Re: How does paleomagnetism help support the Theory of Continental Drift?

The main contribution of paleomagnetism to support the plate tectonics
was the discovered of stripes of normal and reversed magnetization in
the ocean floor (normal means the same as we have today). This
pattern is symmetric if we take the meso-oceanic ridge as a reference.
The only way to obtain this configuration is if the seafloor is
spreading, and the Earth's magnetic field reverses from time to time
(typically, some millions of years), so the same amount of lava that
is erupted from one side is erupted from the other side of the ridge,
and this movement is continuous. This showed that the old continental
drift idea was not so wrong (in fact, plate tectonics is different
from continental drift, but the basic idea of Alfred Wegener in the
early 20th century was the foundation of this theory).

The other use of paleomagnetic data is to reconstruct plate motions
along the time. Suppose you can measure the direction of magnetization
in some lava flow in Africa, and other lava flow in South America,
both geologically the same. Then you can calculate where the virtual
geomagnetic pole was when these rocks were formed. If you know the
age of this volcanism (we can measure this with geochronological methods),
you can rotate one continent around this pole until the magnetization
vector coincides with the magnetization vector of the other continent
lava flow. So, you have the exact position of these continents at that
time. If you have rocks of several ages, you can do that for each
formation, and reconstruct the movement.

If you have any doubt, please feel free to contact me.

Eder C. Molina
eder@iag.usp.br
Dept. of Geophysics
Institute of Astronomy and Geophysics
University of Sao Paulo - BRAZIL