Is the earth an example of constant movement?

6.11.1912

During a meeting of the Geological Society in Frankfurt, the meteorologist and polar researcher Alfred Wegener put forward a daring theory: In his opinion, the continents move on earth. Colleagues in geology are skeptical or even negative.

If Alfred Wegener had claimed that the earth was flat, he would hardly have caused astonishment among his listeners. According to Wegener, all the continents of our earth are said to have been united into a single land mass a long time ago. He calls this supercontinent Pangea, which moved on the Earth's mantle and split into two parts 200 million years ago. These two continents are said to have further divided and shifted. There are clear indications of the breaking and moving of the continents: They fit together like pieces of a puzzle. It is also noticeable that the same animal species occur on different continents.

So Africa and South America should have been one? To the professional world, Wegener's speech sounds as believable as a fairy tale from the Arabian Nights. One is still convinced to this day that the earth's crust is firmly connected to its subsurface. As far as we know, the continents are fixed and were once connected to each other by land bridges. Many geologists still disparagingly refer to Wegener's continental drift as the “geopoetry of a weather frog”. The main thing that remains unclear is the motor of movement: what drives the continents? But research can no longer ignore Alfred Wegener's theory. Can it also be proven?

Alfred Wegener - an airship?

The 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 in the air for over 52 hours. This exceeded the previous world record by 17 hours. But the balloon flight not only served for fame, but above all for science: The Wegener brothers wanted to explore the atmosphere and test methods of location determination. Alfred Wegener's interest is not only in the weather and aviation, but also in the eternal ice. 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 of Pangea

If you take a closer look at a world map, you will see that the shapes of Africa fit North and South America almost as well as pieces of a puzzle. Indeed, the continents are like pieces of a puzzle pushed apart. Only when put together they do not result in a picture, but a single large continent: Pangea.

Pangea existed about 250 million years ago. In this supercontinent all land masses on earth were united and surrounded by a single sea, called Panthalassa. About 200 million years ago Pangea was divided into two parts - Laurasia in the north and Gondwana in the south. The two continents later broke up into even smaller pieces. After that, North and South America, Africa, Asia and Europe could already be recognized in their present form. However, these continents were much closer together then than they are today. It was only in the course of time that they diverged more and more, because a mid-ocean ridge had broken up between America in the west and Africa and Eurasia in the east. A new ocean was created: the Atlantic, which continues to grow today. As a result, North and South America move a few centimeters away from Europe and Africa every year.

The motor for the journey of the continents and the formation of oceans are currents in the hot interior of the earth. These set the plates in motion very slowly. In some cases, the plates soften or break apart, at other points they drift towards one another again.

But not only the shape of the continents tells of how they were once connected. Mountain ranges also indicate where continents were one long ago. The Appalachians in Northeast America are part of a mountain range that stretches across Greenland and Scotland to Norway. The mountains were separated by the North Atlantic, which has slipped between them over time. This mountain range, which was connected millions of years ago, can still be seen well on a world map.

Continents on the move

For a long time it was thought that the land masses of the earth would stand rigidly in place. It later turned out: the opposite is the case. The continents of our planet are moving! Like huge ice floes, they drift in different directions, albeit not very quickly. Their speed corresponds roughly to the growth of a fingernail. But why is it that the continents are constantly on the move?

The earth's crust that envelops our planet is brittle and cracked. It resembles a cracked egg shell and is made up of seven large and many smaller plates. Some of them make up the continents, others make up the ocean floor. These plates of the earth's crust float around on a hot, viscous rock slurry and are driven by movements in the earth's interior, more precisely: by currents in the earth's mantle. Experts also say: you are drifting. All of these processes related to the movement of the earth's plates are called plate tectonics, and the movement itself is also known as plate drift.

The earth is particularly active where the individual plates adjoin one another. At some of these plate boundaries, hot rock penetrates upwards from the earth's mantle and cools down. Here new earth crust forms: the two plates grow and are thereby pushed apart. On the other hand, where two plates collide, the lighter one of them - the continental crust - is crumpled up and unfolded to form mountains. The heavier of the two - the oceanic crust - is slowly disappearing into the depths. Due to the heat in the earth's interior, their rock is melted again. As the edge of the plate sinks into the depth, 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 edges. Most of the unrest on the earth's surface brings with it the largest of its plates: it is the Pacific plate, which is moving northwest at a rate of about 10 centimeters per year. Most of the world's active volcanoes can be found at their edges, and violent earthquakes shake the region. Because of the frequent volcanic eruptions and earthquakes, 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 layer of gray dust. Meteor impacts have torn huge craters in the ground that filled with lava from inside the moon. Around these lava basins, kilometer-high crater edges pile up as mountain rings.

Our blue planet is completely different - if only because three quarters of it is covered by water. The water not only covers a large part of the earth, it also forms its land mass: rivers, glaciers and the surf of the sea process the rock, crush it and move it around. This is how valleys, coasts and ever new layers of rock are created.

The interior of the moon is solid and rigid today. The earth, on the other hand, has a liquid mantle on which movable plates float. The movement of the tectonic plates causes mountains to unfold, deep-sea trenches to form and volcanoes to spew fire and ashes.

Unlike the moon, the earth has a shell of air, the atmosphere. The weather is created in this atmosphere. Wind, rain and snow have worked and shaped the earth's surface over millions of years. The atmosphere also acts as a protective shield that slows down meteorites and lets them burn up.

Because the moon has no such atmosphere, meteorites hit its surface unchecked and suddenly crumble the rock into 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 people to step onto the barren moonscape were astronaut Neil Armstrong and his colleague Edwin E. Aldrin. The footprints they left when they landed on the moon in 1969 can still be seen today - because neither wind nor water cover their tracks on the moon.

What is happening inside the earth?

The lava lamp - cult from the 70s: thick bubbles rise slowly in a viscous liquid, sink back to the ground and bubble up again. A similar circular motion of hot, viscous rock also takes place directly under our feet in the interior of the earth. But what is the reason for this?

Regardless of whether it is 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 upwards. That's 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 huddle together so closely. There are now fewer particles in the same volume than in the vicinity, so it is lighter and rises upwards. There this bubble cools down again and the particles take up less space. The volume piece becomes heavier than the surroundings, sinks again and the cycle starts all over again. When a liquid flows in a circle due to a temperature difference, it is also called convection.

In a lava lamp, the heat from the lamp sets the liquid in motion. In the interior of the earth, the hot, solid inner core of the earth is the heat source. It heats the overlying liquid metal of the outer core of the earth. This rises up and transfers its heat to the earth's mantle, which gradually cools it down. Then it 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 in turn gives off heat. After it cools down, it flows down 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 centimeters per year - a cycle lasts a long time.

Due to the rock currents in the interior of the earth, great heat and pressure act on the thin earth crust. It cannot always keep up: Every now and then it tears open in individual places and hot rock escapes through volcanoes to the surface of the earth.

Where plates scrape past each other

The residents of San Francisco and Los Angeles live on a powder keg: at any moment an earthquake can shake the California coast. The region has already experienced many quakes, one of which was particularly devastating. On April 18, 1906, the earth trembled so badly that entire neighborhoods of San Francisco collapsed, killing around 3,000 people. But why is the danger of earthquakes so great on the west coast of the USA in particular?

Two plates of the earth's crust move past each other along the California coast: the North American and Pacific plates. Both are drifting northwest, but the Pacific plate is a little faster. It is therefore slowly "overtaking" the North American record. So it happens that Los Angeles and San Francisco get closer and closer, by about 6 centimeters every year. If they move at the same pace, Los Angeles will be on the Pacific plate north of San Francisco, which is on the North American plate, in around 12 million years.

Where the plates meet, there is a clearly visible long crack through the land. This San Andreas trench is over 1100 kilometers long. Here, the different speeds of the earth plates cause extremely strong stresses in the rock. Because the two plates don't just slide past each other, they hook into each other. At some point the tension between the rock masses is so great that the faster Pacific Plate moves forward with a jolt. Such jerky movements of the plate are expressed in more or less strong earthquakes. Because of this, California will continue to be shaken by tremors. Some researchers even claim that a tremendous quake would be 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 laid into huge folds - this is how fold mountains are created. What the crumple zone is in a car accident, the mountains are in a collision of plates - only that a car accident takes place in fractions of a second, whereas a plate collision takes many millions of years.

This is exactly how the Alps came into being: Africa pressed against the Eurasian continent and unfolded the mountains. The Himalayas in Asia and the Andes in South America also owe their origins to the collision of migrating crustal plates.

In such a crash, the rock of the lighter plate is pushed upwards, the heavier plate sinks into the depths. This process is called subduction, the area in which the plate descends, the subduction zone. There are often deep gullies along these zones, which is why they are easy to see. The deepest of them is the Mariana Trench in the Pacific Ocean. This deep-sea channel lies where the Pacific plate dips under the Philippine one.

The further the earth's crustal plate disappears in the interior of the earth, the hotter it gets. The rock melts and magma forms in the depths. Due to the increasing pressure, it can be pressed up again. Where it penetrates to the surface of the earth, volcanoes spew lava and ash. There are entire chains of such volcanoes around the Pacific Plate, for example in Indonesia. Because one volcano follows the other, this plate boundary is also called the “Pacific Ring of Fire”.

Not only do volcanoes erupt at such plate edges. The earth also frequently trembles because the movement of the plates creates tremendous pressure and increasing tensions. As soon as these discharge, quakes shake the earth's surface. In Japan, for example, three plates meet: the Pacific, the Filipino and the Eurasian. It is for this reason that violent earthquakes hit Japan so often.

Where plates diverge

A long, deep crack gapes in the earth and is getting wider and wider. Huge forces are tearing the earth's surface to pieces: the East African Rift runs along this break through the continent. Africa began to break up here 20 million years ago. Hot magma from the interior of the earth pushed upwards and tore the earth's crust apart. Since then, the pieces of crust have drifted apart, by about an inch every year. The fact that the earth is very active here can also be seen from the many volcanoes that rise along the rift. Should seawater ever penetrate, the East African Rift will become an ocean. Something similar happened in the Red Sea. The African and Asian continental plates have been separating there for 25 million years. The resulting crack was flooded by sea water.

There where continental Crust breaks apart, one arises Rift valley. Where against it oceanic When pieces of crust move away from each other, mountains grow on the sea floor: the Mid-ocean ridges. They consist of magma that seeps up from the Earth's mantle through the oceanic crust. New sheet material is formed here. It presses itself, so to speak, between two oceanic plates and solidifies to form basalt rock that piles up further and further.

In some places the mid-ocean ridges protrude as islands above sea level. Iceland, for example, and the still young Icelandic island of Surtsey are nothing more than parts of the Mid-Atlantic Ridge. The oceanic crust is constantly growing here due to the replenishment of solidified rock. It not only grows in height, but also to the sides. The two oceanic plates are pushed outwards. Because they spread apart in the process, one also speaks of one Divergence zone.

In this way, new seabed is created and the ocean is slowly getting wider - but only a few centimeters a year. But modern satellites can measure the continents with millimeter precision. From the movement one can calculate that the Atlantic has been 25 meters wider since Columbus' crossing in 1492.

But because the earth as a whole is not getting any bigger, the increase in the seabed has to be compensated for elsewhere. This happens where the oceanic crust is submerged under the continental crust: While the Atlantic continues to grow, the Pacific slowly sinks under the plate margins of America and East Asia.

The outermost shell of the earth

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 - in relative terms - as thick as its skin. Under continents it reaches an average of 40 kilometers deep, under the oceans it is only about seven kilometers.

Below is the outer part of the earth's mantle, which extends to a depth of around 100 kilometers. 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 has broken into slabs of different sizes, which slowly drift around on the hot, viscous mantle of the earth.

Where the rock melt penetrates upwards from the hot earth's mantle, the earth's crust can break up. Then lava flows out, which becomes the new crust of the earth. This mainly happens where the plates of the lithosphere adjoin one another, such as on the mid-ocean ridges.

In Iceland, for example, these plate boundaries are easy to recognize: cracks and furrows run through the earth's crust, 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 in Italy and there are always earthquakes.

The crust is covered by the bottom. The soil of the land masses is formed from weathered rock and remains of animals and plants. The sea floor, on the other hand, develops from deposits such as clay and sunken remains of marine organisms. On the coasts, the sea floor also consists of deposited rubble that was removed from the mainland and washed into the sea.