Rocks never forget!

Figure1. Elephant rock, Castelsardo, Sardinia, Italy (picture by Vid Pogačnik)

Did you know that some rocks can have an incredible “magnetic memory”? The age of rocks can vary from seconds to billions of years, and besides their sometimes very old age they store information that is useful to reconstruct the history of our Planet.

We commonly use the word “memory” referring to our computer storage capacity or our own ability to remember. Rocks store information, but unlike us they are able to do it over longer periods of time. The oldest memory we have is limited to what humankind experienced but some rocks are much older than humans. Therefore it is really important to be able to extract their memories in order to better understand what we didn’t experience ourselves.  

This “magnetic memory” relates to certain minerals in rocks (e.g. magnetite, hematite) able to record the direction and the intensity of the Earth’s magnetic field when they form.

Why is this information about the Earth’s magnetic field important to us?

When preserved, this information can be used to reconstruct the past location of tectonic plates, or more generally, to investigate the metamorphic and the geodynamic history of a rock. You can imagine our Planet as a big disco ball (figure 2) where every facet of this ball represents a rock. Every single one will have its own identification code which serves as reference for their position on the ball. If some of the facets are switched their identification code will allow you to relocate them at their original position. The Earth’s magnetic field will be the phenomenon that assigns the identifications codes. The identification code is what we want to measure.

Figure 2. Disco ball

What do we measure exactly?

We measure something called “natural remanent magnetization (NRM), which is the remaining magnetization of the rock in absence of any external field.

When a rock is in a magnetic field, based on its mineral composition, it can acquire an induced magnetization (proportional to the field strength and amount of magnetic minerals) and/or a remanent magnetization. The first one disappears as soon as the external magnetic field is removed, the second remains, and that is what we want to measure.

There are different types of remanent magnetizations based on the way the magnetization is acquired. One of the most stable over time is the Thermoremanent magnetization (TRM), and it is acquired by a rock during cooling through the Curie temperature of its magnetic minerals. This temperature is the critical temperature above which certain materials lose their permanent magnetic properties. In other words: if we melt a rock which has a certain NRM, the latter will be erased as soon as the Curie temperature of the minerals retaining the remanent magnetization is exceeded. A new magnetization will be acquired when the temperature again drops below the critical Curie temperature of those minerals, and this new information will be stored. Therefore direction and intensity of NRM of rocks thus acquired represent those of the geomagnetic field in situ at the time when magmatic rocks were formed.

Now imagine that our disco ball facets have changed their location on the ball multiple times, the identification code will be always the same. In rocks it is a bit more complicated because they usually save their original identification code (their primary remanent magnetization component) but sometimes they acquire new remanent magnetizations which add to the previous one, these are called secondary magnetization components. The present-day NRM is indeed generally the resultant of different remanent magnetization components acquired at various times during its geologic history.

By investigating these different components it is possible to extract information about the tectonic and the geodynamic history of the rock. However, not all rocks are suitable for these studies. Sometimes rocks have a very complicated history and it is difficult to distinguish between the different magnetization components.

How do we measure the NRM and how do we investigate it?

In my opinion, one of the most interesting applications of paleomagnetic studies relates to the possibility of defining the original emplacement location of a rock. Our planet is very dynamic and plate tectonics is one of the main types of evidence for this dynamic process. Imagine dealing with an old rock that has been moved after its formation over the globe, you would be able to reconstruct its movement.

So… how do we get this information? How can we reconstruct the migration of the plates and not less importantly, what are the limitations?

Let’s go through the main steps needed to get this information out of our rock. For this purpose we will use a specimen of harzburgite from the Late Cambrian Leka Ophiolite (see Barbara’s post) complex in Norway and we will see step by step what information is stored in it.

Step 1_What is the current location of our rock?

In order to answer this question we need to know our rock specimen location on the globe and we need to retrieve the natural system orientation of the rock sample with accuracy. This is the most important step as all the following steps depend on the orientation of the samples. A wrong orientation of the sample will mean a wrong NRM direction.

Step 2_Measurement of the natural remanent magnetization (NRM) of the rock specimen

Figure 3. Spinner magnetometer AGICO JR_6A, NTNU paleomagnetism laboratory in Trondheim (Norway)

The next step involves measuring the NRM of our specimen; this magnetization will be the resultant of possibly several remanent magnetizations. In order to do it we can use an instrument called a magnetometer (figure 3). The instrument is able to measure the resultant of these several remanent magnetizations components, and during the measurements both specimen and sensor are shielded against external magnetic fields to prevent the presence of induced magnetization and noise. As an output, the spinner magnetometer gives intensity and direction of the NRM; the direction is given in terms of inclination, (with respect to horizontal at the collecting location) and declination (with respect to geographic north).




Step 3_ Sample demagnetization

Once we have the NRM we can start investigating the stability of this magnetization as function of an externally applied field or of temperature. In order to do this we demagnetize our specimen in steps and we measure the remanent magnetization after each step. The demagnetization can be done in two ways: applying an increasing magnetic field to the sample or heating up to the Curie temperature of the magnetic minerals within the sample with incremental steps. For our specimen we will use the first approach. The sample is placed in an increasingly strong alternating magnetic field through the Alternating Field Demagnetizer (figure 4a), and its remanent magnetization is measured after each step with the magnetometer. The sample’s remanent magnetization is gradually “erased” starting from the weakest, to the strongest components. When all the components are erased the sample will have only an induced magnetization component and the remanent magnetization will be zero. Figure 4b shows the decay of the intensity of the magnetization after each step. The intensity is normalized to the maximum magnetization which is the starting NRM intensity of our sample and decreases from 1 to 0.

Figure 4. a) Alternating Field Demagnetizer LDA_5 (NTNU paleomagnetism laboratory in Trondheim); b) Decay of the Intensity of the magnetization after each step

Step 4_Data interpretation

It is time to start thinking about the meaning of our results! The results are from a single specimen; this is meaningless from a scientific point of view, however it remains a good exercise to understand how investigate the paleomagnetic history of a rock.

The magnetization is defined in a three-dimensional space, and in order to better visualize demagnetization data, paleomagnetists commonly use the so called Zijderveld diagrams. In addition to the intensity, these diagrams show the direction of the remaining magnetization measured after each demagnetization step. Figure 5 shows the Zijderveld diagram for our specimen; two segmented lines are visible in the plot, representing the projection after each step of the magnetization on two orthogonal planes.

The two segmented lines are straight, meaning that the direction of the magnetization is the same after each step. When more components are present these lines appear segmented in different straight lines with different directions, indicating that there is at least one event in which our sample has experienced an overprinting or a secondary magnetization. This information can be used to separate the primary from the secondary magnetization, and to estimate inclination and declination of the primary magnetization.

If our single specimen magnetization direction was measured in many samples (hence the result would be statistically meaningful), how would we use this information for reconstructing the specimen migration? In other words, where were our samples on the Earth when they acquired the primary magnetization?

Since the inclination of the magnetic field depends on the latitude, rocks magnetized in the same time but at different latitudes have different magnetic directions. It is often preferred to calculate the North magnetic pole position when the rock was magnetized because this is the same for all the rocks of the same age; this position is given by the declination. The inclination can be used instead to determine the ancient latitude of our sample though a simple relation:

tan I = 2 tan λ     

where λ is latitude and I is the Inclination

The combination of the declination with the paleo-latitude is then used to calculate the “apparent pole position”, and consequently the place where the rock was formed with respect to its present-day position.

For our sample from the ophiolite in Leka (Norway) the “apparent pole” shows a different position with respect to the present-day pole, and therefore the original emplacement location was different as shown in figure 5(b).

Figure 5. a) Zijderveld diagram; numbers are for AF steps; b) G represents the actual location of a specimen from the Leka ophiolite complex, P shows instead the past location after calculating the paleopole

Such studies can be really powerful to investigate the dynamics of our Planet. For example, paleolatitudes are used to study past climate, which is strongly dependent on the latitude and directly connected to life development. These ancient rocks can be our eyes in the past and their memory is a precious treasure for Science. However, rocks’ magnetic memory can be overprinted, even in our human time scale! Think of walking with a magnet close to a rock, or of lightning striking (figure 6) … some information can be erased, scrambled or added (chemical alteration can also results in the generation of other magnetic minerals) to their memory!

Figure 6. Cartoon of a rock hit by a lightning strike

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