Geomagnetic reversal
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[[File:Earth seafloor crust age 1996.gif|thumb|right|alt=Chart showing Earth's seafloor crust ages.|Earth's seafloor crust ages.]] | [[File:Earth seafloor crust age 1996.gif|thumb|right|alt=Chart showing Earth's seafloor crust ages.|Earth's seafloor crust ages.]] | ||
| − | During the 1950s-70s, scientists learned that the ocean seafloor is patterned by stripes of magnetism, with alternating sections magnetized north, south, north, etc.<ref name="PWNMS"/><ref name="About">[http://geography.about.com/od/physicalgeography/a/magnetic.htm Rosenberg, Matt. "Magnetic Reversal." About.com. n.d. Web. 05 June 2014.]</ref><ref name="NOAA">[http://www.oceanexplorer.noaa.gov/edu/learning/2_midocean_ridges/activities/seafloor_spreading.html "Seafloor Spreading Activity." NOAA. 12 February 2013. Web. 28 June 2014.]</ref><ref name="CD741">[http://www.talkorigins.org/indexcc/CD/CD741.html Isaac, Mark. "Claim CD741." Talk.Origins Archive. 13 February 2004. Web. 28 June 2014.]</ref> | + | During the 1950s-70s, scientists learned that the ocean seafloor is patterned by stripes of magnetism, with alternating sections magnetized north, south, north, etc.<ref name="PWNMS"/><ref name="About">[http://geography.about.com/od/physicalgeography/a/magnetic.htm Rosenberg, Matt. "Magnetic Reversal." About.com. n.d. Web. 05 June 2014.]</ref><ref name="NOAA">[http://www.oceanexplorer.noaa.gov/edu/learning/2_midocean_ridges/activities/seafloor_spreading.html "Seafloor Spreading Activity." NOAA. 12 February 2013. Web. 28 June 2014.]</ref><ref name="CD741">[http://www.talkorigins.org/indexcc/CD/CD741.html Isaac, Mark. "Claim CD741." Talk.Origins Archive. 13 February 2004. Web. 28 June 2014.]</ref><ref name="SCC">Laurie R. Godfrey (1983). "[[Scientists Confront Creationism]]". W. W. Norton & Company, Canada. Pages 36-37. ISBN 0393301540.</ref><ref group=note>The text of this portion of Scientists Confront Creationism is not online in non-Google Books format, so [[FuzzyCatPotato|I]] typed it up. Here you go:<br>''"We do not need to assume that the drifting of the continents has been at a constant rate over the past 200 million years, for we have a magnetic record of their movements. The molten rock that oozes up in the Mid-Atlantic Ridge is partly magnetized by the magnetic field of the earth, and as the rock cools that magnetism is frozen into it. We can measure the magnetic polarity of the rocks in the sea floor with delicate magnetometers towed on the ocean surface. This would, of course, not be interesting if it were not that the magnetic field of the earth reverses itself every half million or so years; in fact, in the past 76 million years, it has reversed itself 171 times. Ocean floor rocks close to the Mid-Atlantic Ridge carry the magnetism of the present field, but those several miles away on either side have the reversed magnetic polarity of the field of the earth half a million years ago, and those still farther have the polarity of the still earlier field, and so on. Over recent geological times the alternating polarity of the magnetism in the ocean for has reversed many times in consort with the changing field of the earth. Moreover, this pattern of field reversals is consistently reproduced in the ocean floor in widely separated regions of the earth. But how do we know the dates of the magnetic field reversals in the ocean floor? The age of any reversal can be estimated by extrapolation along the sea-floor magnetic strips, assuming constant spreading rates, once a reliable chronology is constructed for a portion of the strip. Such a reliable chronology has been constructed for the most recent segment - up to 5 million years ago. It is based on direct radioactive dating of sequences of lava flows."''</ref> |
| − | This occurs because magma contains iron grains (specifically magnetite<ref name="PWNMS"/>) that orient themselves towards the magnetic north pole, much like a compass. When lava solidifies, the iron grains are no longer able to move freely, and are permanently aligned towards wherever the magnetic north pole was. Thus, when magma from oceanic trenches hardens into seafloor, it will maintain its magnetic alignment.<ref name="PWNMS"/><ref name="About"/><ref name="NOAA"/> | + | This occurs because magma contains iron grains (specifically magnetite<ref name="PWNMS"/>) that orient themselves towards the magnetic north pole, much like a compass. When lava solidifies, the iron grains are no longer able to move freely, and are permanently aligned towards wherever the magnetic north pole was. Thus, when magma from oceanic trenches hardens into seafloor, it will maintain its magnetic alignment.<ref name="PWNMS"/><ref name="About"/><ref name="NOAA"/><ref name="SCC"/> |
| − | Interestingly, the ages of magnetic stripes on both sides of the Atlantic Ocean<ref name="About"/><ref name="CD741"/><ref name="PWNSS">[http://platetectonics.pwnet.org/story_tectonics/theory/seafloor_spreading.htm "Seafloor Spreading." Prince William Network. Our Changing Continent. n.d. Web. 28 February 2014.]</ref> and all over the world<ref name="PWNSS"/> match. This would only occur if all of these locations had experienced the same magnetic field at the same time. This | + | Interestingly, the ages of magnetic stripes on both sides of the Atlantic Ocean<ref name="About"/><ref name="CD741"/><ref name="SCC"/><ref name="PWNSS">[http://platetectonics.pwnet.org/story_tectonics/theory/seafloor_spreading.htm "Seafloor Spreading." Prince William Network. Our Changing Continent. n.d. Web. 28 February 2014.]</ref> and all over the world<ref name="SCC"/><ref name="PWNSS"/> match. This would only occur if all of these locations had experienced the same magnetic field at the same time, which would be incredibly unlikely if magnetic reversals did not occur on a global scale. This indicates that geomagnetic reversals are not local fluctuations, but global events. |
===Volcanic flows=== | ===Volcanic flows=== | ||
| Line 22: | Line 22: | ||
===Seafloor spreading=== | ===Seafloor spreading=== | ||
| − | The theory of [[seafloor spreading]] (and of [[continental | + | The theory of [[seafloor spreading]] (and of [[continental drift]] and [[plate tectonics]]) is supported by the existence of symetrical (in age, polarity, and distance from mid-ocean trenches) magnetic stripes in the rock of the ocean floor.<ref name="NOAA"/><ref name="CD741"/><ref name="SCC"/> No other theory can explain why these stripes exist at all ''or'' their symmetry, making this strong evidence for seafloor spreading. |
===Geodynamo theory=== | ===Geodynamo theory=== | ||
| Line 31: | Line 31: | ||
Because the strength of the geomagnetic field is currently declining, it has become the center of some [[woo]].<ref name="NatGeo"/> It is believed that a geomagnetic reversal or excursion (basically a reversal, with associated decrease of strength, minus the polar flip) may be imminent in the next couple of thousand of years or so.<ref name="GeoMag"/><ref name="NatGeo"/><ref name="About"/><ref name="NOAA"/> | Because the strength of the geomagnetic field is currently declining, it has become the center of some [[woo]].<ref name="NatGeo"/> It is believed that a geomagnetic reversal or excursion (basically a reversal, with associated decrease of strength, minus the polar flip) may be imminent in the next couple of thousand of years or so.<ref name="GeoMag"/><ref name="NatGeo"/><ref name="About"/><ref name="NOAA"/> | ||
| − | Geomagnetic reversals appear to be relatively harmless to living beings.<ref name="GeoMag"/> Considering that around 170 | + | Geomagnetic reversals appear to be relatively harmless to living beings.<ref name="GeoMag"/> Considering that around 170<ref name="About"/> to 171<ref name="SCC"/> reversals are documented in the last 76 million years alone<ref name="SCC"/>, reversals don't seem to pose much of an extinction-level threat. |
==Alternate "interpretations"== | ==Alternate "interpretations"== | ||
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===Talk.Origins=== | ===Talk.Origins=== | ||
The shape of the dipole field is not ideal but is highly distorted by irregularities superimposed on the dipole field. These irregularities, collectively called the nondipole field, are thought to be caused by eddy currents in the liquid core at the Earth’s core/mantle boundary. Like the dipole field, the nondipole field is slowly and constantly changing. The Earth’s magnetic field we actually observe at any spot on the Earth is the sum of dipole and nondipole fields. | The shape of the dipole field is not ideal but is highly distorted by irregularities superimposed on the dipole field. These irregularities, collectively called the nondipole field, are thought to be caused by eddy currents in the liquid core at the Earth’s core/mantle boundary. Like the dipole field, the nondipole field is slowly and constantly changing. The Earth’s magnetic field we actually observe at any spot on the Earth is the sum of dipole and nondipole fields. | ||
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==External links== | ==External links== | ||
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[http://cse.ssl.berkeley.edu/segwayed/lessons/exploring_magnetism/magnetism_on_earth/guide_activity8.pdf] | [http://cse.ssl.berkeley.edu/segwayed/lessons/exploring_magnetism/magnetism_on_earth/guide_activity8.pdf] | ||
[http://www.jstor.org/discover/10.2307/29775784?uid=3739704&uid=2&uid=4&uid=3739256&sid=21104259464483] | [http://www.jstor.org/discover/10.2307/29775784?uid=3739704&uid=2&uid=4&uid=3739256&sid=21104259464483] | ||
| − | |||
[http://www.scientificamerican.com/article/what-causes-the-periodic/] | [http://www.scientificamerican.com/article/what-causes-the-periodic/] | ||
[http://www.scientificamerican.com/article/is-it-true-that-the-stren/] | [http://www.scientificamerican.com/article/is-it-true-that-the-stren/] | ||
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[http://creation.com/the-principle-of-least-astonishment#20120328] | [http://creation.com/the-principle-of-least-astonishment#20120328] | ||
[http://www.icr.org/index.php?module=articles&action=view&ID=371#20120330] | [http://www.icr.org/index.php?module=articles&action=view&ID=371#20120330] | ||
| − | |||
"Impact Theory": | "Impact Theory": | ||
[http://mb-soft.com/public/tecto2.html] | [http://mb-soft.com/public/tecto2.html] | ||
| + | [http://www.newgeology.us/presentation25.html] | ||
==External links== | ==External links== | ||
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[[Science]]: | [[Science]]: | ||
*[[Magnetism]] | *[[Magnetism]] | ||
| + | *[[Seafloor spreading]] | ||
| + | *[[Continental drift]] | ||
| + | *[[Plate tectonics]] | ||
*[[Fun:Scientific Geoterrapinism|Scientific Geoterrapinism]] | *[[Fun:Scientific Geoterrapinism|Scientific Geoterrapinism]] | ||
[[Pseudoscience]]: | [[Pseudoscience]]: | ||
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*[[Hydroplate theory]] | *[[Hydroplate theory]] | ||
*[[Catastrophic plate tectonics]] | *[[Catastrophic plate tectonics]] | ||
| + | |||
| + | ==Notes== | ||
| + | <references group="note"/> | ||
==References== | ==References== | ||
<references/> | <references/> | ||
Revision as of 18:09, 3 July 2014
- Absence of evidence
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“”Magnetism, as you recall from physics class, is a powerful force that causes certain items to be attracted to refrigerators.
|
| —Dave Barry[1] |
A geomagnetic reversal is a shift in Earth's magnetic field in which the positions of the magnetic north (positive) and south (negative) are switched.[2][3] In modern times, a reversal has usually occured every 50,000 to 800,000 years, with an average of 200,000 years.[2][4]
Contents |
Evidence for
Magnetic seafloor stripes
During the 1950s-70s, scientists learned that the ocean seafloor is patterned by stripes of magnetism, with alternating sections magnetized north, south, north, etc.[3][5][6][7][8][note 1]
This occurs because magma contains iron grains (specifically magnetite[3]) that orient themselves towards the magnetic north pole, much like a compass. When lava solidifies, the iron grains are no longer able to move freely, and are permanently aligned towards wherever the magnetic north pole was. Thus, when magma from oceanic trenches hardens into seafloor, it will maintain its magnetic alignment.[3][5][6][8]
Interestingly, the ages of magnetic stripes on both sides of the Atlantic Ocean[5][7][8][9] and all over the world[8][9] match. This would only occur if all of these locations had experienced the same magnetic field at the same time, which would be incredibly unlikely if magnetic reversals did not occur on a global scale. This indicates that geomagnetic reversals are not local fluctuations, but global events.
Volcanic flows
Computer modelling
Importance of
Seafloor spreading
The theory of seafloor spreading (and of continental drift and plate tectonics) is supported by the existence of symetrical (in age, polarity, and distance from mid-ocean trenches) magnetic stripes in the rock of the ocean floor.[6][7][8] No other theory can explain why these stripes exist at all or their symmetry, making this strong evidence for seafloor spreading.
Geodynamo theory
Cause
Impacts of
Because the strength of the geomagnetic field is currently declining, it has become the center of some woo.[4] It is believed that a geomagnetic reversal or excursion (basically a reversal, with associated decrease of strength, minus the polar flip) may be imminent in the next couple of thousand of years or so.[2][4][5][6]
Geomagnetic reversals appear to be relatively harmless to living beings.[2] Considering that around 170[5] to 171[8] reversals are documented in the last 76 million years alone[8], reversals don't seem to pose much of an extinction-level threat.
Alternate "interpretations"
Denialism
Strength fluctuations and geomagnetic decay
Some young earth creationists, such as Walt Brown,[7] Thomas Barnes,[10][11] and Kent Hovind,[12] deny the existence of geomagnetic reversals. Depending on the level of denialism, they either deny that the data exists or consider them to be past fluctuations in the strength of the geomagnetic field.[7]
Why? Because, based on the decline of the magnetic field in the past century or two, they believe that the magnetic field (a) has no generating source and (b) has been in steady, exponential decline, forever.[11] Extrapolating exponentially backwards, YECs conclude that the Earth cannot be more than 10,000 years old, because that would imply
The major problem with this theory is that it assumes that current geomagnetic decay rates are the same as they were thousands of years ago, when there is significant evidence against this.
First, as shown in the image on the right, the geomagnetic field frequently both increases and decreases in strength, with no clear trend either way.[11] The YEC theory has no way to explain any increase, let alone both increases and decreases,[11] while the currently accepted dynamo theory can.[11]
Second, as shown earlier, during geomagnetic reversals and excursions the strength of the field significantly decreases.[2][4][5][6] For example, during the most recent geomagnetic reversal the Earth's magnetic field reduced to 5% of its original strength.[13] The earth's geomagnetic field is believed to be currently undergoing a reversal or an excursion, which would account for the decrease of the strength of the earth's geomagnetic field.[2][4][5][6]
Aside from this, dynamo theory better matches observations than decay. As Brent Dalrymple puts it,[14] "The dynamo theory has gained near-universal acceptance because it is the only proposed mechanism that can explain all the observed features of the Earth's magnetic field. In contrast, Barnes' hypothesis of a freely decaying field cannot explain the existence, configuration, movement, or changes in the nondipole field, the fluctuations in the dipole moment, the reversals in field polarity, or the documentation in the geologic record of the continued existence of the field for more than three billion years."
Rapid reversals
Modern YECs, such as Russell Humphreys,[10] generally accept that geomagnetic reversals exist and have occured. However, needing an explanation that (a) requires Goddidit and (b) allows for a 6,000 year timeframe, they turn to a geomagnetic version of hydroplate theory.
Apocalyptic
Some cranks, including Robert W. Felix and Alan Caruba
Stolen Stuff
Talk.Origins
The shape of the dipole field is not ideal but is highly distorted by irregularities superimposed on the dipole field. These irregularities, collectively called the nondipole field, are thought to be caused by eddy currents in the liquid core at the Earth’s core/mantle boundary. Like the dipole field, the nondipole field is slowly and constantly changing. The Earth’s magnetic field we actually observe at any spot on the Earth is the sum of dipole and nondipole fields.
External links
Google Books: Reversals of the Earth's Magnetic Field Our Magnetic Earth
Useful: [1] [2] [3] [4] [5] [6] [7] [8]
Creationist: [9] [10] [11] [12] [13] [14] [15] [16] [17]
External links
See also
Creationist pseudoscience:
Notes
- ↑ The text of this portion of Scientists Confront Creationism is not online in non-Google Books format, so I typed it up. Here you go:
"We do not need to assume that the drifting of the continents has been at a constant rate over the past 200 million years, for we have a magnetic record of their movements. The molten rock that oozes up in the Mid-Atlantic Ridge is partly magnetized by the magnetic field of the earth, and as the rock cools that magnetism is frozen into it. We can measure the magnetic polarity of the rocks in the sea floor with delicate magnetometers towed on the ocean surface. This would, of course, not be interesting if it were not that the magnetic field of the earth reverses itself every half million or so years; in fact, in the past 76 million years, it has reversed itself 171 times. Ocean floor rocks close to the Mid-Atlantic Ridge carry the magnetism of the present field, but those several miles away on either side have the reversed magnetic polarity of the field of the earth half a million years ago, and those still farther have the polarity of the still earlier field, and so on. Over recent geological times the alternating polarity of the magnetism in the ocean for has reversed many times in consort with the changing field of the earth. Moreover, this pattern of field reversals is consistently reproduced in the ocean floor in widely separated regions of the earth. But how do we know the dates of the magnetic field reversals in the ocean floor? The age of any reversal can be estimated by extrapolation along the sea-floor magnetic strips, assuming constant spreading rates, once a reliable chronology is constructed for a portion of the strip. Such a reliable chronology has been constructed for the most recent segment - up to 5 million years ago. It is based on direct radioactive dating of sequences of lava flows."
References
- ↑ "Magnetism, as you recall from physics class, is a powerful force that causes certain items to be attracted to refrigerators." Just-One-Liners.com. n.d. Web. 04 June 2014.
- ↑ 2.0 2.1 2.2 2.3 2.4 2.5 "Reversals: Magnetic Flip." Natural Environment Research Council. n.d. Web. 04 June 2014.
- ↑ 3.0 3.1 3.2 3.3 "Magnetic Striping." Prince William Network. Our Changing Continent. n.d. Web. 28 June 2014.
- ↑ 4.0 4.1 4.2 4.3 4.4 Roach, John. "Earth's Magnetic Field Is Fading." National Geographic Society. 09 September 2004. Web. 04 June 2014.
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 Rosenberg, Matt. "Magnetic Reversal." About.com. n.d. Web. 05 June 2014.
- ↑ 6.0 6.1 6.2 6.3 6.4 6.5 "Seafloor Spreading Activity." NOAA. 12 February 2013. Web. 28 June 2014.
- ↑ 7.0 7.1 7.2 7.3 7.4 Isaac, Mark. "Claim CD741." Talk.Origins Archive. 13 February 2004. Web. 28 June 2014.
- ↑ 8.0 8.1 8.2 8.3 8.4 8.5 8.6 Laurie R. Godfrey (1983). "Scientists Confront Creationism". W. W. Norton & Company, Canada. Pages 36-37. ISBN 0393301540.
- ↑ 9.0 9.1 "Seafloor Spreading." Prince William Network. Our Changing Continent. n.d. Web. 28 February 2014.
- ↑ 10.0 10.1 Major, Trevor. "The Earth's Young Magnetic Field." Apologetics Press. 1993. Web. 04 June 2014.
- ↑ 11.0 11.1 11.2 11.3 11.4 Isaac, Mark. "Claim CD701." Talk.Origins Archive. 12 May 2006. Web. 28 June 2014.
- ↑ Robbins, Stuart. "Does Earth's Decaying Magnetid Field Mean It Was Created 6,000 Years Ago?". 09 August 2011. Web. 28 June 2014.
- ↑ "Ice age polarity reversal was global event: Extremely brief reversal of geomagnetic field, climate variability, and super volcano." Helmholtz Centre Potsdam. 16 October 2012. Web. 04 June 2014.
- ↑ 1992. "Can Earth be Dated from Decay of its Magnetic Field?" NCSE Reports, Vol.12, No.1 (Spring 1992), pp.16-17 National Center for Science Education, P.O. Box 9477, Berkeley, CA 94709-0477.