However, the geomagnetic field reversal mentioned above is characterized by an asymmetry, manifested in natural magnetization recorded by Keweenawan rocks that crop out around the Lake Superior (e.g., Palmer, 1970; Halls and Pesonen, 1982; Pesonen and Halls, 1983; Schmidt and Williams, 2003).Most but not all of the reversely magnetized (“reversed”) lava flows and dikes of this age consistently have characteristic directions of magnetization that are about 20 to 40 degrees steeper in inclination than their normally magnetized (“normal”) equivalents, while declinations show the expected 180 degree relationship.
Because the ocean floor is mostly composed of basalt, an iron-rich substance containing minerals that align with the magnetic field, they record the alignment of the magnetic fields surrounding oceanic ridges.Based on magnetic records, we know the last magnetic pole shift occurred 781,000 years ago.Earth’s magnetic field also fluctuates in strength every once in a while due to changes in temperature and convection currents at the core.Scientists studied the magnetic signatures of the rocks on the ocean floor and noticed some recorded opposite directions for magnetic field lines even though they were side by side.This likely occurred because magma rose from the ridges in the ocean floor and formed new rock recording a more recent alignment of the magnetic field while pushing old rock with more outdated magnetic records further from the ridge.the normal and reversed field directions are exactly anti-parallel).
The above is the fundamental assumption used to reconstruct continents to their past positions using the ancient magnetic field recorded in rocks (fossil magnetism).
It’s called rock magnetism when rocks record the position of the magnetic field.
The magnetic signature of the rocks allows paleomagnetists to date the rocks and map the position of the field at the time of their formation.
Piecing together the history of Earth’s magnetic field helps us predict its future behavior.
We know from records preserved in rock that Earth’s magnetic field has flipped and reversed in the past.
A more detailed record of Earth’s magnetic past will provide answers regarding the formation of the continents as well as the behavior of our magnetic field.