Planet Earth

Archeology Watch: Magnetic Bishopric

Before Graham Borradaile's discovery, archeologists didn't know that a powerful dating tool la? hidden in every stone building and statue in the world.

By Josie GlausiuszMar 1, 1998 12:00 AM


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THE OLD BISHOP'S PALACE IN LINCOLN, England, built in stages between 1155 and the 1880s, was once the splendid residence of medieval clerics. But over the centuries it fell into such disrepair that at one point hares were breeding in the dining room. Now somewhat restored, it is a favorite haunt of Graham Borradaile, a geophysicist at Lakehead University in Thunder Bay, Ontario. The palace has become an integral part of his research. Its old stone walls and towers have enabled him to develop a new technique that will allow archeologists to date precisely stone monuments and buildings whose ages, until now, have been difficult or impossible to determine.

Borradaile was visiting his parents, who live in Lincoln, when the idea for a new dating method came to him. Every stone in the Bishop’s Palace, he realized, was a kind of fossil compass. Earth’s magnetic field, geologists know, can imprint itself on rocks in two ways. The strongest imprint occurs when rocks first solidify. As molten rocks cool, the electrons of iron atoms line up with Earth’s magnetic field. By examining such rocks, geologists have learned that Earth’s magnetic field flips directions every few million years, since rocks of different ages have different fossil compass headings baked into them.

Earth’s magnetic field can affect rocks in a second, more subtle way. Over hundreds of years, normal ambient temperatures can provide enough energy to jiggle the atoms within a rock. Some will be pulled by Earth’s field and will turn to face north. This form of soft magnetization can partly overlay or obscure the hard magnetization imprinted when the rock solidified. Geologists typically view this overlay as a nuisance. In fact, to uncover the older imprint of past magnetic-field reversals, they usually heat rocks to destroy the more tenuous soft magnetic veneer. (The older fossil magnetic stamp can withstand temperatures that obliterate the more recent imprint.) But where other geologists saw only a nuisance, Borradaile saw a useful tool.

The degree of soft magnetization in a rock increases with time. The longer a rock remains facing in a particular direction, the greater the likelihood a few more of its atoms will have lined up with Earth’s magnetic field. If a rock is turned, say by a stonemason constructing a medieval bishop’s palace, the magnetic realignment begins anew. And the longer the rock is turned, the higher the temperature needed to obliterate this soft fossil compass, because more electrons in iron atoms will have realigned themselves.

The Bishop’s Palace, Borradaile realized, offered a perfect means to calibrate his dating method. Since the founding and various reconstruction efforts of the palace are well documented, he could take a rock from a wall built in, say, 1160, and then measure the temperature required to erase the rock’s soft magnetic memory for the time it had spent in the wall. He could then use that knowledge to date rocks of similar composition anywhere in the world.

So Borradaile took fist-size samples from sections of the palace representing different building eras and carefully noted the stones’ original placement. Back in his lab in Canada, Borradaile placed each sample in a thermal demagnetizer—a kind of electric oven surrounded by a magnetic shield that blocks out Earth’s field. He heated each sample until the overlay of soft magnetization was destroyed.

Borradaile found that rock from a wall built in 1850, for example, needed about 390 degrees of heating before its soft magnetization dissipated, while a stone from Bishop Alnwick’s audience chamber, built around 1440, required 500 degrees. And a wall from a fourth-century Roman settlement for retired legionnaires built on the same site had to be heated to 660 degrees.

Borradaile is planning a trip this summer to date Easter Island’s huge, mysterious statues. It’s thought that they were put there somewhere between the twelfth and the fifteenth centuries, he says, but we’re not too sure, as it is based on an oral history.

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