually decrease until we reach the 75 - 115 Ma bottom line. This period of time is known as the Cretaceous Quiet Interval, or, Cretaceous Normal Polarity Superchron (chron C34N after Harland et al. [1990] and Lowrie [1997]), where normal polarity existed with no reversed polarity transition for about 35 Ma in geologic time. After the Cretaceous Superchron geomagnetic reversals began with long duration polarity chrons which gradually lessened in duration with subsequent reversals until recent times.

See:

Harland, W. B., et al., A Geologic Time Scale 1989, 1990

Lowrie, William, Fundamentals of Geophysics, 1997

Turcotte & Schubert, Geodynamics - Second Edition, 2002

--[edit] - The change in the reversal frequency over time seems to have some kind of rudimentary correlation as shown in the first figure. This may be due to the evolution of the thermal structure at the CMB (core-mantle boundary). Research and computer simulations by Glatzmaier et al.[1999] suggest a correlation between the frequency of geomagnetic reversals and a heterogenous heat flux across the core-mantle boundary (CMB). Lateral variations in temperatures in the lower mantle can vary by hundreds of degrees Kelvin over roughly 1000 km, however convection in the fluid outer core is much more efficient at mixing material and distributing temperatures evenly [Glatzermier, 1999]; lateral temperature variations in the core therefore do not exceed 0.001 K. This causes variation in patterns of heat conduction from the core to the mantle, resulting in slightly cooler core fluid, on average, below cool mantle, and warmer fluid below warm mantle.

These forces, together with Coriolis forces (due to the component of the fluid flow perpendicular to the Earth's rotation vector) and Lorentz forces (due to the component of the electric current perpendicular to the magnetic field), drive complicated time-dependent circulations, providing a convective heat flux within the core that accommodates the imposed non-uniform conductive heat flux out of the core at the CMB. (Glatzmaier et al., 1999)

--Glatzmaier et al. performed computer simulations with eight different, time-independent, patterns of radial heat flux through the CMB. Among the variable CMB heat flow simulations was one case of uniform heat flow and a case with heat flow values based on seismic tomography of the lowermost mantle for today's Earth, the remaining cases imposed simple radial patterns of heat flow. Much of the behavior of the geomagnetic field in several of the simulations resembles that seen in the paleomagnetic record very well (eg. significant decreases in the dipole moment during reversals and excursions). In the simulations there are observed correlations between variable heat flux's and the frequency of geomagnetic reversals over time, however the simulation running times are very short (100,000 - 300,000 Myr) and so the statistical significance of these correlations is speculative.

--It is well substantiated in Glatzmaier et al.(1999) that a variable temperature distribution across the CMB may very well correlate with the changing frequencies of geomagnetic reversals over time. However, that this explains the observed trend seen in the geomagnetic polarity timescale (GPTS) over the 160 Myr time span is questionable. It is important to note that Glatzmaier et al.(1999) for good reason never addresses the problem of explaining the changing frequency of geomagnetic reversals over the 160 Myr time span in their research paper, albeit it has been interpreted by other geoscientists that it does (Buffett, 1999):

...the simulations clearly demonstrate that changes in heat flow conditions can strongly influence the behaviour of the magnetic field. In particular, the change in behaviour appears to be sufficient to explain the variations observed in Fig. 1.[edit - or see the first figure in this post](Buffet, 1999)

--Ocean basins open and close and mantle convection occurs on the same timescales (10^8 years) as the trend in the geomagnetic data appears to operate on. And so the distribution of temperature in the earths mantle may be a cause of the wilson cycle, and more specifically, subducting slabs penetrating deeply into the mantle and possibly down to the CMB.

--This research also opens the door to further research on bahaviour of the geodynamo and geomagnetic reversals during an episode of CPT. The thermal and time constraints on various presupposed parameters relevant to the evolution of the geomagnetic field are pretty anomalous relative to that which would have to be presupposed in any simulation of the geodynamo during CPT. For instance, the total heat flow out of the core is maintained at 7.2 x 10^12 W. The magnetic dipole diffusion time is assumed to be constant at 20,000 years. And for all cases they fixed the peak heat flux variability accross the CMB at ±44.6 mW m-2 relative to the mean. All of these values would be most likely be expected to change significantly a runaway overturning of the earths mantle. The geometry of the outer core also may deform as cold mantle material sank to the CMB and hot upwelling rock buoyed to the upper mantle. Radiogenic heat from accelerated decay would also have to be considered in modeling heat transfer across the CMB, core/mantle viscosity, and thermal heterogeny in the mantle. In one case study, known tomography was imposed on the lower mantle (Glatzermier, 1999), based indirectly on relationships between temperature anomalies and lateral variations in elastic properties to estimate thermal heterogeny above the CMB (Buffett, 1999), but this certainly would not accurately represent temperature distribution during CPT.

--Thought it would be interesting to provide information on this aspect of the geodynamo and this vary intersting behaviour over the course of the Mesozoic and Cenozoic.

See:

Glatzermier, et al., The role of the Earth's mantle in controlling the frequency of geomagnetic reversals, Nature vol. 401, pp. 885-890, 1999

Buffett, Role reversal in geomagnetism, Nature vol. 401, pp. 861-862, 1999

Cheers,

-Chris Grose