Earth is surrounded by a magnetic field that is generated deep down in the Earth’s core. This magnetic field is very important to life on Earth as it protects us from dangerous solar and cosmic rays. More on the magnetic field later.
Earth has an inner solid core, about 2/3 the size of the moon, and made of mostly iron. The iron core is about 5,700 degrees Celsius, about the temperature of the sun’s surface. Gravitational pressure keeps the super-hot iron from becoming a liquid. Surrounding the inner iron core is an outer core that is fluid layer of iron, nickel and other metals about 1,200 miles thick. It’s also very hot, but not as hot as the inner core.
Magnetic fields are generated by rotating electric charges, and that is how Earth generates its magnetic field. According to NASA, Earth’s magnetic field comes from this ocean of iron, which is an electrically conducting fluid in constant motion. Sitting atop the hot inner core, the liquid outer core seethes and roils like water in a pan on a hot stove. The outer core also has “hurricanes”–whirlpools powered by the Coriolis forces of Earth’s rotation. These complex motions generate our planet’s magnetism through a process called the dynamo effect.
The currents created by the inner and outer core are not consistent and thus the Earth’s magnetic field moves a bit year-to-year. The magnetic north pole drifts about 10 miles a year.
Interestingly, hundreds of time over the past 4+ billion years Earth’s magnetic poles have flipped. When the magnetic poles flip it means that a compass that points north will actually be pointing towards Antarctica rather than the north pole.
For the past 20 million years the poles have flipped about every 200,000 to 300,000 years, but it has been about 780,000 years since the last magnetic pole flip. Thus, we are long overdue for a flip based on averages. Also, when the magnetic poles flip the magnetic field weakens and Earth’s magnetic field has weakened about 5% over the past century, leading to some concern that the flip is imminent. On the contrary, here is a recent paper which concludes that the flip is not about to happen: Earth’s Magnetic Field Reversal
Some are concerned that the weakening of the magnetic field during a reversal could be harmful to life on Earth, as the magnetic field protects us from solar and cosmic rays. This is likely alarmist as evidence suggests that prior reversals did not result in extinctions. Another consequence of a reversal is that animals that navigate by the magnetic field such as birds, salmon and sea turtles could get lost during their migrations.
How and why does the magnetic field flip? The swirling liquid iron in the outer core causes parts of the iron to flip in the opposite direction and become “reverse-aligned.” When enough of the iron atoms are reverse aligned the entire magnetic field flips and magnetic north becomes magnetic south. This occurs rather quickly on a geologic scale, but is quite gradual from our perspective, usually taking between 1,000 and 10,000 years. According Monika Korte at the Niemegk Geomagnetic Observatory, “It’s not a sudden flip, but a slow process, during which the field strength becomes weak, very probably the field becomes more complex and might show more than two poles for a while, and then builds up in strength and aligns in the opposite direction.”
I found the article to be very intriguing. I would be interested in knowing how the scientists are able to determine that the flips occurred so long ago.
During volcanic eruptions the magma has iron and nickel in it and also has the directionality of the magnetic field in the cooled metallic magma. So, over time you can see the flips as you look at volcanic flows over time. It looks like this: http://www.astrosciences.info/files/includes/images/magflips_files-image005.jpg
Here is a question which has intrigued me for some time and seems, to me, may be a factor in the magnetic reversal. We, in essence, live on a spinning top. The ice cap at the poles is a relatively large mass which is melting and the mass is being redistributed evenly in the oceans. The mass distribution of the earth is shifting. What impact will this mass redistribution have on the dynamics of our spinning top?