Ficheiro:Ocean floor rift.png

Summary

Rift in the ocean floor through which red-hot magma is squeezed up from the mantle. The magma solidifies with a magnetic polarity corresponding to that of the Earth's magnetic field. After a period of time, the Earth's polarity changes, that is, the magnetic north pole becomes the magnetic south pole, and so the polarity of the newly formed crust changes, too. As new magma is squeezed in, the older crust is moved out from the midocean ridge like a conveyor belt. This produces a series of strips of rock magnetized in opposite senses, with the magnetic stripes parallel and symmetrical to the ridges. Shallow-focus earthquakes occur on the ridge; intermediate and deep-focus earthquakes occur on the downgoing plate as it collides with another plate.

Copyright status

This image is public domain because it was first published by the U.S. Geological Survey

Source

http://earthquake.usgs.gov/learning/topics/plate_tectonics/rift.php