Geology Reference
In-Depth Information
Flip-flopping magnetic poles: Paleomagnetism
and seafloor spreading
To explain how scientists date rocks on the seafloor, I first need to describe an import-
ant characteristic of the earth: magnetism. You are familiar with the earth's magnetism if
you have ever used a compass. The needle in a compass points to magnetic north (or
south if you are in the southern hemisphere).
However, earth's magnetic poles do not remain in the same position all the time. Some-
times the position of the magnetic poles (which right now are very close to the geo-
graphic poles) shifts. The movement of the magnetic poles is called
polar wandering.
Earth's magnetic poles also occasionally switch so that the magnetism recorded from
the current geographic North Pole comes from the geographic South Pole instead. This
phenomenon is called a
magnetic reversal.
Paleomagnetism
is a record of how the earth's magnetic poles have changed
direction over time. Minerals such as magnetite (made of iron) in rocks are
aligned toward the poles when the rock is formed and record a history of these
changes. Rocks formed with their minerals aligned to indicate the same magnet-
ism as the present have
normal polarity,
while rocks created during a reversal
period, with minerals aligned in the opposite direction, have
reversed polarity.
These patterns of polarity are recorded in the minerals of basalt rocks (see Chapter 7)
created along the Mid-Atlantic Ridge of the seafloor (and in basalt formed during volcan-
ic eruptions all over the world). By combining all this data with other methods of geolo-
gic dating (explained in Chapter 16), scientists can create a magnetic time scale for the
earth.
Scientists have mapped parallel patterns of magnetic reversals along each side of the
Mid-Atlantic Ridge. Dates for the rocks indicate that rocks collected from the ridge itself
are younger than rocks located farther from the ridge. The relative ages of the seafloor
rocks are illustrated in Figure 8-7.
Sharks and the earth's magnetism
Researchers have discovered that sharks use the earth's magnetism to help guide them through the
oceans. In the front of their heads, sharks have small electromagnetic sensors that detect the strong
north-south magnetic currents of the earth. These same sensors also pick up the much weaker electro-
