How gravity can cause erosion

Weathering and erosion

Rock is constantly being decomposed, crushed and removed. Both physical and chemical processes play a decisive role in this.

Mountains are nowhere near as hard and stable as they may appear from a distance. Their rock is constantly being decomposed, crushed and removed; it crumbles and crumbles. These processes are fundamental to the appearance of the planet. Without them, mountains - driven by tectonic shifts - would continue to grow higher and higher. Fertile soils, alluvial plains or river deltas could never arise because they would lack the material. Scientists call the decomposition of rock into smaller components "weathering" and the subsequent transport "erosion". Physical mechanisms play an important role in this.

Weathered by sand and wind

Weathering is often caused by cold, for example. In the mountains, rainwater and meltwater penetrate into crevices. If the water freezes, its volume expands and the rock is blasted. The real reason for this is the strong expansion of the water when it freezes: The specific volume, i.e. the ratio of volume to mass, increases by a tenth.

In higher mountain areas, glaciers can also cause weathering: When the ice flows scratch their bedrock, they tear out small and large chunks. This form of weathering is particularly effective when the glacier freezes to the ground. In the Alps, the glaciers usually slide over a thin layer of liquid water on their underside, which reduces weathering.

Temperature fluctuations contribute to weathering not only in humid areas, but also in dry ones. This happens when the temperature fluctuates during the day to such an extent that the strength of the rock can no longer withstand the stresses caused by thermal expansion and contraction. This type of weathering is typical of deserts. In addition, the sand contained in the wind rubs against the rocks there, which creates bizarre shapes. In general, the weathering in dry areas is rather weak. This explains the excellent state of preservation of ancient ruins, for example in Libya or Jordan.

Less known weathering processes

In addition, there are a number of lesser-known weathering processes. One of them is caused by crystals: In rock crevices, new crystals can form from aqueous solutions, for example salt crystals on the seashore. The crystals grow gradually and can - similar to freezing water - burst open the rock. The resulting cavities form a characteristic honeycomb pattern. Interestingly, rock can even weather without any external influence. This happens when rocks that have formed under the weight of overlying layers are gradually released from the pressure. The relief brings tension into the rock, which leads to an independent split.

Weathering from sea salt

Thunderstorm lightning also contributes to weathering, in an indirect way. Because the air temperature in the lightning tunnel can reach up to 30,000 degrees Celsius. If the lightning strikes near a crevice in which there is water, it evaporates suddenly and expands in an explosive manner. The rock bursts open due to the rapid volume expansion. Recent studies suggest that lightning activity could play a much larger role in weathering than previously thought, especially in frost-free areas. In weathering, physical processes are often preceded by chemical processes: unstable minerals are chemically attacked and converted on the rock surface, usually with the participation of water. Ferrous rocks, for example, can rust and turn red.

Further chemical weathering processes lead to a gradual exchange of ions between an aqueous solution on the surface and substances in the rock. Acids dissolve rocks containing carbonate, releasing carbon dioxide. Some plants and animals produce acids that contribute to this type of weathering. In general, the contribution of the biosphere to weathering is an area in which research has recently been increasing.

Ultimately, chemical weathering changes the physical properties of the rock on the surface: fine pores or cracks arise. The physical weathering processes can then attack more easily - it is therefore more likely that parts of the rock will be split off. Conversely, physical weathering by enlarging the rock surface also favors chemical processes. So it is a mutually reinforcing interaction. In general, physical weathering dominates in cold and dry areas, while chemical weathering predominates in humid and warm areas.

The removal

Once the rock has been broken down into individual parts by weathering, it is transported away: erosion. On the coasts of seas and lakes, the waves of the surf slowly pull the material into the depths. Streams and rivers constantly move large amounts of sediment from the mountains down the valley. In this way, they create alluvial plains and river deltas.

Glaciers are not only relevant to weathering, but also to erosion. After they have scraped off rock, they transport it - sometimes over hundreds of kilometers. The boulders in northern Germany, which come from Scandinavia, are impressive evidence of this: These boulders were moved southwards during the last ice ages.

Fossil coral erosion

After the rock material has been crushed into grains of sand, the wind is also involved in the erosion. The dunes of the deserts and beaches attest to the clouds of dust that are thrown up in the Sahara, and sometimes even bathe the sky over Germany in an orange light. The tropical rainforest in South America gets some of its mineral nutrients from the dust of the Sahara that is blown across the Atlantic. As a source of aerosols, the earth's deserts are also an important climate factor. Because the fine particles in the air shade the sun.

Weathering and erosion are not only of interest on geological time scales. This is shown particularly clearly by the example of sediment transport: by building river dikes and dams, humans intervened heavily in erosion within a short period of time. This has changed the sediment budget considerably. In several of the earth's deltas, the ground has sunk by several meters in some places due to lack of material. On the other hand, reservoirs fill with sediment, which also creates problems. Because then the volume of the water reservoir decreases. In order to improve water management in rivers and thus the sediment balance, extensive data on weathering and erosion are required.

Scientists study the two processes in different ways: in the laboratory, through field studies and with the help of remote sensing methods. Satellites in particular are of great help. In addition, one constructs chemical-physical models and calculates the individual processes with computer models. In this way, the researchers try to understand step by step how weathering and erosion work - and how they have changed over the course of the earth's history.