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Rock Magnetism

© Geoff Habiger

The 1960's are arguably the most important decade in the geological sciences. While geological advances and discoveries have been made since the earliest days of civilization, the 1960's saw a culmination of all the previous work. The development of the unifying theory of geology, plate tectonics, was presented in the 1960's and revolutionized the science. Plate tectonic theory was first present in a paper in the British journal Nature in 1965 by J. Tuzo Wilson shows that the Earth is made up of multiple lithospheric plates. These plates move across the planet, forming at spreading centers and colliding together, and can explain most of the geological features seen on the planet.

Plate tectonics depended heavily on a new branch of geology that developed after World War 2, magnetostratigraphy. Magnetostratigraphy is a branch of stratigraphy where rocks of different ages can be correlated together by means of similar magnetic patterns. Stratigraphers, scientists who measure and map the geological record, can use magnetostratigraphy to correlate rocks that occur worldwide and that are independent of the rock type, so an igneous rock can be compared to a sedimentary rock. No other correlation tool available to stratigraphers can do this, making magnetostratigraphy an important tool for geologists.

But surely all the magnetic patterns are the same, right? If you take out a compass, the needle will point toward north, aligning itself with the Earth's magnetic field. That is true today, but several times in the geological past (at least 10 times in the past 5 million years) the Earth's magnetic field has flipped so that magnetic north was at the Earth's south pole. These magnetic flips are called reversals. When studying the rocks a stratigrapher can determine if the rock has a normal polarity (magnetic north lines up with the north pole) or reversed polarity (magnetic north lines up with the south pole). These reversals are nonperiodic and irregular making them useful correlation tools. However, magnetic reversals themselves cannot be used to correlate different rock types and must be used in conjunction with other geological dating techniques such as biostratigraphy and radiometric dating. This is the drawback of magnetostratigraphic data.








The stratigrapher can also use the magnetic minerals to determine where magnetic north is located. If you look at any world map today you will see a notation for both the North Pole and the Magnetic North Pole. Today, magnetic north does not line up with true north (the north pole) but is actually located near the western edge of Queen Elizabeth Islands in Canada. Using magnetostratigraphy geologists have shown that in addition to flipping, the Earth's magnetic poles move through time in a process called polar wandering.

Magnetic TIme Scale

Location of geomagnetic pole.

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