This dynamo, how-ever, changes easily, and for unknown reasons the magnetic field periodically reverses itself – the north and south magnetic poles switch positions.. On average, the mag
Trang 1S G Lucas, New Mexico Museum of Natural History,
Albuquerque, NM, USA
ß 2005, Elsevier Ltd.All Rights Reserved.
Introduction
Most of the dense core of the Earth is iron The outer
portion of the core is liquid, and the motion of this
liquid produces a magnetic field, so that the Earth
behaves like a giant bar magnet This dynamo,
how-ever, changes easily, and for unknown reasons the
magnetic field periodically reverses itself – the north
and south magnetic poles switch positions
On average, the magnetic field reverses itself about
every 500 000 years, though the pattern of reversals is
erratic Flip-flops of the Earth’s magnetic field, when
recorded in a stratigraphical succession of rocks, are
the basis of magnetostratigraphy (a contraction of
‘magnetic-polarity stratigraphy’)
Magnetostratigraphy correlates rocks on the basis
of similarities in their magnetic-reversal patterns and
is generally used to correlate surface exposures of
rocks, though it can also be applied to subsurface
cores As explained below, magnetostratigraphy is
not an independent method of correlating rocks
Nevertheless, it is a powerful tool because
magnetos-tratigraphical correlation is based on matching
mag-netic reversals, which are geologically simultaneous
events worldwide
The Geomagnetic Polarity Time-Scale
Magnetic reversals have occurred frequently but
ir-regularly during Earth history The process of reversal
seems to take about 4000–5000 years The current
state of the magnetic field (in which a compass needle
points towards the north magnetic pole) has persisted
for the last 700 000 years and is referred to as an
interval of normal polarity Geologists refer to periods
when the poles had switched positions (so that a
compass needle would have pointed to the south
magnetic pole) as intervals of reversed polarity
The first attempts at magnetostratigraphy were
made in the 1950s, especially by the Russian scientist
A N Khramov Since the 1960s, geologists have
made a concentrated effort to decipher the history
of the Earth’s magnetic field, and this research is
ongoing
During much of Earth history, the magnetic field
reversed frequently (Figure 1) This has been the case
throughout most of the Mesozoic and Cenozoic, but
during the Late Carboniferous and most of the Permian, an interval of about 70 Ma, the magnetic field was stable (reversed) The pre-Carboniferous nature of the magnetic field is still not as well understood as its later history
Because of plate tectonics and the subduction of oceanic crust, the oldest seafloor preserved on Earth dates from the beginning of the Late Jurassic, about
160 Ma ago Geologists have determined the mag-netic polarities of rocks from the seafloor, which are lavas for which some numerical ages have been calcu-lated Bands of cooled lava on the seafloor adjacent to spreading ridges preserve magnetic stripes that are symmetrical about the ridge This seafloor magnet-ization provides a template that geologists have used
Figure 1 The polarity bias superchrons during the last 700 Ma The polarity history is not well understood before about 350 Ma, and since then it has been mixed (many magnetic reversals) except for two long intervals of polarity stability: the Permo Carboniferous reversed and the Cretaceous normal superchrons.
MAGNETOSTRATIGRAPHY 331