How much limestone on earth
How are caves formed?
Caves have always had something mysterious about humans: fairy tales and legends tell of devils, dragons, dwarfs and other figures who live in dark cavities deep inside the earth. But there are caves not only in the imagination, but also in reality.
What they all have in common is that hardly any sunlight penetrates from outside. That is why it is cool and dark here in the interior of the earth. In order for them to be considered caves, they must be at least large enough for humans to enter and, moreover, have been formed naturally. How this happens, however, is different.
Many caves are formed where water washes away soluble rock. Rock salt and gypsum are among the rocks that can be dissolved in pure water. Limestone, on the other hand, is only soluble in acidic water. The solution is washed away; sometimes entire streams or even rivers flow through the earth's interior. What remains are underground cavities that get bigger and bigger over time. A whole labyrinth of corridors can form here. These processes are also referred to as karst.
Some caves do not appear after, but at the same time as the rock. This is possible after a volcanic eruption. When thin liquid lava cools and solidifies on the surface, hot lava can still flow under this layer. When this lava flow has finally flowed away, tunnels that can be hundreds of meters long remain in the solidified rock. There are such lava caves in Hawaii, for example.
Because little light penetrates into the natural cavities, only a few living things are at home here. Tiny animals such as the olm or the cave shrimp are well adapted to the rather hostile conditions. People, on the other hand, only visited caves from time to time - as a shelter to protect themselves from wind and weather.
It is better not to get lost in its dark corridors: the labyrinth of the Mammoth Cave in the US state of Kentucky extends a full 591 kilometers. That is roughly as far as from Karlsruhe to Kiel. With this size, the Mammoth Cave is the longest known cave on earth.
Its extensive network of corridors and chambers runs through a soft layer of limestone. This is about 400 meters thick and was deposited in a shallow sea a good 300 million years ago. Later the limestone layer protruded from the sea and was leached out by acid rainwater: A hilly karst landscape full of fantastic caves was created, the corridors of which are even distributed over several floors.
The cave has been known to man for thousands of years. Finds of sandals, torches and even mummified human bodies are evidence of this. The mummies come from people who lived more than 2000 years ago and were probably looking for plaster of paris in the cave. They certainly didn't live here in the dark. Because only extremely adapted creatures like the cave blind fish, eyeless crayfish, beetles or crickets can do that.
Today the Mammoth Cave mainly attracts tourists. It is one of the oldest tourist attractions in North America, tours into the belly of the earth have been offered here since 1816.
"Lost John" - the mummy in the mammoth cave
Seven mummies have been found in the mammoth cave to date. All are over 2000 years old and have been preserved thanks to the special cave climate. In 1935, the last mummy to date was discovered and named "Lost John" because of its history: It comes from a man about 1.60 meters tall and about 45 years old who died when the ceiling of the cave collapsed. "Lost John" lived about 2300 years ago and is the only one of the seven mummies that has survived. All others were destroyed after being removed from the conservative atmosphere of the cave.
The four students who were reported missing on Sunday morning have been rescued. They entered the Falkensteiner Höhle near Bad Urach on Saturday. After a thunderstorm, the water level in the cave suddenly rose so much that their way back was blocked. Cave watch divers were able to free the four trapped people on Sunday.
The students were adequately equipped for a normal ascent of the cave: When they set out on Saturday morning at around 9 a.m. they wore helmets, suitable cave clothing, had light and provisions with them and easily reached their destination, the Eisele fall, just under 3 kilometers from the cave entrance. Here the passage ends through a collapse of the cave ceiling.
What the four cave visitors had not expected: Around 5 p.m. a heavy thunderstorm erupted near Bad Urach. Immediately after the heavy rain, the draining water in the cave increased so much that the students could not make the way back without diving suits. Their disappearance was noticed because they were expected at a party that evening. When she did not show up, her acquaintances alerted the police on Sunday morning. At around 10.20 am, four divers from the cave rescue team entered the cave and found the missing persons in the "Reutlinger Halle", an elevation inside the cave. With the help of the divers, they saw the light of day again at 12.45 pm.
The great danger of the Falkensteiner Cave
The Falkensteiner Höhle is traversed by the Elsach all year round. This watercourse is the great danger when entering the cave. After rains or melting snow, the water level can rise so much that large parts of the cave are completely under water. Such floods have already resulted in people being trapped inside on several occasions. In 1964, too, four people were stuck until after two and a half days they could finally be rescued from the interior of the earth by experienced cave divers. If the weather is dry, on the other hand, visitors can safely enter the front part of the Falkensteiner Cave, up to about 150 meters inside. To keep your feet dry, however, rubber boots are recommended.
Workers at the Biggethaler Kalkwerke in Sauerland were amazed when they suddenly looked directly into the belly of the earth on July 19th. A blast in the limestone had exposed a crevice. Driven by curiosity, they crawled in. What they saw inside left them speechless: a fabulous landscape of stalactites and other limestone figures adorned the ceiling, floor and walls. The quarry workers had opened the entrance to an underground magical world.
However, the discovery of the cave was not entirely unexpected. A stalactite cave has been specifically searched for around the Hanseatic city of Attendorn for several years. They wanted to market them profitably as a sight. It was not without reason that people hoped and suspected that there might be such a remarkable cave: While working in the limestone rock holes with stalactites had been found several times, but not of the necessary size.
All the better for the Attendorns that their wishes have now become reality. A name for the subterranean natural wonder has already been found: Atta Cave is to be named after the princess Atta, who gave the city of Attendorn its name.
Initially no access to the Atta cave
Mayor Heim wrote to the district administrator on July 24, 1907 to inform them about the discovery of the stalactite cave and its provisional securing:
“In the limestone quarry of the Biggethaler Kalkwerke a cave has been discovered which, according to several gentlemen who have seen the cave, is characterized by its extraordinarily interesting stalactite formations. Precautions have been taken to ensure that unauthorized persons do not have access to the cave in order to avoid damage to the structure. "
Karst landscape with holes
The landscape of the Swabian Alb is almost as perforated as an Emmental cheese. There are around 2,500 caves here. Some of them can be visited, including the falcon cave in the north or the bear cave in Wolfstal. But why are there so many large holes in the ground in this area of Germany in particular?
The reason for this is that the rock of the Swabian Alb consists of lime. This limestone is soluble in acidic water. Rainwater has the acid it needs in the form of carbonic acid: It is the result of the reaction of water with the gas carbon dioxide in the air. The limestone of the Swabian Alb gradually dissolves in the seeping acidic rainwater, similar to effervescent powder in a glass of water.
The weathering of the limestone begins on the surface of the earth. The acid rain digs gullies and crevices and forms jagged humps that become steeper and steeper. The water penetrates into the earth's interior through cracks and joints until it encounters a water-impermeable layer such as clay. Until then, it loosens the calcareous rock: small holes become large cavities, fine gullies become wide trenches. An extensive network of underground rivers, valleys and caves is created.
There is a lot of lime in the draining water. When this lime-containing water evaporates, the dissolved lime remains and is deposited again. Stalactites grow in such places. Sometimes the cavities also sink or collapse under the weight of the rock cover. In these places hollows can be seen on the surface of the earth, which are called sinkholes.
Such cave landscapes made of weathered limestone can not only be found in the Swabian Alb. They are also found in many other parts of the world, for example in Slovenia. This is where the name for this landscape, which we call karst, comes from. The Slovenian expression for it is something like that, namely Kras.
Some rocks look like they're striped. In the Dolomites, for example, such transverse bands can often be clearly seen. Sandstone or limestone quarries sometimes have similarly pretty patterns.
The "stripe design" is created when the rock is formed. The starting material is weathered debris that is carried away by water or the wind. Rivers, glaciers and dust storms lose their strength at some point: the courses of rivers become slower and slower towards the mouth and finally flow into the sea or a lake. Glaciers are advancing into warmer regions and melting. Dust storms also subside at some point. Then they can no longer move dust, sand and rubble. The crushed rock that is dragged along settles out. Over time, the deposited material forms an ever higher layer - the sediment. Such sediments, including the remains of dead animals or limestone shells, collect particularly on the seabed and on the bottom of lakes, where rivers wash up a lot of material.
Gradually, different sediments are layered on top of each other. A layer can, for example, consist of sandstone: During the dry season, the wind blew desert sand here. If the sea level rises again, this layer is covered by water: the limestone shells of marine animals sink to the sea floor and deposit another layer over the sand. Over millions of years the climate changed again and again and made the sea level fluctuate. This allowed different layers to deposit.
Over time, the sediment cover becomes thicker and thicker. Under the weight of one's own weight, the initially loose sediments are compressed more and more, small cavities disappear, the mass condenses. Further layers are deposited over it, the sediment becomes more and more solid and finally becomes sedimentary rock under pressure. This process is also called diagenesis in geology. For example, if the shells of tiny marine animals are pressed into stone, limestone is created. Fine grains of sand made of quartz cement together under the high pressure to form sandstone.
In addition to rubble, dead animals also settled, for example fish on the sea floor. Their bones and scales remained hermetically sealed and petrified. Such fossils are immortalized in the stone. Even after millions of years they reveal a lot about the time in which the sediment was formed. Therefore, geologists can read in the rock layers like a history book.
Usually only the top layer is visible to us. However, when a river digs its way through the sedimentary rock, lifts it up during mountain formation, or blasts it free in a quarry, we get a glimpse of the cross-section. The individual layers of sediment can then be easily recognized as "stripes" or bands in the rock.
From rock to grain of sand - weathering
Today the north of Canada is a gently undulating landscape. However, many million years ago there was a mountain range here. In fact, even high mountains can turn into small hills over a very long time.
The reason for this transformation: The rock on the earth's surface is constantly exposed to wind and weather. For example, if water penetrates into cracks in the stone and freezes, it splits the stone apart. This process is called frost blasting. The rock also becomes brittle through temperature changes between day and night and through the power of water and wind. In other words: it weathers. This process can also be observed in buildings or stone figures. During the weathering, the rock breaks down into smaller and smaller components up to fine grains of sand and dust. Different rocks weather at different rates: Granite, for example, is much more resistant than the comparatively loose sandstone.
Some types of rock even completely dissolve when they come into contact with water, for example rock salt and lime. Rock salt is chemically the same as table salt - and that already dissolves in ordinary water. Lime is somewhat more stable, but limestone also dissolves in acidic water. Acid is formed, for example, when rainwater in the air reacts with the gas carbon dioxide. This “acid rain” attacks the limestone and dissolves it over time. The weathering leaves rugged limestone landscapes on the surface of the earth, and caves are formed below the surface.
But not only solution weathering, heat and pressure also wear down and crumble rock under the earth's surface. Wherever plants grow, roots dig in, break up the rock piece by piece and also ensure that it is removed millimeter by millimeter.
In this way, weathering not only works on individual rocks, it gnaws at entire mountain ranges. It will take a few million years for the Black Forest to be as flat as northern Canada.
How do stalactites get into a cave?
It is dark, cool and often damp inside a stalactite cave. It's not particularly cozy here, but with a little lighting you can see fantastic structures on the ceiling and on the floor: the stalactites. But how do they get into the cave?
The answer to this already reveals its name: stalactites are created by dripping water. It is crucial that it is not pure water. The water droplets contain lime that has loosened from the surrounding limestone. When the water saturated with lime evaporates, lime residue remains. We know similar traces of lime from areas in which “hard water”, ie water with a high lime content, flows out of the tap. There, limescale deposits settle on the sink or on the bottom of a kettle. But what causes ugly stains there creates fantastic shapes in the caves: The limescale deposits grow slowly and gradually as moist, shiny cones from the ceiling: the dripstones or stalactites.
When calcareous water drips down from the stalactites, stalactites also form directly beneath them, like in a mirror image: the stalagmites. If stalactites and stalagmites are big enough, they grow together and form a column. In addition to the stalactites, there are other interesting structures in such a cave. Where water runs along the ceiling, traces of lime remain, which look something like curtains. They are therefore also called "stone curtains".
On average, stalactites grow about a millimeter in ten years. How fast it actually goes depends on the amount of water dripping down and how much lime is dissolved in it. The temperature in the cave also plays a role: the warmer it is, the more water evaporates. This leaves more lime and the stalactites grow faster.
If you want to visit a stalactite cave, you can go to the Swabian Alb: There are many of them in this karst area. But stalactite caves can also be found in other areas of Germany, for example the Atta cave in the Sauerland or the Iberg stalactite cave in the Harz Mountains.
Consequences of volcanic eruptions
Volcanic eruptions can have dire consequences. Hail of rock, ash rain, poisonous gases and glowing lava flows have already killed hundreds of thousands of people. During the eruption of Vesuvius in 79 AD alone, during which the cities of Pompeii and Herculaneum were buried, around 5000 people died. An entire city was also wiped out in Colombia: the eruption of the icy Nevado del Ruiz volcano in 1985 triggered several mudslides. The avalanches buried the city of Armero, 47 kilometers away, and 25,000 residents.
Tsunamis can also arise from volcanic eruptions: The explosion on the volcanic island of Krakatau in 1883 caused a tidal wave that flooded regions thousands of kilometers away. Even earthquakes sometimes follow such an explosive volcanic eruption. During these quakes, built-up tensions in the earth are discharged.
In Iceland, the eruption of over a hundred volcanoes in the Laki Fissure in 1783 triggered a famine. The outbreak released toxic gases into the air. The poison settled and contaminated the sheep pastures. The animals died from the poisoned food, an estimated ten thousand people because of the famine that followed.
The “laki fires” on Iceland were followed by a cooling that could still be felt far away. The rising ash cloud darkened the sky, strong winds came up and the temperature dropped. The whole of Northern Europe then experienced an unusually cold winter. In fact, volcanic eruptions change the climate. This is mainly due to the sulfur gases emitted, which form fine sulfuric acid droplets in the air that float in the atmosphere for a long time. The sunlight is scattered by the droplets and partly reflected back. This can lower the average temperature all over the world.
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