The above cartoon shows how ocean crust is created at a
midocean ridge through volcanic eruption and how the Earth's magnetic field
is recorded
by the volcanic rock as it cools. The crust is transported laterally away
from the spreading center by the process of seafloor spreading (see Oceanus
article on paving the seafloor). The geomagnetic
field is a dynamic property of the
Earth varying on timescales from seconds to
millions of years. One of the more unusual properties of the
geomagnetic field is it's ability to reverse polarity, so that the magnetic
north pole is at the geographic south pole.
This reversal in polarity has happened quite often in the past and has been
recorded in the rocks of the ocean crust which give rise to variations in
our measurements of Earth's magnetic field called "magnetic anomalies". Scientific research on
magnetism here in the Geology
and Geophysics Department at Woods
Hole Oceanographic Institution centers on various aspects of how the
magnetism is recorded by the crust, what the magnetic recording can tell
us about past history of Earth's tectonic plates and how Earth's magnetic
field itself has varied in the past.