Paleomagnetism

Our August table’s theme was magnets and electromagnetism. We brought out lots of demonstrations, including a ferrofluid tube, and we had many great interactions with everyone who stopped by our booth. There is one major part of magnetism that we left out, however, so this blog post is dedicated to Earth’s magnetic field. 

While less than 1/100th of the strength of a refrigerator magnet, Earth’s magnetic field is still an important, useful tool. Most of us have used a compass before, so we know that Earth’s magnetic field can be used to find our way. We understand that the magnet inside of the compass will rotate so that the red arrow will always point a certain way. That red arrow is the north part of the magnet, and will always align so that it, indeed, points towards north. It seems trivial: the north part of the magnet points north, and the other end points south. 

Except that it hasn’t always been that way—and it won’t be that way in the future. Using rocks to study how Earth’s magnetic field changed over time is a scientific discipline called paleomagnetism.

The Mid-Atlantic Ridge spans from the Arctic to the Antarctic. It is a series of divergent and transform plate boundaries.

Evidence from the geologic record proves that Earth’s magnetic field has reversed itself many times over Earth’s 4.5 billion year history. Most of this evidence comes from the seafloor in the Atlantic ocean, specifically the Mid-Atlantic Ridge (see picture above). The Mid-Atlantic Ridge stretches from the Arctic to the Antarctic along the bottom of the ocean, and is actually a series of connected divergent plate boundaries (but we’ll get into plate tectonics later). This means that every year, the continents North and South America and Europe and Africa get a little farther away.

At the Mid-Atlantic Ridge, the forces that push the continents apart also send melted rock, magma, up from inside Earth to the ocean floor. This is how new oceanic crust is created, and is some of the youngest rocks on Earth. This deep-ocean crust is also rich in heavy metals, such as iron, because it is formed from magma that comes from inside the Earth, where there is a higher concentration of such elements. Many of these metals are also magnetic. 

While these rocks are still hot, the magnetic dipoles inside of them will act as the needle in a compass does, and align themselves with Earth’s magnetic field. When they cool down to a certain temperature, called the Curie temperature, the rock will maintain the orientation of Earth’s magnetic field. As time passes, more rock is created, pushing the old rock to the side. 

When Earth’s magnetic field switches, the new rocks record an opposite orientation. In the picture below, the black bars represent periods of the current magnetic field's orientation, and white bars represent periods of reversed orientation. Geologists who study paleomagnetism measure the ages of the rocks and the orientation of Earth’s magnetic at the time those rocks cooled. By doing so, geologists piece together the timing of the reversals in Earth’s magnetic field. Their work on rocks from the Atlantic seafloor has revealed that Earth’s magnetic field has reversed orientation many times in the past 180 million years. Curiously, there is no regular pattern to the switches, as they appear to happen rapidly and at irregular times. What causes these reversals is still an open question in geology.

The paleomagnetic record

It is not clear when the next reversal will take place, or how long it will take when it starts to happen. It is an event that has never been observed by humans. Like so many other events in Earth history, the only record we have of this phenomenon is the rocks they left behind. Through geology, scientists discover the history of Earth and how it’s changed, and use it to predict what may happen to Earth in the future. For now, however, simply holding a compass is enough to observe Earth’s magnetic field. 

Sources: http://web.ics.purdue.edu/…/teach…/eas450/paleomagnetism.pdf
http://www.grisda.org/origins/10066.htm
https://www.tes.com/lesso…/QzyoecA6gyWRFw/mid-atlantic-ridge