Answer:
The orientation of the magnetic field
Step-by-step explanation:
The earth's magnetic field is a region of magnetic force surrounding the earth. It is widely accepted that it originates from the motion of iron and nickel in the outer core.
The magnetic field of our earth has both North and South Pole. The North Pole is around the geographic north while the South Pole lies very close to the Geographic South.
From careful studies of sedimentary rocks and some igneous rocks, it has been observed that there has been changes in the magnetic field directions through geologic time. This change is based on the strength of the magnetic field.
We have a normal polarity when magnetic lines of force leaves the South and enters the North and it is reversed when the direction changes.
Some magnetic minerals like magnetite in rocks would align their magnetic domains as they crystallise out of magma to the prevailing magnetic field of the earth. When the Curie temperature is reached, the alignment stops and the domains have been aligned.
For a magma that crystallise when the polarity was normal, the magnetic minerals will bear the records. When the polarity reverses in the presence of crystallising magnetic minerals, the record is also taken.