Reversals of magnetic poles in 115-120 ka BP and ~150Ma. Image courtesy: Jacobs, J.A. Reversals of Earth's Magnetic Field after Laj et al. (1991)
Reversals of Earth's Magnetic field
The magnetic field is created within Earth's core and may undergo changes so called reversals overtime.
More than a century ago, it was believed that the magnetic field was unchangeable. In 1904 and 1906, David and Brunhes discovered for the first time that rocks can be magnetized not only in the direction of the present magnetic field, but also in the opposite direction. Indeed, they found lava flows with reversed magnetization. This gave birth to numerous questions in the scientific world and in a brand new yet undeveloped topic.
The study of the Earth's magnetic field reversals can be made by examining the orientation of rock's magnetism (J.A. Jacobs The Magnetic Field Reversals). Reversals can be affected by core-mantle boundaries as well as exchange tectonic movements. Some biological events can be linked to the reversals as well. For instance, some animals use magnetism to orientate their movements (and W.Wiltschko R., Magnetic orientation in animals).
We will try to explain quickly, clearly and simply what a reversal in the magnetic field is and what is it due to.
More than a century ago, it was believed that the magnetic field was unchangeable. In 1904 and 1906, David and Brunhes discovered for the first time that rocks can be magnetized not only in the direction of the present magnetic field, but also in the opposite direction. Indeed, they found lava flows with reversed magnetization. This gave birth to numerous questions in the scientific world and in a brand new yet undeveloped topic.
The study of the Earth's magnetic field reversals can be made by examining the orientation of rock's magnetism (J.A. Jacobs The Magnetic Field Reversals). Reversals can be affected by core-mantle boundaries as well as exchange tectonic movements. Some biological events can be linked to the reversals as well. For instance, some animals use magnetism to orientate their movements (and W.Wiltschko R., Magnetic orientation in animals).
We will try to explain quickly, clearly and simply what a reversal in the magnetic field is and what is it due to.
❍ What is it?
❍ Why/How?
❍ How are they recorded
❍ How are they dated?
❍ How are they recorded?
❍ Major proof: the Oceanic ridge
❍ Chronology
❍ Why/How?
❍ How are they recorded
❍ How are they dated?
❍ How are they recorded?
❍ Major proof: the Oceanic ridge
❍ Chronology
What is it?
Earth's magnetic poles may undergo inversions, that is to say the poles are reversed: the Northern pole becomes the Southern pole and vice versa. In other words, when the magnetic field is reversed, the magnetic Northern pole moves to the Southern geographical pole.
There are different types of reversals. These are distinguished by their period:
⟐ the shortest one: 30 000years < cryptochron
⟐ 100 000years> microchron
⟐ 100 000years <subchron <1M years
⟐ 1M years <chron <10M years
⟐ 10M years <superchron <100M years
⟐ the longest one: 100M years <megachron <1billion years
When a reversal occurs, magnetic intensity decreases of 10% (according to J.A. Jacobs). It also occurs relatively quickly with respect to the geological scale and rocks are unable to record this intensity variation well. Never the less, Van Zijl et al (1962) and Kaporovich et al. (1966) have confirmed that the intensity of magmatism decreases before a reversal. By the end of the reversal, intensity reaches its initial value.
There are different types of reversals. These are distinguished by their period:
⟐ the shortest one: 30 000years < cryptochron
⟐ 100 000years> microchron
⟐ 100 000years <subchron <1M years
⟐ 1M years <chron <10M years
⟐ 10M years <superchron <100M years
⟐ the longest one: 100M years <megachron <1billion years
When a reversal occurs, magnetic intensity decreases of 10% (according to J.A. Jacobs). It also occurs relatively quickly with respect to the geological scale and rocks are unable to record this intensity variation well. Never the less, Van Zijl et al (1962) and Kaporovich et al. (1966) have confirmed that the intensity of magmatism decreases before a reversal. By the end of the reversal, intensity reaches its initial value.
Why/How?
Polarity transitions are generated by atomic motion within our planet's core. Indeed, the latter is composed of Iron and nickel which form a liquid outer core and solid inner core. These elements interact and move irregularly within the core which generates reversals.
Each part of the core has its own magnetic field, so
Each part of the core has its own magnetic field, so
- When both the inner and outer core undergo similar changes and when the transition is completed, we get a complete reversal of magnetic poles. They move along tropics crossing the Equator, and finally end up at the opposite geographical pole. In this case, we enter in a reversed magnetic period, or
- When inner and outer core undergo opposite changes, inner core resists to outer core's movements, poles return to their initial position, i.e. North magnetic pole remains in the North. This is a renewal of the magnetic intensity and is called “excursion”.
How are they recorded?
Reversals are recorded in sedimentary rocks and in both intrusive and extrusive igneous rocks. A transition is best recorded when contemporary sedimentation is fast, hence these are used the most. Igneous rocks cool down and also make a good record of the transition but are not commonly used. Dunn et al. in 1971 and Dodson et al in 1978 are two of the few who have used these rocks to study magnetic variations.
How are they dated?
In 1959, Rutten used the Potassium-Argon (K-Ar) method to date differently magnetized rocks for the first time. Overtime, his work has been slightly modified and improved as a better approximation of radioactive decay constants of the concerned elements were found. The United States and Australia used K-Ar dating to produce a polarity time-scale. Thus, the first polarity time-scale was created by Cox et al in 1963. We could therefore finally describe how the magnetic field has evolved and changed over geological time-scale. It is believed that the Earth has always possessed a magnetic field. The polarity time-scale ends at 180 000 000, the age of the oldest magnetized rock found on Earth.
Reversals can be clearly displayed at the oceanic ridge which has been the basis of all studies related to the magnetic field.
Reversals can be clearly displayed at the oceanic ridge which has been the basis of all studies related to the magnetic field.
Major proof: the Oceanic ridge
The mid-ocean ridge is a boundary between two tectonic plates: the Afro-European plate and American plate which are moving away from each other. Matter is constantly uplifted and forms a new crust. Once formed, the material moves away from the oceanic ridge perpendicularly to its axis while being cooled; when a rock cools, it records the magnetic field contemporary to the moment it solidified. Bands parallel to the oceanic ridge can be found with alternation of normal magnetization (i.e. oriented like in the present day) and reversed (when the magnetic field is reversed). These bands have been studied many times and have been recorded by specially designed vessels; a magnetometer attached to a boat can record magnetism from the surface (more common in the past), or near the bottom of the ocean (nowadays). Data recorded at the surface of the ocean, due to a lack of technological tools, confused scientists as records always changed. Moreover, the path of the vessls were not precisely recorded and could not be properly linked to the magnetometer's data.
The discovery of alternating magnetized bands gave birth to the idea of tectonic plates by McKenzie and Parker in 1967, Morgan in 1968 and Le Pichon in 1968. This idea was a big step in the knowledge of Earth Sciences. According to R.T.Merrill and M.W. McElhinny, the magnetic field of the Earth was mentioned for the first time in Heirtzler et al. work in 1968. In 1978, Johnson and Merrill drilled a deep hole into the Atlantic ocean and confirmed the phenomenon of magnetic reversals.
The lithosphere, i.e. the crust and the upper mantle, is considered to be the conveyor belt system for magnetism.
The discovery of alternating magnetized bands gave birth to the idea of tectonic plates by McKenzie and Parker in 1967, Morgan in 1968 and Le Pichon in 1968. This idea was a big step in the knowledge of Earth Sciences. According to R.T.Merrill and M.W. McElhinny, the magnetic field of the Earth was mentioned for the first time in Heirtzler et al. work in 1968. In 1978, Johnson and Merrill drilled a deep hole into the Atlantic ocean and confirmed the phenomenon of magnetic reversals.
The lithosphere, i.e. the crust and the upper mantle, is considered to be the conveyor belt system for magnetism.
Chronology
Over Earth's History, inversions occurred randomly. The study of the oceanic crust near the ridge has allowed the identification of distinct events:
i.Cretaceous Normal Superchron, C34
120-83 million years: Cretaceous (Aptien - Santonien)
ii. Superchron Kiaman
312-262 million years: end Carboniferous - beginning Permian
iii. Superchron Moyero
285-463 Ma in the Ordovician
Chrons, i.e. longer intervals were named after scientists Bruhnes, Matuyama, Gauss and Gilbert (marked 1 on the image), whereas subchrons, i.e. shorter intervals were given names of the places where they were discovered (marked 2 on the image).
Geomagnetic polarity time-scale. Black represents normal polarity,
white represents reversed polarity. Image courtesy: McDougall, 1979
white represents reversed polarity. Image courtesy: McDougall, 1979