What is the Source of the Earth’s Magnetic Field?
The natural magnetic field is generated by the Earth’s electric-dynamo system. It is
believed that 99 % of the Earth’s magnetism and its energy is the result of the fluid
motions in the Earth’s core. The molten nickel and iron spread spinning in the
outer core are capable of generating electrical currents where the mechanical
energy is transformed into electromagnetic energy. This is the same as in a car
battery where the generator transforms mechanical energy into electricity, and this
is called the dynamo effect. A switch of polarity in a dynamo will flip the direction
of the current. This also happens in the interior of our planet and it is called the
reversal of the magnetic field when the poles change polarity from north to south.
We do not know why the field changes its polarity. The last switch of polarity
occurred about 780,000 years ago and it is not known when the next one will take
place. The flip of the magnetic field could be due to the change in direction of the
convection currents moving the molten Fe–Ni-enriched matter in the Earth’s core
(Olsen and Mandea 2008).
The first striking evidence of sea floor spreading was observed when analyzing
the magnetic data obtained by magnetic sensors (called magnetometers) towed by
the a research vessel in order to measure the fields of magnetic intensity on the sea
floor. Vine and Matthews published this finding in 1963. Their paper talked about
the existence of long stripes of normal and reversed magnetization running parallel
to each other on each side of the spreading ridge, like a mirror image. Molten
material containing magnetite and titanomagnetite had magnetized and upon
solidification these elements pointed towards the direction of the Earth’s magnetic
field. This observation on the magnetism that was seen on the mirror image stripes
is a way of showing that the solidified material is moving away from the axis of the
ridge as more magma is injected to take its place. The magnetized formations
reported by Vine and Matthews consist essentially of dykes and basaltic flows. It is
believed that the Earth’s magnetic field reverses itself (the North Pole becomes the
South Pole and vice versa) about once every one million years. Therefore,
the width of the stripes of magnetized anomalies will depend on the extent of the
magmatic material injected into the crust and will be comparable to the rate of
lithospheric opening (Harrison 1968).
References
Bell DR, Rossman GR (1992) Water in earth’s mantle: the role of nominally anhydrous minerals.
Science 255(5050):1391–1397
Ben-Avraham Z, Nur A, Jones D, Cox A (1981) Continental accretion: from oceanic plateau to
alloctonous terrains. Science 213:47–54
Boyet M, Carlson RW (2005) 142Nd evidence for early ([4.53 billion year) global differentiation
of the silicate earth. Science 309:576–581
Broecker WS (1985) How to build a habitable planet. ELGEO Press, New York, 291 p
What is the Source of the Earth’s Magnetic Field?
49
The natural magnetic field is generated by the Earth’s electric-dynamo system. It is
believed that 99 % of the Earth’s magnetism and its energy is the result of the fluid
motions in the Earth’s core. The molten nickel and iron spread spinning in the
outer core are capable of generating electrical currents where the mechanical
energy is transformed into electromagnetic energy. This is the same as in a car
battery where the generator transforms mechanical energy into electricity, and this
is called the dynamo effect. A switch of polarity in a dynamo will flip the direction
of the current. This also happens in the interior of our planet and it is called the
reversal of the magnetic field when the poles change polarity from north to south.
We do not know why the field changes its polarity. The last switch of polarity
occurred about 780,000 years ago and it is not known when the next one will take
place. The flip of the magnetic field could be due to the change in direction of the
convection currents moving the molten Fe–Ni-enriched matter in the Earth’s core
(Olsen and Mandea 2008).
The first striking evidence of sea floor spreading was observed when analyzing
the magnetic data obtained by magnetic sensors (called magnetometers) towed by
the a research vessel in order to measure the fields of magnetic intensity on the sea
floor. Vine and Matthews published this finding in 1963. Their paper talked about
the existence of long stripes of normal and reversed magnetization running parallel
to each other on each side of the spreading ridge, like a mirror image. Molten
material containing magnetite and titanomagnetite had magnetized and upon
solidification these elements pointed towards the direction of the Earth’s magnetic
field. This observation on the magnetism that was seen on the mirror image stripes
is a way of showing that the solidified material is moving away from the axis of the
ridge as more magma is injected to take its place. The magnetized formations
reported by Vine and Matthews consist essentially of dykes and basaltic flows. It is
believed that the Earth’s magnetic field reverses itself (the North Pole becomes the
South Pole and vice versa) about once every one million years. Therefore,
the width of the stripes of magnetized anomalies will depend on the extent of the
magmatic material injected into the crust and will be comparable to the rate of
lithospheric opening (Harrison 1968).
References
Bell DR, Rossman GR (1992) Water in earth’s mantle: the role of nominally anhydrous minerals.
Science 255(5050):1391–1397
Ben-Avraham Z, Nur A, Jones D, Cox A (1981) Continental accretion: from oceanic plateau to
alloctonous terrains. Science 213:47–54
Boyet M, Carlson RW (2005) 142Nd evidence for early ([4.53 billion year) global differentiation
of the silicate earth. Science 309:576–581
Broecker WS (1985) How to build a habitable planet. ELGEO Press, New York, 291 p
What is the Source of the Earth’s Magnetic Field?
49