Table

Some Common Types of Remanent Magnetization (RM) in Rocks

Acronym Type of Magnetization

Description

NRM Natural Remanent

Magnetization TRM Thermoremanent Magnetization

CRM Crystallization (or Chemical)

Remanent Magnetization DRM Detrital (or Depositional) Remanent Magnetization PDRM Post-Depositional Remanent Magnetization

IRM Isothermal Remanent Magnetization

VRM Viscous Remanent

Magnetization ARM Anhysteretic Remanent Magnetization

The RM acquired by a sample under natural conditions

The RM acquired by a sample during cooling from a temperature above the Curie temperature in an external field (usually in a weak field such as that of the Earth) The RM acquired by a sample during a phase change or formation of a new magnetic mineral in an external field The RM acquired by sediments when grains settle in water in the presence of an external field The RM acquired by the magnetic alignment of sedimentary grains after deposition but before final compaction

The RM acquired in a short time at one temperature (usually room temperature) in a external field (usually strong)

The RM acquired over a long time in a weak external field

The RM acquired when an alternating magnetic field is decreased from some large value to zero in the presence of a weak steady field

In nature isothermal remanent magnetization (IRM) usually refers to that magnetization acquired by rocks from lightning strikes, although it generally refers to that acquired in laboratory experiments aimed at determining the magnetic properties of samples (see §2.1.4 and §3.5.2). Viscous remanent magnetization (VRM) refers to the remanence acquired by rocks after exposure to a weak external magnetic field for a long time. Examples include that acquired by a sample after collection and before measurement or that acquired from deep burial and uplift (see §2.3.8). Anhysteretic remanent magnetization (ARM) is that produced by gradually reducing a strong alternating magnetic field in the presence of a weak steady magnetic field. To avoid heating samples (see §3.5.3), it is often used in laboratory experiments as an analog of TRM.

The component of NRM acquired when the rock was formed is termed the primary magnetization; this may represent all, part, or none of the total NRM. Subsequent to formation the primary magnetization may decay either partly or wholly and additional components may be added by several processes. These subsequent magnetizations are referred to as secondary magnetization. A major task in all paleomagnetic investigations is to identify and separate all the components be they primary or secondary.

1.2.3 Geocentric Axial Dipole Hypothesis

On the geological time scale the study of the geomagnetic field requires some model for use in analyzing paleomagnetic results, so that measurements from different parts of the world can be compared. The model should reflect the long-term behavior of the field rather than its more detailed short-term behavior. The model used is termed the geocentric axial dipole (GAD) field and its use in paleomagnetism is essentially an application of the principle of uniformitarianism. It is known from paleomagnetic measurements (see §6.3) that when averaged over a sufficient time interval the Earth's magnetic field for the past few million years has conformed with this model. However, there are second-order effects that cause departures from the model of no more than about 5%. Such an averaged field is referred to as the time-averaged paleomagnetic field. A basic problem that arises is to decide how much time is needed for the averaging process. In the early days of paleomagnetism it was generally thought that times of several thousands of years were sufficient, but it is now thought that much longer times may be required, possibly on, the scale of hundreds of thousands of years (see discussion in Merrill et al., 1996).

The GAD model is a simple one (Fig. 1.10) in which the geomagnetic and geographic axes coincide, as do the geomagnetic and geographic equators. For any point on the Earth's surface, the geomagnetic latitude A. equals the

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