Sea floor spreading

Black lines denote tectonic plate boundaries

The Earth’s crust is broken into many slowly moving plates. Sea floor spreading occurs at the mid-ocean ridge where two plates are moving away from each other. 

Magma (hot, molten, underground rock material) rises up into the gap from below and cools to form new seafloor rock. The sea floor spreads out to either side. This spreading occurs at about the same rate as your fingernails grow! 

Sea floor spreading in the Atlantic Ocean

The pattern of sea floor spreading can be observed by studying the magnetic field of the rock on the sea floor. At the mid-ocean ridge, magma rises up from the mantle below and cools. As it continues to cool, iron in the rock aligns itself with the magnetic field of the Earth, much like the needle in a compass. When the rock solidifies, this magnetic “signature” is locked in place.

Iron filings align along a magnetic field

White strips show periods of reversed polarity

A geomagnetic reversal or polar reversal is a change in the orientation of Earth's magnetic field.The positions of magnetic north and magnetic south become interchanged. Reversals are not predictable. The latest reversal occurred 780,000 years ago. In the last 10 million years, there have been, on average, 4 or 5 reversals per million years. At other times in Earth's history, for example during the Cretaceous era, there have been much longer periods when no reversals occurred. Most scientists believe that reversals occur due to convection of molten iron within the Earth's core which generates electric currents and that reversals are inherent to that process. 

Geomagnetic reversal "flips" north and south poles

This chart shows the last 150 million years of Earth's polarity.
The black bars represent polarity as it is now.
The white  bars represent periods of reversed polarity.

The chart below shows the geomagnetic reversals over the last 2 million years. 

Additional reading:

Do you think the Earth will experience a geomagnetic reversal in 2012? Click here to read a 2008 article by Universe Today. 

Fossil evidence for plate tectonics: Click here to read about "The curious case of the Mesosaurus"

Click here to watch a 10 minute video "Ask a physicist, Geomagnetic reversal, the end of the world?" He talks about solar wind, northern lights, magnetic fields, and geomagnetic reversal. He also shows a clip from the Hollywood movie The Core, where they are experiencing a geomagnetic reversal in overly dramatic fashion.

Assignment for Thursday September 22, 2011

For 5 possible points write at least 3 sentence comment on the subject of plate tectonics, sea floor spreading, or geomagnetic reversals. Your comment should include one question. You need to state some facts and/or scientific principles. Click below on comments to get started!When you're finished, click here to practice some Mac keyboard shortcuts