Geology rewards an active imagination. It gives us a lot of tantalizing clues about very different times and places in Earth’s history, leaving us to try to answer “Man, what would that be like?” One of the things that's tough to imagine involves changing something that most of us never give a second thought—the fact that compasses point north. That’s plainly true today, but it hasn’t always been.

What we call the “north” magnetic pole—the object of your compass’ affection—doesn’t need to be located in the Arctic (it noticeably wanders there, by the way). It feels equally at home in the Antarctic. The geologic record tells us that the north and south magnetic poles frequently trade places. In fact, the signal of this magnetic flip-flopping recorded in the seafloor was the final key to the discovery of plate tectonics, as it let us see how ocean crust forms and moves over time.

That the poles flip is interesting in itself, but “Man, what would that be like?” Does the magnetic pole slowly walk along the curve of the Earth over thousands of years, meaning your compass might have pointed to some part of the equator for long stretches of time? Do the poles weaken to nothing, disappearing for a while before re-emerging in the new configuration? Do they somehow flip in the blink of an eye? Given the number of species that use the Earth’s magnetic field to navigate—especially for seasonal migrations—this is more than an academic curiosity.

These natural questions aren’t the easiest to answer because it’s hard to find records with sufficient detail to resolve time periods short enough to capture a flip. A newly published study led by Leonardo Sagnotti of the Italian National Institute of Geophysics and Volcanology takes advantage of a wonderfully high-resolution record in Italy to reveal something about the last pole flip around 780,000 years ago—it probably happened within a human lifetime.

That last flip is called the Matuyama-Brunhes reversal. From studies of ocean cores and lava flows, it’s thought to have lasted less than a thousand years, a figure that has informed the computer models of Earth’s magnetic field reversals. But in Italy’s Apennine Mountains, researchers found some sediments deposited at the bottom of a lake around the time of the Matuyama-Brunhes reversal. The lake accumulated sediment much more quickly than the ocean typically does, meaning that the time of the reversal is “stretched out” over a thicker stack of sediment. That allows measurements to zoom in on smaller chunks of time. On top of that, there are also a fortuitous number of volcanic ash layers within the lake muds—and the age of an ash can be determined using radiometric dating.

Some of those ash layers have been examined before, but the researchers collected samples to precisely date three of the layers closest to the magnetic reversal. They collected between 25 and 75 crystal grains from each layer and sent them off to two separate laboratories to ensure the most accurate and reliable results possible. The dates they got actually imply that the reversal may have occurred a few thousand years earlier than the current best estimates.

With the dates of three layers around the magnetic reversal pinpointed, they could calculate the average rate of sediment accumulation between those times—a little over 20 centimeters of sediment per thousand years during the reversal.

Sediment samples were also analyzed by machines that detect the orientation of magnetic mineral grains, which are influenced by the Earth’s magnetic field at the time they’re deposited. Today, those minerals tend to align pointing north, like the magnetite needle in your compass. Before the Matuyama-Brunhes reversal, however, they tended to point south. The analyses also measure the strength of this tendency, which reflects the strength of the Earth’s magnetic field back then.

Those measurements show that the magnetic reversal occurs in an interval of sediment about two centimeters thick. From the calculated sedimentation rate, that equates to less than a century, which means that the magnetic poles traveled to the opposite geographic poles going at least two degrees latitude per year. No intermediate positions were detectable, though, so we can’t say much more than that—it could have been nearly instantaneous.

These rocks tell us that at least one reversal happened ten times faster than previous estimates. That’s pretty important information for researchers trying to work out how the geodynamo of Earth’s liquid outer core flips its magnetic poles.

Unfortunately, we could still use even higher resolution records that catch more details of the magnetic reversal—the kinds of things that might allow us to better guess how migratory birds would have dealt with the change. But this study certainly puts some constraints on what that would be like.

Geophysical Journal International, 2014. DOI: 10.1093/gji/ggu287 (About DOIs).