Find out how a buried log unearthed in New Zealand may hold clues to understanding Earth’s magnetic field.
When we took possession of the lot where our family cottage is, my dad wasn’t all that sure what he’d bought. It was covered with very thick bush, and we had no neighbours. We managed to find the survey stakes, and we had a copy of the plan of subdivision. The drawing showed the compass bearings for the lot lines. We decided to indicate where our lot lines were by marking the trees. That’s how I first learned to use a compass.
One of the tricks to using a compass well is realizing that the magnetic north pole and the true north pole aren’t in the same place. Allowing for that is called declination. What makes it even trickier is that the magnetic north pole doesn’t stay in one place. It’s continuously in random motion. To be entirely accurate with a compass, we need to keep updating our maps to keep up with the latest declination adjustment.
The Earth’s magnetic field is still a mystery
We say that a compass “points north,” but it’s more accurate to say that it aligns with the Earth’s magnetic field. The Earth’s magnetic field is still a mystery. For something we use every day, it’s surprising that we don’t know more about it. It comes from our planet’s iron core.
The outer part of Earth’s central core consists of molten iron. Our understanding of the dynamics of that core is sketchy at best. William Gilbert was the first scientist to try to explain Earth’s magnetic field formally. He published a paper called De Magnete in 1600. It was only in the 20th century that a coherent model of it began to take shape.
Today, most scientists agree that the churning motion of this liquid iron in the Earth’s outer core acts as a dynamo, generating a magnetic field known as the magnetosphere. This dynamo is very unstable. The poles of the magnetic field are always shifting. They’ve been particularly volatile lately.
The poles have entirely reversed several hundred times
The north magnetic pole has been drifting by 55 km per year over the last 20 years. Scientists are unable to predict these movements, even over five years accurately. Weirder still, scientists can tell that the poles have entirely reversed several hundred times over the past 3 billion years.
Those reversals were in the news in New Zealand over the summer. The Nghawa Generation company was excavating some land near Nghawa Springs, New Zealand, to build a new geothermal power plant.
During the dig, they unearthed a massive, old kauri tree. The tree is 16 metres long and weighs 60 tonnes. They called in Waikato University’s Waikato Radiocarbon Dating Laboratory, who were able to carbon date the tree to between 41,000 and 42,500 years ago.
The only tree that was alive when the magnetic poles reversed.
This was exciting for the scientists because it means that it is the only known tree on Earth that was alive the last time the Earth’s magnetic poles reversed. They believe that the tree’s rings will contain clues as to what happens to the ecosphere during one of these shifts.
Our magnetic field shields us from the continuous flow of charged particles emanating from the sun. That’s what causes the Northern Lights. Scientists believe that when the poles of Earth’s magnetic field shift, this weakens the magnetosphere and allows more of the solar wind to reach the Earth’s surface. They think this harms the climate.
They also believe that if our magnetic poles reversed again, it would disrupt our telecommunications network. Understanding more about what happens during one of these shifts would help us to be better prepared for the future.
clues about what life was like with a weak magnetosphere.
The reversal this tree lived through is called the Laschamp Event. The gradual shift lasted about 440 years, which is brief by geological standards. The polarity was reversed entirely for only about 250 years. At that point, the strength of the Earth’s magnetic field was only 5% of today’s levels. Scientists are eager to study this tree’s rings to look for clues about what life was like with such a weak magnetosphere.
We tend to think of ecology as involving the “web of life”—the interrelationship of various living species and their diversity. We should bear in mind that ecology also considers what scientists call “abiotic factors.” These are the non-living parts of the environment that can influence the life forms in an ecosystem. Studying this tree will help scientists understand the degree to which our magnetic field is an important abiotic factor for the Earth’s biosphere.
From time to time, we come across commonplace experiences that science still can’t explain very well. The magnetosphere is one of those phenomena. We need to learn how and why the Earth’s magnetic poles suddenly shift. We also need better ways of forecasting the drift in their location to better support navigation.
Our magnetic field has been decaying over the last 3,000 years, and many scientists predict that it may reverse again sometime in the coming century. We would then be faced with the disruptions to the environment and the telecommunications system mentioned above. Hopefully, this tree can provide some of the answers we need to mitigate the consequences of such a reversal.
We always have more to learn if we dare to know.
New Zealand Herald