Measuring the geomagnetic field in the Botanic Gardens

The big fort in the middle of the Botanic Gardens forms the backdrop for research into the Earth's magnetic field. Lennart de Groot explains why he considers the fort's gunpowder stores the ideal spot for his research.

When you visit the Botanic Gardens in the Utrecht Science Park you can not possibly miss it: the large, stone fort in the centre of the Gardens. In the late nineteenth century, Fort Hoofddijk, part of the New Dutch Waterline defence line, was used to protect our country against invasions from neighbouring countries. The use of aircraft during the Second World War rendered the defence line redundant, and Fort Hoofddijk was left vacant. This ended in 1963, however, when new residents moved in: the soldiers were replaced by scientists studying the Earth's magnetic field.

Fort Hoofddijk

The Earth is one big magnet

It is no coincidence that the earth scientists have settled there, explains Lennart de Groot. De Groot himself conducts his research in the fort too. “To understand why we work here you need to know what it is that we are studying. Put simply, the Earth is one big magnet with a north pole and a south pole. As a result, it is surrounded by an enormous magnetic field which protects us from harmful radiation from, for example, the sun.”

The poles shift

“The north and south poles, however,” De Groot continues, “are not the same as the poles you see on a map. They even shift relative to the geographical poles. Being able to track this shift is important, since it affects how our compasses work. And to measure this movement really minutely, we need to get to know the historic movements of the magnetic north and south poles.”

Measuring lava

De Groot: “To a greater or lesser degree, almost all kinds of rock are magnetic, and they contain information about the past direction and intensity of the geomagnetic field. Lava, for example, contains many ferriferous minerals that register the geomagnetic field at the time it solidifies. Our advanced measuring equipment allows us to analyse this solidified lava, which allows us to learn what the geomagnetic field was like in the past.”

Metal distorts the results

We need extremely sensitive equipment to be able to study minute magnetisations in rock. This equipment is so sensitive that the presence of metal in a building is capable of distorting the measurements. This is why De Groot's research must be conducted in an environment in which metal is largely absent. “In a modern building, for example, we would be able to measure the passage of the lift, but hardly make any worthwhile measurements in terms of our rock samples.”

Fortunately, the nineteenth-century builders of the fort had also considered the presence of metal in the former gunpowder store at Fort Hoofddijk: to prevent sparks in the vicinity of the gunpowder, no metal was used in the construction of the old gunpowder store. This makes the gunpowder store the perfect place for De Groot to conduct his research in. What is more, the Botanic Gardens surrounding the fort act as a further protective layer: there is very little heavy traffic nearby the fort, for example.