Moving earth magnetic fieldGeology and Geography
The earth’s magnetic field behaves much like that of an ordinary bar magnet. However, there are also decisive differences. For example, the earth’s magnetic field is not rigid, but dynamic. Its magnetic poles are constantly in motion. At present, the magnetic south pole is close to the geographic north pole. At about 40 kilometres a year, it moves to the northwest. The magnetic north pole in Antarctica is also shifting, away from the geographic south pole.
So the magnetic poles are on the move. But that’s not all: in the course of history, the Earth’s magnetic field has completely reversed its polarity several times. On average, this has happened every 250,000 years. The last pole reversal was about 780,000 years ago. So is another pole reversal “overdue”? For some years now, experts have been measuring that the Earth’s magnetic field is weakening. They see this observation as a sign that the Earth’s magnetic field is indeed slowly reversing: at some point, the magnetic south pole will be in Antarctica, the magnetic north pole in the Arctic. Scientists assume that it will take about 2000 years until the polarity is completely reversed.
The evidence that the polarity of the Earth’s magnetic field has already reversed several times is immortalised in the rocks. This is particularly visible at the mid-ocean ridges, i.e. at the places where the ocean floor grows: here, glowing hot rock mush, which also contains iron, constantly escapes. As long as this rock mush is liquid, its iron components align themselves with the Earth’s current magnetic field. When the rock cools and solidifies, this alignment remains permanently “frozen” in it. Because it is known how strongly the ocean floors grow, the magnetic alignment of this rock can be used to calculate approximately when and how often the Earth’s magnetic field has already reversed polarity.
On an Arctic expedition, the polar explorer James Clark Ross discovered the magnetic South Pole. His measuring instruments had shown him the way. The magnetic pole is located on the Canadian mainland, about 2300 kilometres from the geographic North Pole.
In May 1829, the British polar explorer John Ross and his nephew James Clark Ross set out on an expedition to the Arctic. The goal of the two explorers was the Northwest Passage. It is a sea route north of the American continent that leads through the middle of the icy Arctic Ocean. When the two explorers reached a peninsula with their sailing ships, they realised that they had reached the northernmost point of the American continent. Because of the enormous ice masses and technical problems with the ships, they were stuck there. While exploring the mainland, James Clark Ross realised that they were near the magnetic South Pole. With the help of local Inuit, he set off on sledges and reached the magnetic South Pole on 1 June 1831. The geographic North Pole was about 2300 kilometres away from them. This makes the Briton James Clark Ross the first European to stay at the magnetic South Pole.
When and how the expedition with the Ross research duo will return to Europe is not yet known due to ongoing technical problems.
The Boothia Peninsu
Boothia is the name of the peninsula that polar explorer John Ross discovered in the north of the Canadian mainland. He named it after his friend, the gin producer Felix Booth. This wealthy English businessman had largely paid for the polar expedition.
The Boothia Peninsula is barren: tundra and bare frost heaps and rocky outcrops dominate the landscape. This is the home of the Inuit, an ethnic group that is also native to Greenland. They live mainly from hunting seals, whales or polar bears and from fishing. Their means of transport are kayaks and sledges pulled by dogs. It was only with the support of these indigenous people that James Clark Ross succeeded in reaching the magnetic South Pole.
We don’t notice it, but the compass needle shows us clearly: the earth is a huge magnet. It has two magnetic poles, a north pole and a south pole. And like all magnets, the earth is surrounded by a magnetic field: the earth’s magnetic field.
In the area of its magnetic field, a magnet exerts force on other magnets, for example on a compass needle. The effect of a magnet can also be made visible by fine iron filings: they arrange themselves around the magnet and point in the direction of its two poles. A line-like pattern is created that indicates the magnetic forces. The lines of this magnetic field are the so-called field lines.
The earth’s magnetic field also has such field lines. They emerge from the earth near the south pole, run outside the earth to the north pole and disappear into the earth again there. They are thus arranged as if a huge bar magnet were running through the middle of the earth.
The south pole of this imaginary bar magnet points approximately to the geographical north pole, its north pole to the geographical south pole. What sounds confusing at first has a simple explanation: the north and south poles attract each other. That is why the north pole of the compass needle points to the magnetic south pole of the earth, and the south pole on the needle points to the magnetic north pole.
However, the Earth’s magnetic field is not only used for orientation on this planet. Together with the atmosphere, it also protects us from threats from space. One of these threats is a charged particle stream that the sun constantly emits in all directions. This so-called solar wind is deflected by the Earth’s magnetic field. Like a capsule, the Earth’s magnetic field deflects the charged particles so that they fly past the Earth and can no longer be dangerous to us.
Why is the earth magnetic at all?
The fact that the Earth has a magnetic field is very practical: among other things, it protects us from charged particles from space (the “solar wind”) and was – at least before GPS – an important aid to navigation at sea and in unknown terrain. But why is the earth magnetic at all?It is not easy to explain this exactly – scientists are still researching the details. One thing is clear: the Earth’s magnetic field originates in its core. This consists mainly of the metals iron and nickel and is over 5000 degrees Celsius hot. In the outer core of the Earth, the metals are molten and therefore liquid, and even further inside, the pressure is so high that the inner core is solid.
The solid inner core acts like a hotplate: it heats the liquid above it, the heated liquid rises and finally meets a somewhat cooler layer. There it passes on its heat and cools itself down a little in the process. As a result, it sinks back down again. This cycle is called “convection flow”.
In the outer core of the Earth, there are currents of iron – a conductive material. You can almost imagine this as a wire that moves. And we know from a wire moving in a magnetic field that a voltage is generated (“induced”) in it. This voltage in turn causes an electric current to flow, which in turn generates a magnetic field.
While the iron masses in the earth’s core move, the earth also rotates on its own axis. This causes these fluid currents to be additionally twisted. With the right combination of flow movement and Earth rotation, this can result in the magnetic field generated being aligned in such a way that it supports and amplifies the original magnetic field. And this strengthened magnetic field induces a stronger voltage, which causes a stronger electric current to flow, which further strengthens the magnetic field. In this way, the magnetic field can eventually keep itself stable.
So in the beginning, a small magnetic field must have existed by chance. Driven by the earth’s rotation and the earth’s heat, this mechanism caused it to strengthen itself more and more. So strongly, in fact, that a magnetic field with a uniform direction gradually prevailed throughout the Earth’s core. We can then measure this on the surface as the “Earth’s magnetic field”.
However, it can also happen that the flow conditions in the core change a little. Then this mechanism, in which the magnetic field maintains itself, no longer works so well. As a result, the Earth’s magnetic field can become weaker overall – and it is even possible that the opposite direction suddenly gains the upper hand in one part of the Earth’s core and that this gradually prevails in the entire core. In the end, the Earth’s magnetic field has completely reversed: the North Pole has become the South Pole and vice versa. Scientists have found that such a “pole reversal” has happened many times in the past, on average about every 250,000 years.
The surface of the oceans glistens in dark blue. It is hard to believe that the seabed lies many kilometres deeper in places and that a spectacular underwater landscape is hidden down there. For the seabed is not as smooth as the bottom of a swimming pool: On the seabed there are high mountains, deep trenches and lava-spewing volcanoes as well as vast plains.
The water of the oceans is not equally deep everywhere. Around the continents lie the shallow shelf seas. Here the seabed slopes gently downwards from the coastline until it reaches a depth of about 200 metres below sea level. The bottom of the shelf seas consists of continental crust. Therefore, it actually belongs to the mainland, even though it is covered by seawater.
Only many kilometres from the coast, on average after 74 kilometres, does the shallow shelf area end with the shelf edge. From this edge, it descends steeply, like a slide, to a depth of about four kilometres. This steep slope forms the transition to the deep sea, into which no more light penetrates. That is why no plants grow down there. Only some animal species have been able to adapt to this habitat, despite the hostile conditions.
In the midst of the oceans, mountains rise up into the air, the mid-ocean ridges. These underwater mountains stretch over long distances through all the world’s oceans. In some places they rise above sea level as islands. Iceland, for example, lies directly on the mid-Atlantic ridge, the longest mountain range in the world.
Deep trenches also run through the oceans. Most of them are in the Pacific. Among them is the Mariana Trench, the deepest trench in the world. It reaches down to 11,034 metres below sea level. Only two people have ever been down there: The marine explorer Jacques Piccard and his companion Don Walsh on their record-breaking dive in 1960.
Where slabs diverge
A long deep crack gapes in the earth and grows wider and wider. Huge forces are tearing the earth’s surface into pieces: The East African Rift runs through the continent along this fracture. Africa began to break apart here 20 million years ago. Hot magma from the earth’s interior pushed upwards and tore the earth’s crust apart. Since then, the pieces of crust have been drifting apart, by about a centimetre every year. The fact that the earth is very active here can also be seen from the many volcanoes that rise along the trench. Should seawater penetrate at some point, the East African Rift will become an ocean. Something similar happened at the Red Sea. There, the African and Asian continental plates have been separating for 25 million years. The resulting rift was flooded by seawater.
Where continental crust breaks apart, a rift valley forms. In contrast, where oceanic crust pieces move away from each other, mountains grow on the sea floor: the mid-ocean ridges. They consist of magma that penetrates upwards from the Earth’s mantle through the oceanic crust. New plate material is formed here. It squeezes between two oceanic plates, so to speak, and solidifies into basalt rock that continues to pile up.
In some places, the mid-ocean ridges rise above sea level as islands. Iceland, for example, and the still young Icelandic island of Surtsey are nothing other than parts of the Mid-Atlantic Ridge. Due to the supply of solidified rock, the oceanic crust here is constantly growing. It not only grows upwards, but also to the sides. The two oceanic plates are pushed outwards. Because they spread apart in the process, this is also called a divergence zone.
In this way, new seabed is created and the ocean slowly widens – albeit only a few centimetres a year. But modern satellites can measure the continents with millimetre precision. From the movement, it can be calculated that the Atlantic has already become 25 metres wider since Columbus’ crossing in 1492.
But because the Earth as a whole is not getting any bigger, the increase in seabed must be compensated for elsewhere. This happens where oceanic crust submerges under continental crust: While the Atlantic continues to grow, the Pacific is slowly sinking under the plate margins of America and East Asia
Polar Regions – Arctic and Antarctic
The largest ice surfaces on earth are around the North Pole and the South Pole. Because of their special location, the polar regions receive very little sunlight and solar heat, and the summers are particularly short there. That is why it is always extremely cold there – temperatures of up to minus 70 degrees Celsius prevail throughout the year. The cold has allowed huge masses of ice to form in the polar regions.
The Arctic ice around the North Pole covers a large part of the Arctic Ocean in winter. It then covers an area of several million square kilometres. For the most part, this is a layer of ice that floats on the sea. In addition, the Arctic ice covers the northern areas of Europe, Asia and North America.
In contrast, the South Pole is located on a continent, Antarctica. Antarctica is the coldest place on earth. Its land mass is almost completely buried under a shell of ice and snow up to 4 kilometres thick. Almost three quarters of the fresh water on Earth is stored in this ice.
Humans, animals and plants have adapted to life in the “eternal ice”. Polar bears or reindeer, for example, protect themselves against the cold with a layer of fat and thick fur. Only a few people inhabit the Antarctic, the Arctic is somewhat more densely populated. The best-known inhabitants of the Arctic are the Inuit in North America and Greenland, but there are also the Lapps in northern Scandinavia and indigenous peoples in northern Siberia. In the past, they lived there as nomads and moved around with dog sleds. Today they use snowmobiles and many of them live in cities.
Hardly anything grows in the ice deserts around the poles because of the great cold. The ground between the polar regions and the cold temperate zone is permanently frozen to great depths. This ground is therefore also called permafrost after the Latin word “permanere” for “to last”. It only thaws slightly a few months a year. Then particularly hardy plants such as mosses, lichens or dwarf shrubs can grow on it. This region around the polar regions is also called subpolar tundra.
The polar regions are the coldest areas on earth. It is also here that it is apparent that the Earth is heating up: for some years now, researchers have been observing that the ice masses of the Arctic and Antarctic are melting. The consequences of this warming cannot yet be precisely estimated. But it is already clear that many habitats are threatened by the melting of the poles.
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