Bold theory: the earth’s parts are moving!

During a conference of the Geological Society in Frankfurt, the meteorologist and polar researcher Alfred Wegener put forward a daring theory: According to him, the continents move on the earth. Colleagues of the geology express themselves skeptically to rejecting. If Alfred Wegener had claimed that the Earth was a disc, he would hardly have caused more astonishment among his listeners. According to Wegener all continents of our earth should have been united long ago to a single land mass. Pangaea he calls this supercontinent, which moved on the earth’s mantle and broke into two parts 200 million years ago. These two parts of the earth are supposed to have further divided and shifted. There would be clear indications of the breakup and movement of the continents: they fit into each other like puzzle pieces. It is also striking that the same animal species are found on different continents. So Africa and South America should have been one? For the experts Wegener’s speech sounds as credible as a fairy tale from thousand and one night. To this day, people are still convinced that the earth’s crust is firmly connected to its subsoil. According to current knowledge, the continents are fixed and were once connected by land bridges. Many geologists still disparagingly refer to Wegener’s continental drift as the “geopoetry of a weatherman”. For what remains unclear above all is the motor of the movement: What drives the continents? But research can no longer ignore Alfred Wegener’s theory. Can it be proven?

Alfred Wegener – an aeronaut?

Meteorologist Alfred Wegener became famous for a record he set in balloon flight: On April 5, 1906, he ascended with his brother Kurt and stayed aloft for more than 52 hours. This beat the previous world record by 17 hours. But the balloon flight was not just for fame, but above all for science: The Wegener brothers wanted to explore the atmosphere and test methods for determining locations. Alfred Wegener’s interest was not only in the weather and aviation, however, but also in the perpetual ice. Still in the year of his world record he set out to explore Greenland. He returned from this Greenland expedition in 1908. Since then, the 32-year-old scientist has been a lecturer in meteorology, astronomy and physics at the University of Marburg.

The supercontinent Pangaea

If you look at a world map a little closer, you’ll notice that the shapes of Africa fit North and South America almost as well as pieces of a jigsaw puzzle. And indeed, the continents are something similar to puzzle pieces pushed apart. Only when put together, they do not form a picture, but a single large continent: Pangaea.

Pangaea existed about 250 million years ago. In this supercontinent, all land masses of the earth were combined and surrounded by a single sea, called Panthalassa. About 200 million years ago, Pangaea broke into two parts – Laurasia in the north and Gondwana in the south. The two continents later broke into even smaller pieces. After that, North and South America, Africa, Asia and Europe were already roughly recognisable in their present form. However, at that time these continents were much closer together than they are today. Only in the course of time did they become more and more distant from each other, because a mid-ocean ridge had broken up between America in the west and Africa and Eurasia in the east. A new ocean was formed: the Atlantic Ocean, which continues to grow today. North and South America therefore move away from Europe and Africa by a few centimetres every year.

Currents in the earth’s hot interior are the motor for the travel of the earth’s parts and the formation of oceans. These set the plates in motion very slowly. In some places, the plates move away or break apart, in others they drift back towards each other.

But it is not only the shape of the continents that tells us how they were once connected. Mountain ranges also indicate where parts of the earth were one long ago. The Appalachian Mountains in the north-east of America, for example, are part of a mountain range that stretches across Greenland and Scotland to Norway. The mountains were separated by the North Atlantic Ocean, which has slid in between over the course of time. This mountain range, which was connected millions of years ago, can still be seen clearly on a map of the world.

Continents on the move

For a long time, it was thought that the earth’s land masses stood rigidly in place. Later it turned out that the opposite is the case. The continents of our planet are moving! Like huge ice floes, they drift in different directions, albeit not very fast. Their speed is about the same as the growth of a fingernail. But why are the continents constantly on the move?

The earth’s crust that envelops our planet is brittle and cracked. It resembles a cracked eggshell and is composed of seven large plates and many smaller ones. Some of them form the continents, others the ocean floor. These plates of the Earth’s crust drift around on a hot, viscously flowing rock mush and are driven by movements in the Earth’s interior, or more precisely: by currents in the Earth’s mantle. Experts also say: they drift. All these processes surrounding the movement of the Earth’s plates are called plate tectonics, and the movement itself is also called plate drift.

Where the individual plates border on each other, the Earth is particularly active. At some of these plate boundaries, hot rock from the Earth’s mantle penetrates upwards and cools down. New crust forms here: the two plates grow and are pushed apart as a result. In contrast, where two plates collide, the lighter of the two – the continental crust – is crumpled and folded into mountains. The heavier of the two – the oceanic crust – slowly disappears into the depths. The heat in the Earth’s interior melts its rock again. While the edge of the plate sinks into the depths, it pulls the rest of the plate behind it and thus additionally drives the plate movement.

Volcanic eruptions, earthquakes, long mountain ranges and deep ocean trenches accumulate along such plate margins. Most of the turmoil on the Earth’s surface is caused by the largest of its plates: it is the Pacific Plate, which is moving northwest at a rate of about 10 centimetres per year. Most of the earth’s active volcanoes are found at its edges, and violent earthquakes shake the region. Because of the frequent volcanic eruptions and quakes, this plate boundary is also called the “Pacific Ring of Fire”.

Why does it look different on Earth than on the Moon?

It doesn’t look very inviting on the moon: The surface is dry and covered with a grey layer of dust. Meteorite impacts have torn huge craters into the ground, which filled with lava from the interior of the moon. Around these lava basins, kilometre-high crater rims tower up as mountain rings.

Our blue planet is completely different – if only because three quarters of it is covered by water. But water not only covers a large part of the earth, it also shapes its land mass: rivers, glaciers and the surf of the sea work the rock, break it up and rearrange it. This is how valleys, coasts and ever new layers of rock are formed.

The interior of the moon today is solid and rigid. The Earth, on the other hand, has a liquid mantle on which movable plates float. The movement of the Earth’s plates causes mountains to fold up, deep-sea trenches to form and volcanoes to spew fire and ash.

Unlike the moon, the earth has an air envelope, the atmosphere. It is in this envelope of air that the weather is created. Wind, rain and snow have worked and shaped the earth’s surface over millions of years. The atmosphere also acts as a protective shield, slowing down meteorites and preventing them from burning up.

Because the moon has no such atmosphere, meteorites strike its surface unchecked and suddenly crumble the rock to dust. But meteorites are the only forces that shape the lunar landscape. Because there is no water, no atmosphere and no plate tectonics, the influences that make our earth’s surface so varied are missing.

The first humans to set foot on the barren lunar landscape were astronaut Neil Armstrong and his colleague Edwin E. Aldrin. The footprints they left behind when they landed on the moon in 1969 can still be seen today – because neither wind nor water cover the tracks on the moon.

What happens inside the earth?

The lava lamp – cult from the 70s: Thick bubbles slowly rise in a viscous liquid, sink back to the ground and bubble up again. A similar circular movement of hot, viscous rock also takes place directly beneath our feet in the earth’s interior. But what is the cause of this?

Whether it’s a lava lamp, water in a saucepan or the Earth’s mantle, the reason is always the same: when a liquid is heated, warm bubbles rise to the top. This is because the tiny particles that make it up move back and forth more and more as the temperature increases. To do this, they need more space and no longer crowd together so closely. There are now fewer particles in the same volume than in the surrounding area, so it is lighter and rises upwards. There, this bubble cools down again and the particles need less space. The volume becomes heavier than the surroundings, sinks again and the cycle begins again. When a liquid flows in a circle because of a temperature difference, this is also called convection.

In a lava lamp, the heat from the lamp causes the liquid to move. In the earth’s interior, the hot, solid inner core is the heat source. It heats the liquid metal of the outer earth core above it. This rises upwards and passes its heat on to the earth’s mantle, causing it to cool down gradually. It then sinks back down, where it heats up again.

A second, similar cycle takes place in the Earth’s mantle: Its heated rock moves upwards from the core towards the Earth’s crust, to which it again gives off heat. After it has cooled, it flows downwards to the Earth’s core, where the cycle begins again. Because the Earth’s mantle rock is very tough, the convection current only moves a few centimetres per year – so a cycle takes a long time.

The rock currents in the Earth’s interior exert great heat and pressure on the thin Earth’s crust. It cannot always withstand this: From time to time, it cracks open in individual places and hot rock escapes to the earth’s surface through volcanoes.

Where plates scrape past each other

The inhabitants of San Francisco and Los Angeles live on a powder keg: an earthquake can shake the California coast at any moment. The region has already experienced many quakes, one of which was particularly devastating. On 18 April 1906, the earth trembled so strongly that entire quarters of San Francisco collapsed, killing around 3000 people. But why is the danger of earthquakes so great on the west coast of the USA?

Along the Californian coast, two plates of the Earth’s crust move past each other: the North American Plate and the Pacific Plate. Both are drifting northwest, but the Pacific Plate is moving a little faster. It is therefore slowly “overtaking” the North American Plate. As a result, Los Angeles and San Francisco are moving closer together, by about 6 centimetres every year. If they move at the same pace, in about 12 million years Los Angeles will be on the Pacific Plate north of San Francisco, which is on the North American Plate.

Where the plates meet, a long rift runs through the land, clearly visible. This San Andreas Fault is over 1100 kilometres long. Here, the different speeds of the Earth’s plates cause extremely strong tensions in the rock. This is because the two plates don’t just slide past each other, they get caught up in each other. At some point, the tension between the rock masses is so great that the faster Pacific plate moves forward with a jerk. Such jerky movements of the plate manifest themselves in more or less strong earthquakes. For this reason, California will be shaken by earth tremors again and again. Some researchers even claim that a huge quake is already imminent in a few years. But no one can predict exactly when that will be.

Where plates collide

When two vehicles collide, their sheet metal is crumpled together. Something similar happens when two plates of the earth’s crust collide. Then their rock is pushed together and very slowly folded into enormous creases – this is how fold mountains are formed. What the crumple zone is in a car accident, the mountain range is in a plate collision – except that a car accident takes place in a fraction of a second, whereas a plate collision takes many millions of years.

The Alps were formed in exactly the same way: Africa pressed against the Eurasian continent and folded up the mountains. The Himalayas in Asia and the Andes in South America also owe their origin to the collision of moving crustal plates.

In such a crash, the rock of the lighter plate pushes upwards, the heavier one sinks into the depths. This process is called subduction, and the area where the plate dives is called a subduction zone. Along these zones there are often deep gullies, which is why they are easy to recognise. The deepest of these is the Mariana Trench in the Pacific Ocean. This deep ocean channel is located where the Pacific Plate dips below the Philippine Plate.

The further the earth’s crustal plate disappears into the earth’s interior, the hotter it gets. The rock melts and magma forms in the depths. The growing pressure can force it upwards again. Where it reaches the earth’s surface, volcanoes spew lava and ash. There are whole chains of such volcanoes around the Pacific Plate, for example in Indonesia. Because one volcano follows another here, this plate boundary is also called the “Pacific Ring of Fire”.

Not only do volcanoes erupt at such plate edges. Often, the earth also shakes because the plate movement causes enormous pressure and growing tensions. As soon as these are discharged, quakes shake the earth’s surface. In Japan, for example, three plates meet at once: the Pacific, the Philippine and the Eurasian. This is why Japan is so often hit by violent earthquakes.

Where slabs give way

A long deep fissure gapes in the earth and grows ever 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 up 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.

The Earth’s Outermost Shell

Like an egg from an eggshell, the Earth is also surrounded by a hard shell. This outermost layer surrounds the Earth’s mantle and is called the Earth’s crust. If you compare the earth to a peach, the earth’s crust is – relatively speaking – as thick as its skin. Under continents, it reaches an average depth of 40 kilometres, under the oceans even only about seven kilometres.

Below this lies the outer part of the Earth’s mantle, which reaches down to a depth of about 100 kilometres. It is also solid, but consists of heavier rock. The earth’s crust and this outermost part of the mantle together are also called the “lithosphere”. This solid layer of rock is broken into plates of different sizes that drift very slowly on the hot, viscous mantle.

Where the molten rock from the hot mantle penetrates upwards, the earth’s crust can break open. Lava then flows out and becomes new crust. This mainly happens where the plates of the lithosphere adjoin each other, such as at the mid-ocean ridges.

In Iceland, for example, these plate boundaries are clearly visible: Cracks and furrows run through the earth’s crust here, where the Eurasian and North American plates drift away from each other. There is also a plate boundary in the Mediterranean region. Because the African plate is pressing against the Eurasian plate here, there are many volcanoes and earthquakes in Italy.

The crust is covered by the soil. The soil of the land masses forms from weathered rock and the remains of animals and plants. The seabed, on the other hand, develops from deposits such as clay and the sunken remains of marine organisms. On the coasts, the seabed additionally consists of deposited debris that has been eroded from the mainland and washed into the sea.