What is fossil magnetism?

Answer 1

In hot Iron bearing rocks or magmas the iron minerals they contain align themselves parallel to the Earth's magnetic field. When they cool below about 500 degrees Celsius the the iron minerals 'freeze' in this magnetic orientation and are no longer able to adapt to any changes in the Earth's field (which occasionally reverses polarity and wanders). This frozen magnetic alignment is a fossil of the ancient magnetic field and can be 'read' to find out what the Earth's magnetic field has been doing in the past.

Answer 2

It refers to molten rock which aligned with the then-current magnetic field of Earth and then solidified, "locking in" that orientation. This is how scientists know that the Earth's magnetic field has switched directions before (we don't really know why, though).

Answer 3

Fossil magnetism refers to the natural magnetization that certain rocks acquire when they are formed. It occurs when minerals within a rock align themselves with the Earth's magnetic field at the time of the rock's formation, creating a record of the Earth's past magnetic field. By studying this fossil magnetism, scientists can gain insight into the history of the Earth's magnetic field and the movement of tectonic plates over time.