2. THE REVERSING MAGNETIC FIELD OF THE EARTH
In molten lava, there are magnetite crystals which can move in any direction.

3. In the 1950s, scientists began to study the magnetism of lava flows
In the 1950s, scientists began to study the magnetism of lava flows. As expected, they found that many lava flows contained magnetite crystals that were lined up with the Earth’s magnetic field.
However, some lava flows contained magnetite crystals which were lined up in the opposite direction.

4. Using these observations, scientists inferred that the Earth’s magnetic field has reversed during the Earth’s past. Scientists estimate that the Earth’s magnetic field has reversed at least 170 times in the last 100 million years.
The magnetic poles of the Earth move slowly all the time. They change position by about 20 kilometres every year. But a magnetic field reversal is a large change.
In a magnetic field reversal, the magnetic poles ‘switch’ ends.

5. Scientists were able to work out the
dates for the magnetic field reversals
for the last ten million years.
This information is shown on
the diagram.