Magnetization and Magnetic Minerals - Quantitative Plate Tectonics - page 80

3

Magnetization and Magnetic Minerals

Abstract
This is the first of four chapters devoted to the techniques for collecting and
analysing kinematic data. The fundamental source of data for determining
past plate motions is furnished by rock magnetization. In this chapter,
I describe the different classes of rock magnetism, the main magnetic
minerals, and the origin of ferromagnetism.

dV ,havingbase dS and height equal to the dis-
tance h travelled by the charge in the direction n .
Clearly, h D ( v n ) dt . Then:

3.1 Electric Currents

An electric current is a material flow of electric
charges. These charges may be single particles
(e.g., electrons) that move through a medium or
in the empty space or, in a continuous mechanics
representation (see Sect. 2.1 ) , we can think about
an electric current as a flow of volume elements
dV having electric charge. Given an arbitrary
surface S in the physical space, the current I ( S )
across this surface is defined as the charge q that
walks through S in the unit time:

dq D ¡dV D ¡. v n/dtdS

D ¡ v dSdt j dSdt (3.2)

Here, the scalar quantity ¡ and the vector
j D ¡ v are respectively the density of charge and
the density of current . Combining this expression
with ( 3.1 ), we have:

Z

I.S/ D j dS

(3.3)

dq
dt

I.S/ D

(3.1)

S

Let us consider an infinitesimal surface ele-
ment of S . This quantity can be represented by
a vector d S D n dS , whose versor n is normal
to the surface, and having magnitude dS equal
to the infinitesimal area of the surface element
(Fig. 3.1 ).
The charge dq crossing d S in an infinitesimal
time interval dt coincides with the electric charge
that originally was contained in a small cylinder

The electric charge is expressed in Coulomb
[C], while currents are measured in Ampere
[1A D 1 ı Cs 1 ]. Magnetic fields are force fields
generated by charges in motion and that exert a
force only on moving charges. The most simple
example is represented by the field generated by
a flow of electrons through a copper cable. If
I is the current through the conductor, the law
of Biot-Savart allows to determine the magnetic