Why do hard rocks form steep cliffs?

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.


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.


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.

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.

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.

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.

Glaciers shape the landscape

Wherever glaciers move, they shape the landscape. Stones enclosed in the ice act like coarse sandpaper: They grind rock from the subsurface at the edges of the glacier. The ice masses carry away the rubbed off rubble. The glacier scrapes out the rock. This creates valleys that look round like a U in cross-section and are therefore called U valleys.

Sand and boulders that are dragged along by the glacier ice remain at the edges and at the bottom of the glacier on the way down and form small and large hills. Such boulders on the edge of the glacier are called moraines.

If it is very cold for a long time, the glaciers grow and advance further and further into the landscape. If, on the other hand, it becomes warmer, the ice masses melt and the glaciers retreat. The moraines of rubble remain, however. Centuries later you can still tell from them how far the glacier had penetrated. The place that the glacier once excavated and covered with its ice is shaped like a tongue. One therefore speaks of a tongue pelvis.

How are valleys formed?

River and valley are inseparable. But why? How do these elongated hollows, which are called valleys, even come about? Wherever water runs off in small streams or large rivers, a valley forms. This is because flowing water digs deeper and deeper into the subsoil. The soil on the sides slides down towards the river bed. A slope forms to the right and left of the watercourse; this creates a valley along the river.

Valleys can look very different: steep walls or gentle slopes, wide valley floors or just enough space for the river. The shape depends on how strongly the water attacks the bottom and the side walls and how stable the rock is.

It's steep in the mountains, at the headwaters of a river. The water shoots down the mountain with force. Because of its high speed, it transports a lot of sand and debris there. With this rubble, it grinds the ground heavily and can dig itself deep. This creates rather narrow, deep valleys.

Towards the mouth, the river widens and carries more and more water. As the terrain becomes flatter, the water flows more and more slowly. For this reason, the lower reaches of the river gradually deposit the cargo it has carried along with it on the ground again. Erosion takes place here more on the side walls, so that broad, flat valleys are created.

The rock through which the river flows is also responsible for the different valley shapes: water and rubble dig into solid rock without a lot of rock sliding down the sides. This creates valleys with steep or even almost vertical walls. Soft rock layers, on the other hand, slide quickly and lead to flat slopes.

Valleys are divided into different types based on their shape: Narrow valleys with steep walls are called canyon, in the case of vertical walls one speaks of one Klamm. Narrow valleys with gentler slopes are called Kerbtal or V valley designated. If, on the other hand, the valley floor is significantly wider than the river, it is a Sohlental, or - with steep walls - by one Kastental.

A special form of valleys are Canyons. Here the water has dug its way through different layers of rock that lie on top of each other like several layers of cake. Some layers were easily removed by the river, they were washed out wide and round, the more resistant layers broke off steeply and angularly. The result is a valley, the side walls of which slope down like steps towards the river. A famous example of such a valley is the Grand Canyon in the US state of Arizona.

Earth in motion - landslides, landslides and landslides

Suddenly the earth starts moving: tons of rock, mud and rubble slide or tumble down the mountain into the valley. Destructive and unstoppable, the earth masses with them everything that gets in their way.

Erosion can progress very slowly, but sometimes it can happen suddenly. If the ground becomes very soft and heavy enough after a heavy downpour, an entire slope can slide. Such a landslide transports large amounts of earth and debris down into the valley. At the foot of the slope, the loosened rock collects in cones of rubble and heaps.

Whether there is a landslide depends on the gradient: the steeper the slope, the more likely it is that the earth will slide. How tightly the layers of earth stick together also plays a role. If the slope is overgrown with plants, the roots provide more support. If nothing grows on the slope or trees have been cleared, the roots that hold the ground are missing. Then a landslide occurs more easily.

A landslide can look different: the entire slope can slide downwards over a large area. Or earth and mud flow like a river through a valley or dig one, then one speaks of a mudslide.

In a very short time, huge masses of rock fall off in a landslide. Rubble and rocks tumble down within a few seconds. Most of the time, landslides happen in places where different layers of rock meet. Heavy precipitation, the alternation of heat and cold or earthquakes can cause these layers to diverge. Large blocks of rock break off in a landslide. Due to global warming, layers of rock that used to be held together by ice are now thawing. As a result, such landslides are becoming more and more common.

Landslides and landslides are very dangerous. They have already cost many lives and destroyed entire towns. In the Goldau landslide in Switzerland, for example, forty million cubic meters of rock fell, buried several villages and buried hundreds of people.

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.

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.