Interactive Animation of Seafloor Spreading and Magnetic Field Reversals

Earth's magnetic field reverses itself from time to time; North becomes South and South becomes North. Lava that cools and becomes rock at a given time in Earth's history stores a record of our planet's magnetic polarity at the time of the rock's formation. Rocks on the seafloor on either side of a mid-ocean spreading ridge preserve a record of the Earth's magnetic field over time. The discovery of this phenomenon was an important bit of evidence that helped confirm the theory of plate tectonics.

The interactive animation below illustrates this concept. Drag the compass, which represents a magnetometer, to the right and left. Can you find places where the magnetism of the rocks reverses? This section of seafloor is in the North Atlantic Ocean near Iceland. The rate of plate movement in this area is about 25 millimeters (1 inch) per year, or about 25 km (16 miles) every million years. Can you determine when the field reversals occurred?

Click the "Distance" checkbox to view a distance scale. Click the "Age" checkbox to see the age of the seafloor on either side of the ridge.

(Note: If you cannot see the animation below, or it is not working properly, you may need to download the latest Flash player.)

On this section of the seafloor, the history of Earth's magnetic field that is revealed includes the following periods:

  • Brunhes normal - present time to 730 thousand years ago
  • Matuyama reverse - 0.73 to 2.48 million years ago (ma)
  • Gauss normal - 2.48 to 3.40 ma
  • Gilbert reverse - 3.40 to 5.3 ma

If you want to build a physical model of this system, check out the seafloor spreading section of our "Magnetometer Extensions" activity.

Last modified March 29, 2004 by Randy Russell.

You might also be interested in:

Traveling Nitrogen Classroom Activity Kit

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

Mid-Ocean Spreading Ridge

As the Earth cools, hot material from the deep interior rises to the surface. Hot material is depicted in red in this drawing, under an ocean shown in blue green. The hotter material elevates the nearby...more

Plate Tectonics

The main force that shapes our planet's surface over long amounts of time is the movement of Earth's outer layer by the process of plate tectonics. This picture shows how the rigid outer layer of the Earth,...more

Volcanic Ash

Ash is made of millions of tiny fragments of rock and glass formed during a volcanic eruption. Volcanic ash particles are less than 2 mm in size and can be much smaller. Volcanic ash forms in several ways...more

Cinder Cones

Cinder cones are simple volcanoes which have a bowl-shaped crater at the summit and only grow to about a thousand feet, the size of a hill. They usually are created of eruptions from a single opening,...more

Flowing Lava

Lava can move in broad flat lava flows, or it can move through tight channels or tubes. Lava flows tend to cool quickly and flow slowly. The fastest lava outside of channels moves at about 6 mi/hr an easy...more

How Do Plates Move?

Plates at our planetís surface move because of the intense heat in the Earthís core that causes molten rock in the mantle layer to move. It moves in a pattern called a convection cell that forms when...more

Clues to Plate Movements

Many kinds of surface features are clues that our lithosphere is sliding. Two types of features can form when plates move apart. At mid ocean ridges, the bottom of the sea splits apart and new crust is...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA