Download How do we know the age of the seafloor?

How do we know the age of the seafloor?
Scientists can determine the age of the seafloor thanks to the changing magnetic field of our planet.
Every once in a while, the currents in the liquid core, which create the Earth’s magnetic field,
reverse themselves: it is called a geomagnetic reversal. This has happened many times throughout
Earth's history.
When scientists studied the magnetic properties of the seafloor, they discovered normal and
reversed magnetic stripes with different widths. These magnetic patterns are parallel to the midocean ridges and symmetrical on both sides. As rocks crystallize from lava at the ridges, they
literally record the magnetic field of the Earth at the time of their creation.
These stripes of normal and reverse magnetic fields with different sizes can be matched with the
geomagnetic reversals records obtained from continental rocks already dated: this is how
scientists get the age of the seafloor.
How to form m agnetic striping: new oceanic crust form s continuously at the m id-ocean ridges. W hile it
cools dow n, it records the m agnetic field during its form ation. The tw o parts of the oceanic plate are
pulled apart, and m agnetic stripes becom e older as they m ove aw ay from the m id-ocean ridge. The
figure show s the spreading ridge about 5 m illion years ago (a), about 2 to 3 m illion years ago (b) and
now adays (c). Courtesy of the U.S Geological Survey.
To confirm the ages obtained with magnetic records, and get an absolute age of the seafloor,
scientists use the radioactive dating technique. When the lava solidifies at the ridges to form the
new seafloor, radioactive elements coming from the mantle are trapped in it. These elements, like
238U (Uranium) or 40K (Potassium) are unstable, and decay with a very precise rate to become
what is called daughter products: 206P (Lead) for Uranium and 40Ar (Argon) for Potassium. By
measuring the amount of remaining radioactive elements and daughter products in the seafloor,
scientists can determine when the magma crystallized, and thus know the absolute age of the
seafloor.