Bottom formation

Plants rarely grow on bare rock. They need a soil from which they can draw nutrients and in which they can form roots. For such soil to develop, weathering is necessary: rain and oxygen, heat and cold, water and wind grind the rock and thus grind even hard granite into smaller and smaller grains. What emerges is the so-called weathering debris.

But thousands of years 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 them. Dead plant remains, animal carcasses and faeces gradually mix with the crushed rock. From this mix, with the help of fungi and bacteria, the upper soil layer of fertile soil develops, on which plants can thrive. Below that are other layers, for example of sand or clay. At the very bottom is the rock from which the soil develops.

Depending on which rock is weathered, how wet it is, which plants grow and what temperatures prevail, different soils with different properties and colours are created. Whether weathered rock is washed away or deposited also plays a role.

In our temperate latitudes, brown earths are common. They develop on rock with little or no limestone in a humid climate. Dark coloured is the rendzina, a soil that forms on limestone. Because it is so stony, it is difficult to cultivate crops 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 also pitch black.

From rock to grain of sand – weathering

Today, northern Canada is a gently undulating landscape. Many millions of years ago, however, a mountain range stood here. In fact, over a very long time, even high mountains can turn into small hills.

The reason for this transformation: the rock on the earth’s surface is constantly exposed to wind and weather. If, for example, water penetrates into cracks in the rock and freezes, it blasts the stone apart. This process is called frost blasting. Changes in temperature between day and night and the force of water and wind also cause the rock to become friable. In other words, it weathers. This process can also be observed on buildings or stone figures. During weathering, the rock breaks down into smaller and smaller components down 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 dissolve completely 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 resistant, 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. On the earth’s surface, weathering leaves behind fissured limestone landscapes, while caves form underground.

But it is not only solution weathering, but also heat and pressure that wear down and crumble rock beneath the earth’s surface. Where plants grow, roots dig in, blast the rock apart piece by piece and also ensure that it is eroded millimetre by millimetre.

In this way, weathering not only works on individual rocks, it gnaws away at entire mountain ranges. But it will take a few million years before the Black Forest is as flat as northern Canada.

What causes erosion?

When rock weathers, it rarely stays in its original place. Often, rock debris rolls down the slope, is washed away by water or pushed away by ice masses. Fine rock dust or sand can also be carried by the wind. Whether the rock is carried away by water, ice, wind or gravity, all these processes are called erosion.

The erosion caused by flowing water is particularly drastic. Streams and rivers dig a bed in the ground, rock slides down and a valley forms. When a glacier rolls down the valley, it planes the valley wider with the debris it has dragged along. Such trough valleys show that there was a glacier here long after the ice has melted. The surf of the sea, on the other hand, attacks the coast. Steep cliffs are eroded 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 carry away. A sandstorm gradually grinds away obstacles made of solid rock like a sandblaster.

When rain and wind wash or blow away the soil cover on large areas, this is called soil erosion. Landslides on slopes are also referred to as soil erosion. The problem is that 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 wind and water from carrying it away. However, if the plant cover is destroyed, for example by deforestation, the soil lacks this hold and is eroded away.

Erosion by wind – Of shifting sand dunes and mushroom rocks

Wherever wind sweeps over sandy dry ground, it carries fine grains with it and later lets them fall 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 Desert or the Namib Desert. Their dunes can be over 200 metres high and many kilometres long.

But you don’t have to go to the desert to see a dune: There are also dunes on the coasts, in Germany for example on the North Sea or Baltic Sea coast. The sand that is blown off 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 at all, for example when they are overgrown with beach grass. Others, however, roll forward in the direction of the wind, similar to the waves of the sea, the shifting dunes.

Dunes have different shapes. Some are curved like crescents or sickles – the sickle dunes. Others form a wall across the wind direction, the transverse dunes. Both rise slightly on the windward side. On the side away from the wind, they drop steeply downwards. And some dunes even sing their own song: When sand avalanches break loose from the dune and the grains of sand collide, they make humming or buzzing noises: the dune “sings”!

But wind and sand do not only shape dunes. Flying grains of sand can abrade rocks in the landscape like sandpaper. Even hard rock can be given a new shape by this wind abrasion: Rising rocks are scraped and hollowed out at their base over time. Finally, they rise up like mushrooms – a mushroom rock has been created.

Earth on the move – landslides, debris flows and rock avalanches

Suddenly the earth starts to move: tons of rock, mud and debris slide or tumble down the mountain into the valley. Destructive and unstoppable, the earth masses sweep away everything in their path.

Erosion can proceed very slowly, but sometimes it happens suddenly. If, after a heavy downpour, the soil softens considerably and is heavy enough, an entire slope can start to 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 debris cones and heaps.

Whether a landslide occurs depends on the slope: The steeper the slope, the more likely the earth is to slide. How firmly the layers of earth hold together also plays a role. If the slope is overgrown with plants, the roots provide more grip. If nothing grows on the slope or trees have been cleared, the roots that hold the soil in place are missing. Then it is easier for a landslide to occur.

A landslide can look different: The whole slope can slide downwards over a large area. Or earth and mud flow like a river through a valley or carve one out, then it is called a mudflow.

In the shortest possible time, huge masses of rock go down in a landslide. Debris and rocks tumble down within a few seconds. Mostly, landslides happen at places where different rock layers meet. Heavy precipitation, the alternation of heat and cold or earthquakes can cause these layers to separate. In a landslide, large blocks of rock break off. Due to global warming, layers of rock that used to be held together by ice are thawing today. As a result, such landslides are occurring more and more frequently

Cycle of the rocks

No rock on earth is made to last forever. It weathers on the surface, is transported away and deposited again. When two plates collide, sedimentary layers are compressed and folded into high mountains. The rock of submerging plates melts in the earth’s interior and forms the source of volcanoes. Lava spewed out by a volcanic crater cools in turn and solidifies back into rock.

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

When magma cools, the hot mass solidifies into magmatic rock. This can happen both on the Earth’s surface and in its interior. Where layers of eroded rock debris accumulate, on the other hand, the sediments are compressed under the weight of their own weight. This pressure causes them to solidify into sedimentary rock. High pressure and great heat in the earth’s interior, in turn, cause rocks to transform and form another. Geologists then speak of metamorphic rock.

These three rock types are closely connected: Each type can change into any other. This rock cycle will go on and on as long as the Earth exists.

What is rock?

In some places it peeps out from under a thin cover of plants, elsewhere it rises up as a steep rock face: the bare rock. It is the building material that makes up the earth’s crust and mantle. Rock, however, is not a uniform mass. Similar to a cake dough – only much harder – it is a mixture of different ingredients: the minerals.

Rock therefore consists of different minerals. Depending on their composition, the minerals combine to form certain types of rock. Granite, for example, is a rock that consists of the minerals feldspar, quartz and mica. The fact that granite is made up of different minerals is evident from the fact that it is speckled: it contains lighter and darker parts that owe their different colours to three different minerals. The darker parts come from the mineral mica. The quartz mineral often appears whitish to grey. The third mineral, feldspar, can take on all kinds of colours, even pink. Unlike the hard granite rock, the softer sandstone consists almost entirely of quartz. For this reason, sandstone looks more uniform than the speckled granite.

Almost all minerals arrange themselves according to a certain lattice pattern into uniform shapes, the crystals. For example, the mineral rock salt grows into a cube. However, the regular arrangement also results in other shapes with smooth surfaces, as can be clearly seen in a rock crystal. This consists of particularly pure and therefore transparent quartz. If, on the other hand, liquid is enclosed in the quartz, it turns a milky, cloudy colour. Geologists then speak of milky quartz.