How does water affect the weathering of stones?

From rock to grain of sand - weathering

Today the north of Canada is a gently undulating landscape. However, many millions of years ago there was a mountain range here. In fact, even high mountains can turn into small hills over a very long period of 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.

30.9.2008

It is a sensation for science: In the north of Canada, geologists have come across the oldest rocks ever discovered. They are part of the Nuvvuagittuq greenstone belt on Hudson Bay and are over four billion years old.

An international team of researchers has now dated the rocks in northern Canada to 4.28 billion years. That would make it just 300 million years younger than our solar system. Now the scientists are investigating whether the ancient rocks are a remnant of the very first crust that was once separated from the earth's mantle. Then the discovery could help unravel some of the secrets of the very early history of the earth. Perhaps the rocks reveal something about where and when life began? The researchers also hope to be able to read in the rock how the atmosphere changed and when the first continent of our earth was formed.

Incidentally, the name greenstone belt comes from the color of its metamorphic rock. It is the minerals contained in the rocks that give them a greenish color in some places.

Age ranking of the "veterans"

They are all old and wrinkled. But the chunks of the oldest rocks are separated by millions of years. Until recently, when the rock from the Nuvvuagittuq rock belt was dated an impressive 4.28 billion years ago, another rock in northern Canada was considered the oldest rock on earth: the so-called Acasta gneiss in the northwest of the country. After all, it is 4.03 billion years old. With its old age, the old gneiss surpasses a formation of ribbon iron in Greenland: This now ranks 3rd on the age scale of the rocks. The rocks in Greenland are “only” 3.9 billion years old!

15.1.1990

For hours yesterday, two tourists were trapped on a rock in the raging sea. One of the two arches of the rock sculpture “London Bridge” on Australia's famous “Great Ocean Road” suddenly collapsed. As a result, the way back was cut off for the visitors. They had to be rescued by a helicopter.

The young couple had walked to the end of the second arch to enjoy the fantastic view of the sea and coast. Once there, they heard an ominous crunch. When they looked around, the arch had already collapsed, cutting off the connection to the bank. Fortunately, no one was on the first arch, and there were no other victims. After five hours of waiting, the couple were happily brought back ashore by helicopter.

The double arch of "London Bridge" was one of the most famous rock formations on Australia's south coast. Wind and waves carry away this coast more and more and made sure that part of the tourist attraction collapsed. After the collapse, the "London Bridge" was renamed without further ado: It is now called "London Arch".

On the "Great Ocean Road"

The surf on Australia's south coast, along which the famous Great Ocean Road runs, is wild. The stormy sea has already claimed many victims here: Over a hundred ships have already crashed into the rocky coast. Wind and waves grind everything that gets in their way. And that is above all the relatively soft limestone with its bizarre rock colossi: London Bridge was just one of them, the "Twelve Apostles" or the "Island Archway" are also world-famous. The collapse of “London Bridge” shows how fragile the coast is: The rock crumbles in the raging sea, almost like sugar in hot tea. Without a break, the forces of nature gnaw at the coast and redesign it. So if you want to see the twelve apostles in full splendor, you should hurry.

2.9.1806

Terrible devastation was caused by a landslide in the Schwyz district on September 2nd. After heavy rains, a rocky peak of the Rossberg broke off in the late afternoon. The masses of earth below began to slide and in a few minutes buried the villages of Goldau and Röthen as well as parts of Lauerz and Buosingen. 457 people were killed in the natural disaster.

In the weeks before, it had rained almost continuously and the layers of the earth softened. At around 5 p.m., the rock loosened and thundered down into the valley with force. Blocks of stone weighing several pounds were thrown through the air, dragging houses, people and cattle. The destructive masses of earth buried the neighboring villages under a layer of rubble ten to fifty meters high and even rolled up the opposite Rigiberg. Some of the rock masses thundered into the Lauerzer See. This triggered a tidal wave that also claimed several lives.

A total of 457 people died in the Goldau landslide. Including seven people from a Bern travel group who reached Goldau at the time of the landslide, of all times. 111 houses, 2 churches and 2 chapels were buried, 220 barns and stables were destroyed and 323 head of cattle were killed. In addition to the Basel earthquake of 1356, the Goldau landslide is the largest natural disaster in Switzerland to date.

Appeal for donations

The Swiss Confederation has already reacted to the Goldau disaster: yesterday, the neighboring cantons of Zug and Lucerne sent aid workers to the affected region. Delegates from Zurich and Bern are expected to arrive soon to support the rescue work and the reconstruction of the villages. Rapid help for the survivors is now required. Switzerland is therefore calling for donations across the country. You too can help the victims of the Goldau landslide! You can get addresses and information directly from the editorial office.

31.8.2002

In the north of Jutland, a huge dune has set out to migrate. Their masses of sand now spilled the local lighthouse. Its operation was given up years ago. Now the drifting sand museum, which was located in one of its outbuildings, is also closing.

The lighthouse sent its first signal out to sea on December 27, 1900. But on the rock at his feet a huge dune grew: the Rubjerg Knude. Over time the sea drew closer, the wind blew large amounts of sand up the cliff. Beach grass was planted to stop the drifting sand, but the dune remained unimpressed and grew bigger and bigger. When the light from the lighthouse could no longer be seen from the sea, the 23-meter-high tower was shut down on August 1, 1968.

In contrast, a museum opened in an outbuilding of the tower that provided information about the drifting sand. When the fight against the drifting sand around the tower was given up in 1994, the wandering of the dune began. Now it moves forward about 10 meters per year. And the museum is now falling victim to the quicksand itself. Three of the tower's outbuildings have already sunk, and now the museum has to close too. But the dune keeps moving and at some point will release the building again. However, in the not too distant future they will fall victim to erosion on the coast and fall into the sea.

A whole village gone with the wind

The inhabitants of Rubjerg have long since left the worst drifting sand behind them: More than 300 years ago it was very cold here, as it was in all of Northern Europe, the "Little Ice Age" prevailed. The sea level is about a meter lower, the beaches are wide. Your sand can easily be blown away by the gale winds. The devastating sandstorms make it more than uncomfortable for the residents of the area: They flee. What remains is a lonely church in a desert environment. The believers continue to come to the service, but the path through the dunes becomes too difficult for them. In 1904 they completely demolished the church and rebuilt it two kilometers away in their new parish.

24.8.2010

It crumbles, splinters and weathered: the ravages of time are gnawing at Cologne Cathedral. Acid rain has already eroded the famous building. Air pollution laws have been reducing pollution for a number of years. But pigeon droppings, exhaust fumes and the weather continue to affect the old walls and never let the craftsmen of the cathedral builder become unemployed.

The foundation stone of Cologne Cathedral was laid on August 15, 1248. Since then, around fifty different rocks have been set in the Rhenish sand here. The builders only built many of them on a trial basis; not every stone could withstand the weather. In addition, the rock had to come from close by, as transport was incredibly expensive in the Middle Ages. As a result, the cathedral consists mainly of trachyte, shell limestone, sandstone and basalt. The calcareous sandstones and shell limestone are particularly susceptible to weathering and environmental influences. These are already badly pitted. In order to save the sensitive shell limestone from weathering, various protective coatings were tried out. That should at least slow down the crumbling. In contrast, the trachyte from the Drachenfels has held up well. The basalt rocks are also weatherproof and in good condition to this day.

Despite all efforts, components have to be replaced again and again. Every year 15 to 20 cubic meters of natural stone are used to preserve the famous church building. Even if the Cologne Cathedral was already completed in 1880: The stonemasons of the cathedral building have their hands full to this day!

Stone intolerance

The Cologne Cathedral evidently weathers more than other comparable buildings. Conservationists now have a guess as to why that is so: The cathedral is built from many different types of stone. And not all rocks get along with each other. For example, the damage is particularly severe where trachyte from Drachenfels meets sandstone from Obernkirchen. A research group is now to find out whether and why some rocks actually damage each other and which of them can get along well with each other.

What causes erosion?

When rock weathers, it seldom remains in its original location. Rock debris often rolls down the slope, is washed away by the water or pushed away by masses of ice. The wind can also carry fine rock dust or sand with it. Regardless of whether the rock is removed by water, ice, wind or gravity, all of these processes are called "erosion".

The erosion by running waters is particularly drastic. Streams and rivers dig a bed in the ground, rock slides down, a valley forms. If a glacier rolls down the valley, it planes this valley wider through the scree it has carried along with it. Long after the ice has melted, you can tell from such trough valleys that there was a glacier here. The surf of the sea, on the other hand, attacks the coast. Steep cliffs are hollowed out and collapse, sandy beaches are washed away by the waves. In deserts, the wind sweeps away large areas of sand. The harder it blows, the more sand it can take with it. A sandstorm gradually removes obstacles made of solid rock like a sandblasting fan.

When rain and wind wash or blow away the soil cover over large areas, we speak of soil erosion. Soil erosion is also used in the case of landslides on slopes. The problem: The fertile upper layer of the soil disappears. In the worst case, it can no longer be used for agriculture.

If the soil is overgrown with plants, this slows down erosion. The roots of the plants hold the soil in place and prevent the wind and water from carrying it away. If the plant cover is destroyed, for example by deforestation, the soil lacks this support and it is eroded.

Wind erosion - From shifting dunes and mushroom rocks

Wherever wind sweeps over sandy, dry ground, it drags fine grains with it and later drops them again. In this way, sand hills pile up - the dunes. Such sand dunes are mainly found in arid deserts such as the Sahara, the Gobi or the Namib Desert. Their dunes can be over 200 meters high and many kilometers long.

To see a dune, you don't have to go into the desert at all: there are also dunes on the coasts, in Germany for example on the North or Baltic Sea coasts. The sand that is blown away from the beach by the wind piles up inland to form dunes. If you want to go to the beach, you often have to find a way through or over the dunes.

Some dunes hardly move from the spot, for example when they are overgrown with beach grass. Others, on the other hand, roll forward in the direction of the wind, similar to the waves of the sea, the shifting dunes. The Rubjerg Knude on the coast of Denmark is a particularly fun dune. This almost 100 meter high dune is moving towards the northeast and has even rolled over a lighthouse on its journey.

Dunes have different shapes. Some are curved like crescents or sickles - the sickle dunes. Others form a wall across the direction of the wind, the transverse dunes. Both rise slightly on the windward side. On the side facing away from the wind, they fall steeply downwards. And some dunes even start their own song: When sand avalanches break out of the dune and the grains of sand collide, they make humming or humming noises: The dune is "singing"!

But wind and sand don't just form dunes. Flying grains of sand can grind rocks in the landscape like sandpaper. Even hard rock can get a new shape through this wind cut: Towering rocks are scraped off and hollowed out at their feet over time. After all, they tower up like mushrooms - a mushroom rock has emerged.

constant dripping wears away the stone

Deep gorges in the mountains, wide sandy beaches by the sea and wide rivers that meander through meadows and fields - all of these are landscapes that we know well. Because they are so varied, we find them impressive and beautiful.

The sculptor of all these landscapes is the water cycle. Sooner or later, water forms the surface of the earth more strongly than any other force. It washes away soil after a downpour. It digs into the ground and loosens parts of the rock. It carries earth and weathered rock debris with it down into the valley. Where the water drains off more slowly, it lets go of its burden of silt, sand and rubble. When there is high water, it floods the flat areas of a valley, the river floodplains. Here, too, it deposits fine mud. When the water finally flows into the sea, it works the coasts and forms very different landscapes, for example cliffs or long sandy beaches.

Water also shapes the landscape in the form of ice. If water freezes in cracks in the stone, it bursts the stone. As a glacier, it planes out notch-shaped river valleys to form round trough valleys. And the moraine landscape in the foothills of the Alps with its boulders and boulders is the result of glaciers that formed the subsoil a long time ago.

Carved in stone - landscapes made of sedimentary rock

Like the layers of a cake, different sedimentary rocks can be stacked on top of each other. If the subsoil beneath the layers rises, they are tilted. When tectonic plates collide, they are compressed and unfolded like in the Alps. Weathering and erosion have gnawed these sediment layers over millions of years.Depending on the hardness of the sediment, the forces of water, cold and wind leave their traces and carve impressive landscapes into the rock.

A famous example of this is the Grand Canyon in Arizona: Here the Colorado River has dug a channel through various layers of rock. It is easy to see how soft and harder rock alternate: The soft rock gives way quickly, creating sloping slopes, the harder rock remains standing and forms steep, almost vertically sloping walls. These sediment steps lead down like a staircase to the current course of the river and offer the visitor a spectacular sight.

A well-known large landscape in Germany also formed from sediment layers: the south-west German layer level country, which extends from Baden-Württemberg via Hesse and Bavaria to Thuringia. After the Upper Rhine Graben collapsed, the sediment layers stood here at an angle. Depending on the hardness of the rock, the individual layers were removed to different degrees. Hard limestone formed steep steps, while soft, clayey layers were washed out more strongly and now make up the landscape as gentle slopes and wide stepped areas. On the left side of the Rhine this landscape is - almost mirror-inverted - facing the northern French plains.

Cycle of rocks

No rock on earth is made to last. It weathers on the surface, is removed and redeposited. When two plates collide, layers of sediment are compressed and unfolded to form high mountains. The rock of submerged plates melts in the earth's interior and forms the source of volcanoes. Lava that spits out from a volcanic crater cools down and solidifies again into rock.

It is an eternal cycle that ensures that even the hardest rock is constantly changing and new things are created from it. The transformation does not happen overnight, of course, but over millions of years. "Players" in this cycle are three groups of rocks, each of which is formed under different conditions:

When magma cools, the hot mass solidifies igneous rock. This can happen both on the surface of the earth and inside the earth. On the other hand, where layers of excavated rock pile up, the sediments are compressed under the weight of their own weight. This pressure causes them to solidify Sedimentary rock. In turn, high pressure and great heat in the earth's interior ensure that rock is transformed and another is created. Then geologists speak of transformation or of metamorphic rock.

These three types of rock are closely related: each type can transform into any other. This rock cycle will continue as long as the earth exists.

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. In addition, the atmosphere 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 that 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.

Bottoming out

Plants rarely grow on bare rock. They need a soil from which to draw nutrients and in which to form roots. Weathering is necessary for such a soil to develop: rain and oxygen, heat and cold, water and wind grind the rock and grind even hard granite into smaller and smaller grains. The result is what is known as weathering debris.

But thousands of years will pass before it becomes living soil. Bacteria, fungi and lichens are the first to settle on the rock; the first soil animals are attracted to it. Dead plant remains, animal carcasses and excrement gradually mix with the crushed rock. From this mix, with the help of fungi and bacteria, the upper soil layer develops from fertile soil on which plants can thrive. There are other layers underneath, such as sand or clay. At the very bottom lies the rock from which the soil develops.

Depending on which rock is weathering, how moist it is, which plants are growing and which temperatures are prevailing, different soils with different properties and colors are created. Whether weathered rock is washed away or deposited also plays a role.

In our temperate latitudes there are often the brown earths. They develop on rock with little or no lime in a humid climate. The Rendzina, a soil that forms on limestone, is dark in color. Because it is so rocky, it is difficult to farm on it. And on the Italian island of Stromboli there are very special sandy soils: Because the lava rock that comes from the Stromboli volcano is dark, the sandy beaches on the volcanic island are pitch black.