published on 9 February 2021 in earth
At the origin of rocks: the secrets of paleomagnetism
Dear rock, tell me how your magnetic crystals are oriented and I’ll tell you when, under what conditions and in what part of the world you were formed. It’s not magic, but a subject known to geologists as paleomagnetism. Every rock, in fact, contains a veritable repository of information about its birth, history and evolution. But let’s try to understand in which language these databases are written and how we can translate and interpret them.
The Earth’s magnetic field
The outer core of the Earth, the one closest to the surface, more fluid, is characterised by the presence of convective currents of iron, nickel and other lighter elements. These currents generate a magnetic field – the Earth’s magnetic field – which can be considered as a dipole. Simplifying, the Earth’s magnetic field can be compared to that generated by a large magnet placed in the centre of the Earth, whose axis and – consequently – poles are in constant motion, with an inclination of about 11.5° with respect to the Earth’s axis of rotation.
The name that scientists have given to this dipole is GAD (Geocentric Axial Dipole) and conventionally its flux lines point to the North pole (see figure below). However, studying the magnetisation of the rocks that make up the ocean ridges, scientists have discovered that the Earth’s magnetic field has reversed its sign (normal polarity and reverse polarity) several times in the geologic history of the planet (with a frequency of between 0 and 4- 5 magnetic reversals every million years).
Minerals respond differently to the stresses of a magnetic field and have accordingly been divided into diamagnetic (such as quartz), paramagnetic (such as calcium) and ferromagnetic (as hematite) minerals. While the first two categories have a rather limited magnetisation intensity that returns to zero once the magnetic field is removed, ferromagnetic substances have the ability to “record” the direction of the magnetic field applied to them in their fabric. And it is precisely this latter feature – also known as permanent magnetisation – that makes them particularly attractive in the eyes of a paleomagnetologist.
When an external magnetic field (in our case, the Earth’s magnetic field) is applied to a ferromagnetic material, the magnetic units comprising it will gradually tend to align parallel to the field applied and, therefore, parallel to each other. In this way, the body in question (in our case, the rock) acquires a magnetisation which, in ferromagnetic substances, persists even after removal of the external magnetic field applied. When a rock is formed – for example from cooled magma or from sediments – the ferromagnetic minerals comprising it are statistically oriented according to the flux lines of the Earth’s magnetic field present at that time, “memorising” the direction and intensity. It follows that the magnetization of rocks can provide a series clues as to the conditions, place and date on which a rock was formed.
What do the magnetic crystals of rocks tell us?
One of the best known contributions of paleomagnetism to the history of science is related to the development of the theory of expansion of the ocean floor, the same that led to understand that the ridges that cut oceans in two draw the boundary between two different plates. Around the middle of the last century, scientists observed that the magnetic polarity (normal or reverse) of the rocks comprising the ocean floors varies symmetrically on either side of an ocean ridge. From this data, geologists speculated that the oceans expand precisely in correspondence with the ridges and that the emission of basalt and the formation of new oceanic crust takes place there. Having confirmed this hypothesis, the oceans have become veritable “registers” of reversals in the Earth’s magnetic field and, in a certain sense, calendars (Watch the video).
But that’s not all. Because the magnetic crystals, in addition to telling us if a rock was formed in a period of normal or reverse polarity, are also able to tell us – based on their orientation – the latitude at which the rocks were formed. Depending on the direction of the magnetic fabric of a rock, we are therefore able to reconstruct its movements in the North-South direction. This is valuable information, which has led to the refinement, along with other subjects, of our knowledge on the theory of continental drift and on the geological and climatic history of the planet.
Finally, paleomagnetism can provide us with vital information on the directions of the fluxes (magmatic, ignimbritic, turbiditic, etc.) that led to the formation of the rocks. The orientation of the magnetic fabric of a rock can in fact be determined by the action of the fluxes that act on the particles of a sediment or a magma before they become rock. This means that in the microscopic world of crystals, a kind of “autobiography” of the rock is preserved, written by the dynamic phenomena that formed it and which can be interpreted by studying the preferential orientation of ferromagnetic minerals.
Depending on the direction they assume, the magnetic crystals of rocks provide us with important clues concerning the conditions in which these rocks were formed. The geologist then has the daunting task of collecting them, putting them together and… interpreting them!
By Anna Pellizzone
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