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Earth's Magnetic Field: Formation, Changes & Impact

Instructor: Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

Earth has a magnetic field surrounding it. In this lesson, we will go through what creates it, how it changes, and what impact the change has on our planet.

Earth's Magnetic Field

Did you know that what we call the North Pole is also near the South Pole? Confusing, but true. The North Pole is a geographic place, and the South Pole I am talking about is the Earth's southern magnetic pole. Opposite ends of magnets attract each other, and the north end (usually red) of a compass points north towards Earth's southern magnetic pole. In addition, the North Pole is in the Arctic Ocean and is aligned with the North Star. Earth's south magnetic pole is drifting around somewhere in Northern Canada.

Some might think that there is a giant bar magnet deep down in the Earth which isn't the case as far as we know, but it serves as an analogy in explaining the magnetic field lines. Let's learn more about Earth's magnetic field.


Analogy of what causes the magnetic field
magnetic_field

Formation

Studying earthquakes and seismic waves has led to the discovery that the Earth has different layers: the crust, mantle, outer and inner core. The two cores are the innermost layers, and the outer core is responsible for the formation of the Earth's magnetic field.

The outer core is predominantly composed of molten iron while the inner core is solid iron, and nickel. The intense mass of material above the inner core is responsible for its solid state. The pressure on the outer core isn't enough to make it solid. We know the Earth is spinning because we experience daytime and nighttime. The outer core spins too, but it may spin at a different rate the rest of the earth.

Also, radioactive decay adds to the extreme heat at this depth causing convection currents. Convection currents are all about density differences caused by heat differences. Think of see-through pot on the stove. Pour in water, some small, frozen vegetable cubes, and turn on the heat. When the water boils you see the vegetables cycling from the bottom of the pot to the top, and then sinking.

The movement of liquid iron in the outer iron core generates an electric current, which induces a magnetic field.

Changes in Magnetic Field

Evidence exists in the rocks formed at the Mid-Atlantic Ridge indicating that Earth's magnetic field reverses roughly 200,000 years, but it is not at constant intervals. The Mid-Atlantic Ridge is a divergent plate boundary where molten rock high in iron content reaches the surface and solidifies. The iron particles align with the Earth's magnetic field before the lava solidifies. The newly formed rocks get pushed away leaving room for more lava to solidify, and a banding pattern is formed. Some rocks indicate the North Magnetic Pole was the North Geographic Pole, and others show the North Magnetic Pole was the South Geographic Pole.

What causes this?


Mid-Atlantic Ridge
MOR

What causes these reversals is difficult to determine. We can't even drill through the earth's crust, so getting to the outer core is impossible.

Scientists have used a computer model of the Earth-driven dynamo (electricity generator). The model has gone through 500,000 years, and once in a while an instability arises causing sections of the outer core to reverse magnetic polarity. Physical models are also made in addition to computer models. Using these models in tandem might lead to insights on why instabilities occur.

It is theorized that when a lot of instabilities occur at the same time the effect is the Earth's magnetic field flips. As of 2014, scientists believe that magnetic north is moving towards Siberia. Overall, the Earth's magnetic field is decreasing by 5% a decade, which is 10 times faster than previously thought.

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