The "pattern" of reversals is anything but regular. During one span of more than 40 million years in the Cretaceous Period the magnetic field's polarity remained unchanged. At other times, the field has flipped back and forth several times within a million years.

Earth's magnetic field has been in its current alignment, called the Brunhes normal, for the past 780,000 years. Click here for a closer look at magnetic reversals over the most recent 5 million years, or here to see the 5 million year and 160 million year timelines side-by-side.

Sometimes Earth's magnetic poles switch places. That is called a magnetic reversal. The graph below shows how often magnetic reversals happen. The graph starts (left side) 160 million years ago in the Jurassic Period when dinosaurs roamed the Earth. The right side of the graph shows today.

The black and white stripes along the bottom of the graph show the direction of Earth's magnetic field. Black stripes mean Earth's magnetic field was "normal" (like it is today). White stripes show times when the field was reversed.

Did you notice that there isn't much of a pattern? Sometimes the magnetic field goes 40 million years or longer without changing once. Look at the Cretaceous Period around 100 million years ago. Other times the magnetic field changes pretty often. Sometimes it flips back and forth several times within a million years.

Earth's magnetic field has been "normal" for the past 780,000 years. Scientists call this time period the Brunhes normal. Click here for a closer look at magnetic reversals over the past 5 million years. Click here to compare the past 5 million years with the past 160 million years.

Sometimes Earth's magnetic poles switch places. That is called a magnetic reversal. The graph below shows when magnetic reversals happened in the past. The graph starts (left side) 160 million years ago in the Jurassic Period. There were dinosaurs back then. The right side of the graph shows today.

The black and white stripes along the bottom of the graph show the direction of Earth's magnetic field. Black stripes mean Earth's magnetic field was "normal" (like it is today). White stripes show times when the field was reversed.