seafloor spreading

Seafloor spreading

New seafloor material is created when tectonic plates move apart. Molten rock rises between the plates, cools, and solidifies. This process is called seafloor spreading - a consequence of the solid mantle moving.

Magnetic field reversal

Seafloors spread by about 10cm per year. This spreading leaves a characteristic pattern of magnetism in the rocks on the ocean floor that can be "read" by scientists.

The Earth's magnetic field has not always had the same North-South alignment. Every so often it reverses direction - for thousands or millions of years. Iron-rich minerals in molten magma line up in the magnetic field, and this alignment is preserved when the magma solidifies. Scientists can then "read" the pattern of magnetic field reversal that forms.

Lava pouring thorugh gap in crust, Earth's magnetic field in one directon		Lava pouring thorugh gap in crust, Earth's magnetic field in opposite directon

The patterns on either side of a mid-ocean ridge are mirror images of each other. This provides evidence for seafloor spreading and plate tectonics in general.