# How do rocks store information about Earth's magnetic field?

When a particular rock is being formed in a rift (by the solidification of magma, which is composed of ferromagnetic materials), the rock "stores information" about the Earth's magnetic field at the time of its formaiton.

My textbooks talk about the materials becoming "aligned with the magnetic field" while they're being formed, if the magma reaches a certain temperature, and that that information is "stored" in the rock, but it doesn't really explain what they mean by "alignment".

I've read somewhere that this is related to electron spin, but, from a physics standpoint, how does this happen? How does the electron spin change during cooling and why is it required for the materials to be at a specific temperature for the informaiton to be stored?

The field direction is stored in the magnetization direction of ferromagnetic minerals in the bedrock when it cools down. Those minerals are typically iron oxides like magnetite Fe$$_3$$O$$_4$$ etc or iron sulphides like pyrrhotite Fe$$_7$$S$$_8$$, with admixtures of other elements.

Such minerals are often not really hard ferromagnets, but their coercive field is large enough that the magnetization does not follow a switch of the Earth's magnetic poles.

One can do similar things with pottery or with the magnetization near hearths in archeological digs. This can give an indication of the direction and strength of the magnetic field in recent times, at many sites.

The critical temperature is the Curie temperature of the material. Above that, there is no ferromagnetic magnetization, the material is in a paramagnetic state, where the spin moments do not have long-range order. When the material cools below the Curie temperature, magnetic order will appear with a magnetization direction dependent on the external field.

• I think you're missing a critical final sentence, something along the lines of When the material cools below the Curie temperature, magnetic order will appear with a magnetization direction dependent on the external field. If the temperature remains below the Curie temperature, that alignment will be permanent, even if the external field changes direction. Jan 25 '20 at 15:05
• @DavidHammen I had mentioned that the coercivity was large enough to keep the alignment.
– user137289
Jan 26 '20 at 0:07
• but how does electron spin come into this? and what exactly do you mean by "magnetization direction"
– Rye
Jan 28 '20 at 20:08
• @Rye The iron ions have localized moments. For example, the Fe(III) ions have five $3d$ electrons with parallel spins, so a magnetic moment of five Bohr magnetons. Below the Curie temperature, the moments on different iron ions align into long-rage order, in a certain direction that depends on the external field. (I disregard the complication of ferrimagnetism here.)
– user137289
Jan 28 '20 at 20:23
• Coercivity is the field required to reduce a magnetization to zero. In small minerals, it is determined by magnetic anisotropy, and that is also what keeps the field aligned. See my answer for more details. Jan 30 '20 at 18:55

The answer by Pieter is a start, but it’s incomplete. Magnetization is a local average of the spins, and it can vary from point to point. In larger minerals, it is organized into regions called domains. Inside each domain, the magnetization can be in one of 2-8 possible directions (easy directions) that are determined by anisotropy in the mineral’s magnetic properties. In minerals below about 0.1 microns in diameter, there is only one domain. If an applied field is not aligned with an easy axis, the magnetization rotates towards it; but when the field is removed, the magnetization rotates back to an easy direction. In general, this is not in the direction that the field was in. Only the sum of all the magnetic moments is in the direction of the field.

Without the anisotropy, there is no magnetization in zero field (what paleomagnetists call remanence). However, even with anisotropy, thermal fluctuations in the magnetization can lead to jumps between remanent states. Just below the Curie temperature, jumps occur too frequently to allow a remanence to establish itself, and the minerals are superparamagnetic (like paramagnetic, but with electron spins replaced by moments of entire crystals). The only thing stopping them from jumping from one state to another is the energy barrier in between. This is determined by the anisotropy and the size of the particle. As the temperature decreases to room temperature, there may come a point where the barrier is too large for jumps to occur at an appreciable rate, and the remanence is blocked. The blocking temperature is different for each mineral, and the smallest minerals remain superparamagnetic at room temperature. Because of the thermal fluctuations above the blocking temperature, there is some probability of the magnetization ending up in any one of the easy directions. Thus, this kind of remanence (known as thermoremanent magnetization) is a statistical average of magnetizations that can be in all directions. That is what is really meant by "aligned with the magnetic field".