A vast city of 10,000 once stood within the grounds of Tel Zafit National Park. Now it is an archaeological dig, nothing but burned mud bricks, a crumbling break in the city’s defenses, and weapons cobbled together at the last minute from animal bones. What happened here? What force brought this great city to its end?  

According to the Bible, Gath was one of the main Philistine cities and the home of Goliath the Giant. Its destruction is glossed over, described in less than one verse of the Bible, in the book of 2 Kings. 

Archaeologists have long worked to figure out what happened to the ancient city of Gath, and just as important, when it happened. But dating sites like this is no straightforward task. Recently, a team of scientists led by Yoav Vaknin of Tel Aviv University tried a new method to date archaeological digs like Gath: They used the Earth’s magnetic field. Their results recently appeared in Proceedings of the National Academy of Sciences.

Pole reversal

Deep within the Earth, thousands of miles below the crust, lies the boundary between the planet’s solid inner core and the molten outer core that surrounds it. That boundary burns at 6,000° C, hotter than the surface of the Sun. It is the hottest place on Earth.

In the outer core, massive currents of molten iron convect in giant cells around the inner core. Imagine water boiling on your stove — only on a much larger scale, and warmed by superheated liquid rock. This rock is made up of iron and nickel, and its motion creates the Earth’s protective magnetic field.

We can thank that magnetic field for a lot of things, including life. It helps shield the Earth, deflecting radiation up and around the planet. It protects our planet from solar winds and coronal mass ejections from our sun, as well as from cosmic rays. 

But since motion within the core is chaotic, the magnetic field is dynamic, too. In fact, the magnetic field of the Earth completely reverses — the North and South take turns being the pole of attraction. Sometimes the switch happens after tens of thousands of years, sometimes after tens of millions, but even when it is not reversing, the magnetic field of the Earth is constantly in flux. The North Pole travels about 45 km per year, its intensity gradually varying. 

The history of changes in the magnetic field is recorded in rock. Perhaps the most well-known record is etched in stone at the mid-Atlantic ridge. Here, new seafloor is constantly being created as the tectonic plates spread along fault lines as long as the ocean. As these molten rocks orient themselves, cool, and solidify, they record the direction and intensity of the magnetic field. A search of the seafloor allows us to read the history of the Earth’s magnetic field itself.

Surprisingly, this method can also be used for ar