One of the keys to plate tectonics was the discovery that the Earth's magnetic field has reversed its polarity 170 times in the last 80 million years. As new basaltic material is squeezed up into the midocean cracks and solidifies, it is magnetized according to the polarity of the Earth's magnetic field. If thefield reverses its polarity, the strip of new material is magnetized in an opposite sense. As the oceanic floor continues to spread, the new strips of rock are carried away on either side like a conveyer belt.

Using these magnetic strips as evidence of movement, it became obvious that the Earth's surface consisted of a mosaic of crustal plates that were continually jostling one another.

NASA's Mars Global Surveyor has discovered surprising evidence of past movement of the Martian crust, further evidence that ancient Mars was a more dynamic, Earth-like planet than it is today. Scientists using the spacecraft's magnetometer have discovered banded patterns of magnetic fields on the Martian surface. The adjacent magnetic bands point in opposite directions, giving these invisible stripes a striking similarity to patterns seen in the crust of Earth's sea floors. On the Earth, the sea floor spreads apart slowly at mid-oceanic ridges as new crust flows up from Earth's hot interior. Meanwhile, the direction of Earth's magnetic field reverses occasionally, resulting in alternating stripes in the new crust that carry a fossil record of the past hundreds of million years of Earth's magnetic history, a finding that validated the once-controversial theory of plate tectonics.

The discovery of this pattern on Mars could revolutionize current thinking of the red planet's evolution, if the bands on Mars are an imprint of crustal spreading, they are a relic of an early era of plate tectonics on Mars. However, unlike on Earth, the implied plate tectonic activity on Mars is most likely extinct. Alternate explanations for the banded structure may involve the fracturing and breakup of an ancient, uniformly magnetized crust due to volcanic activity or tectonic stresses from the rise and fall of neighboring terrain.

The bands of magnetized crust apparently formed in the distant past when Mars had an active dynamo, or hot core of molten metal, which generated a global magnetic field. Mars was geologically active, with molten rock rising from below cooling at the surface and forming new crust. As the new crust solidified, the magnetic field that permeated the rock was "frozen" in the crust. Periodically, conditions in the dynamo changed and the global magnetic field reversed direction. The oppositely directed magnetic field was then frozen into newer crust.

Like a Martian tape recorder, the crust has preserved a fossil record of the magnetic field directions that prevailed at different times in the ancient past. When the planet's hot core cooled, the dynamo ceased and the global magnetic field of Mars vanished. However, a record of the magnetic field was preserved in the crust and detected by the Global Surveyor instrument.

The map also identifies an area in the southern highlands as the oldest surviving unmodified crust on Mars. This area on Mars is where the magnetic stripes are most prominent. The bands are oriented approximately east-to-west and are about 160 kilometers (100 miles) wide and 965 kilometers (600 miles) long, although the longest band stretches more than 1,930 kilometers (1,200 miles).

The bands are wider than those on Earth, perhaps for a couple of reasons. The Martian crust could have been generated at a greater rate, causing a given magnetic field to be imprinted over a wider area before it reversed direction. Second, the Martian magnetic field may have reversed direction less frequently, which would have given more time for any one field direction to imprint itself in the steadily moving crust, resulting in wider bands.

In order to call this pattern a crustal spreading center like that observed in the mid-oceanic ridges on Earth, a point of symmetry has to be identified, where the pattern on one side matches the pattern on the other. We have not yet found evidence of this type of symmetry.