A material that obtains a record of the Earth's magnetic field through the application of heat is said to hold a thermo-remanent magnetisation (TRM) signal.
The dated event is the last time the feature was heated and cooled over approximately 400°C. Image description: If the material is heated, but the temperature does not exceed approximately 400°C, some magnetic particles will realign with the Earth's magnetic field but the more stable particles will retain their original position.
Most dating applications use the direction of the magnetic field to provide a date.
The material must therefore remain in the same position as it was when it was fired or deposited.
This process locks in the orientation of the magnetic particles within the sediment and is the basis of the DRM signal.
The types of deposits that record a DRM signal includes sediments found in lakes, ponds, within ditches and pits.
Lower temperature reheating may also be detectable in some situations.
As the material cools the magnetic field recorded within the material is fixed, thereby locking in a record of the location of magnetic north at that time.[top] A material must contain magnetised particles of the right type and in sufficient quantities for a material to record the Earth's magnetic field.This includes most materials of geological origin, such as rocks and clay.[top] When a magnetic particle is suspended in water with a slow rate of flow, it is free to align with magnetic north and is referred to as depositional remanent magnetisation (DRM).As the particles settle out of the water they are deposited in layers; the weight of the sediment accumulating on top squeezes the water out of the material so that the magnetic particles are no longer free to move.