What supercontinents existed before Pangea?
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 each other again.
But not only the shape of the continents tells of how they 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 the years. This mountain range, which was connected millions of years ago, can still be seen well on a world map.
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 up and movement 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. According to current knowledge, 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 - a Luftikus?
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.
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 drift 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”.
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.
Oceanic and continental crust
The earth's crust is not built up in the same way everywhere. The earth's land masses consist of continental crust, the sea floor of oceanic crust. One of the differences is that in addition to oxygen, the continental crust mainly contains silicon and aluminum. The oceanic crust, on the other hand, also has a high proportion of magnesium. But that is by no means the only difference:
Oceanic crust forms on the sea floor, where magma rises and solidifies along the mid-ocean ridges. Since the crust is constantly growing back here, the two lithospheric plates are pressed outwards. The oceanic crust is therefore getting older and older towards the coasts. Some of the oldest pieces are around 200 million years old. They lie in the Atlantic off North America and east of the Mariana Trench in the Pacific. The five to eight kilometers thick oceanic crust does not get any older: because it is heavier than the continental one, it submerges in the event of a collision and is melted again in the interior of the earth.
The continental crust is lighter, but thicker than the oceanic crust: on average, it extends 40 kilometers, under mountains it can even be up to 80 kilometers deep. When exactly it was formed is still a mystery even to science. Evidence of this is provided by the oldest known rock on earth: It was found in northern Canada, is over four billion years old and is believed to be a remnant of the very first crust of the earth.
The ocean floor
The surface of the oceans glitters in a dark blue. It is hard to believe that the sea floor is sometimes many kilometers deeper and that a spectacular underwater landscape is hidden there below. Because the sea floor is not as smooth as the bottom of a swimming pool: On the sea floor there are high mountains, deep trenches and lava-spewing volcanoes as well as extensive plains.
The water in the oceans is not the same depth everywhere. The shallow shelf seas lie around the continents. Here the seabed slopes gently down from the coastline until it reaches a depth of around 200 meters below sea level. The bottom of the shelf seas consists of continental crust. Therefore it actually belongs to the mainland, even if it is washed over by sea water.
Only many kilometers away from the coast, on average after 74 kilometers, does the flat shelf area end with the shelf edge. From this edge it goes down steeply like a slide to a depth of about four kilometers. This steep slope forms the transition to the deep sea, into which no light can penetrate. That's why no plants grow down there. Only a few animal species were able to adapt to this habitat, despite the hostile conditions.
In the midst of the oceans rise mountains, the mid-ocean ridges. These underwater mountains stretch across the world's oceans for long stretches. In some places they protrude as islands above sea level. Iceland, for example, lies directly on the mid-Atlantic ridge, the longest mountain range in the world.
Deep trenches also crisscross the oceans. Most of them are in the Pacific. One of them is the Mariana Trench, the deepest trench in the world. It reaches down to 11,034 meters below sea level. Only two people have ever been down there: the oceanographer Jacques Piccard and his companion Don Walsh on their record dive in 1960.
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