Global Magnetic Field

Ocean Floor Magnetism

New electronic magnetometers developed in the 1950s allowed scientists (who were looking for oil) to map the magnetism of rocks. On land the pattern seemed jumbled, with no meaningful order. But when they mapped the ocean floor, the magnetism was in long strips running parallel to the mid ocean ridges (discovered by Harry Hess). Further, the magnetic strips were the same on both sides spreading out from the ridge. The sea-floor was giant tape-recording of the Earth's global magnetic field - every half million years or so, the Earth's magnetic poles reverse. Nobody knows why. But the last reversal was more than 800,000 years ago.

How do rocks become magnetised?

When molten rock cools and turns solid, the iron crystals will align themselves with the direction of the Earth's magnetic field, becoming weakly magnetised. Many rocks contain iron (Fe) particularly basalt. Do you remember the composition of oceanic crust? Learn more here.

Magnetic pole reversals

Earth's magnetic field is generated by convection currents in the liquid outer core (which is a good conductor of electricity). This is called dynamo theory and is not completely understood, but recently scientists have modelled what happens to the magnetic field during a pole reversal.

While it's extremely difficult to model the dynamo forces inside the Earth's core, we can demonstrate the relationship between electricity and magnetism and force (motion). The first electrical motor was built in 1821 by Michael Faraday, known as an homopolar motor. While the activities are simple, the explanations of why they work are not - you'll learn more in year 12 physics.

TASK 19: Make a homopolar motor using a battery, electrical wire, neodymium magnet and a screw. Describe the process and what you observed. You can find instructions here, and here (and some extra motor designs using copper wire).

Protection from the solar wind and atmosphere stripping

The solar wind streams off the Sun in all directions as a high-speed stream of charged particles (mostly protons and electrons).

The global magnetic field deflects most of the dangerous radiation away from the Earth's surface, and prevents the atmosphere from been blown away. Without magnetic fields, Venus and Mars's atmospheres are constantly being stripped away into space.

The solar wind is responsible for blowing tails away from comets as they pass through our solar system, and the auroras at the North and South poles.

What happened on Mars?

Watch the video Mars Magnetic Field: Where did it go?

Next we'll look at atmospheres, where they came from and how they can disappear.