Geology Reference
In-Depth Information
4
The Geomagnetic Field
Abstract
The Earth's magnetic field represents the ultimate cause of rock
magnetization. Here I describe the main internal source, or geodynamo,
associated with convective currents in the external core, as well as the
crustal field, generated by magnetized rocks, and the external magneto-
spheric and ionospheric sources. The concepts discussed in this chapter
are essential for the correct use of magnetic data in plate kinematics. In
particular, it is considered the secular variation of the core field, which rep-
resents the ultimate cause of most difficulties in collecting and analysing
paleomagnetic data on continents. Furthermore, the concepts of polarity
inversion and chron are introduced, along with the techniques used to build
geomagnetic timescales. These topics are particularly important for the
analysis of marine magnetic anomalies, a major source of data in plate
kinematics. Finally, this chapter introduces the potential field technique,
which represents the fundamental mathematical tool for describing the
Earth's magnetic field.
first realistic models about the
origin
of the
field. Joseph Larmor first proposed in 1919 that
the geomagnetic field originates from electric
currents associated with fluid motions in the
Earth's core. Today we know that the observed
magnetic field at the Earth's surface (or close
to it) is mainly produced within the
external
core, but also includes a small component of
crustal origin and an even smaller “external”
component, whose source is in the Earth's
ionosphere (between 85 and 600 km altitude)
and in the magnetosphere (a region that extends
beyond the atmosphere). On average, the field
that originates from magnetization of crustal
rocks is two orders of magnitude lower than
4.1
Source of the Main
Geomagnetic Field
The first systematic studies on the Earth's
magnetic field go back to the beginning of
the 1800s. Gauss, von Humbold, and Weber
in Germany, and Arago in France, were the
pioneer of the modern techniques of observation
of the Earth's magnetic field time variations
(Courtillot and Le Mouël
1988
). In addition,
Gauss introduced a new theoretical method
to represent the field in terms of
spherical
harmonics
, a technique that is still in use.
However, another century occurred to have the
