Stone corrosion on Cologne Cathedral

It is crumbling, splintering and weathering: the ravages of time are gnawing away at Cologne Cathedral. Acid rain has already eaten away at the famous building. Laws on air pollution control have been reducing the pollution levels for some years now. But pigeon droppings, exhaust fumes and the weather continue to attack the old walls and never leave the craftsmen of the cathedral’s construction hut out of work.

The foundation stone of Cologne Cathedral was laid on 15 August 1248. Since then, about fifty different types of stone have been set into the Rhenish sand here. Many of them were only used by the builders on a trial basis; not every rock withstood the weather. In addition, the stone had to come from nearby, because transport was fabulously 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 heavily pitted. To save the sensitive shell limestone from weathering, various protective coatings have been tried. This should at least slow down the crumbling. The trachyte from Drachenfels, on the other hand, has held up well. The basalt rocks are also weather-resistant and are still in good condition today.

Despite all efforts, components have to be replaced again and again. Every year, 15 to 20 cubic metres of natural stone are used to maintain the famous church building. Even though Cologne Cathedral was already finished in 1880: the stonemasons of the Dombauhütte still have their hands full today!

Stone incompatibility

Cologne Cathedral apparently weathers more than other comparable buildings. Preservationists now have a guess as to why this is so: the cathedral is built of many different types of stone. And not all stones are compatible with each other. For example, the damage is particularly severe where trachyte from Drachenfels meets sandstone from Obernkirchen. A group of researchers is now trying to find out whether and why some rocks actually damage each other and which of them can work well together.

Constant dripping wears the stone

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

The sculptor of all these landscapes is the cycle of water. More than any other force, water sooner or later shapes the surface of the earth. It washes away soil after a downpour. It burrows into the subsoil and loosens parts of the rock. It carries soil and weathered rock debris down into the valley. Where the water flows away more slowly, it releases its load of silt, sand and debris. At high tide, it floods the flat areas of a valley, the river floodplains. Here, too, it deposits fine silt. 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. When water freezes in rock cracks, it blasts the stone. As a glacier, it planes notch-shaped river valleys into round trough valleys. And the moraine landscape in the foothills of the Alps with its scree hills and boulders is also the result of glaciers that shaped the subsoil long ago.

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. A particularly wandering dune is the “Rubjerg Knude” on the coast of Denmark. This almost 100-metre-high dune moves in a north-easterly direction 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 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.

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 by dragging debris with it. 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.

Sedimentary rocks

Some rocks look as if they are striped. In the Dolomites, for example, such transverse bands are often clearly visible. Sandstone or limestone quarries also sometimes have similarly pretty patterns.

The “stripe design” is already created during the formation of the rock. The starting material is weathering debris that is carried away by water or wind. Rivers, glaciers and dust storms lose their power at some point: river courses slow down towards their mouths and eventually flow into the sea or a lake. Glaciers advance into warmer regions and melt. Dust storms also weaken at some point. Then they can no longer carry dust, sand and debris. The crushed rock that is dragged along settles. Over time, the deposited material forms an ever higher layer – the sediment. Especially on the seabed and on the bottom of lakes, where rivers wash up a lot of material, such sediments accumulate, including remains of dead animals or calcareous shells.

Gradually, different sediments are layered on top of each other. One layer may consist of sandstone, for example: In dry times, the wind has blown desert sand onto it. When the sea level rises again, this layer is covered by water: calcareous shells of marine animals sink to the seabed and deposit another layer on top of the sand. Over millions of years, the climate changed again and again, causing the sea level to fluctuate. This allowed different layers to be deposited.

Over time, the sediment cover becomes thicker and thicker. Under the weight of their own weight, the initially loose sediments are compressed more and more, small cavities disappear, the mass thickens. Further layers are deposited on top, the sediment becomes increasingly solid and finally, under pressure, sedimentary rock. In geology, this process is also called diagenesis. If, for example, the shells of tiny marine animals are pressed into stone, limestone is formed. Fine grains of quartz sand cement together under high pressure to form sandstone.

In addition to debris, dead animals, for example fish, also settle on the seabed. Sealed airtight, their bones and scales were preserved and fossilised. Such fossils have immortalised themselves in the stone. Even after millions of years, they reveal a lot about the time when the sediment was formed. This is why geologists can read the rock layers like a history book.

Normally, only the top layer is visible to us. However, when a river digs through the sedimentary rock, it is uplifted during mountain building or blasted free in a quarry, we get a view of the cross-section. The individual sediment layers are then clearly visible as “stripes” or bands in the rock.

Magmatic rocks

To bite on granite means that something is hopeless. Because of its great hardness, however, granite is not only used as a figure of speech, but also as a paving stone or for building walls. Granite is a rock that lies more than two kilometres below the earth’s surface and occurs frequently in the earth’s crust.

Granite is formed when molten magma solidifies as it cools. The dark mottled gabbro or monzonite are also formed from slowly cooling magma. When this process takes place deep inside the earth, geologists speak of deep rock, also called plutonite.

If, on the other hand, the hot rock mush penetrates outwards during a volcanic eruption and pours over the earth’s surface, it is called effusive rock or volcanite. Volcanites include light pumice, porous tuff or rhyolite, which was formed from the same material as granite but has a different structure and is less hard because it cools more quickly at the earth’s surface than granite at depth. Basalt is also a volcanic rock. Sometimes it solidifies into hexagonal, closely spaced columns that look as if they have been cast into shape. Basalt forms on the earth’s surface from the same mass as the gabbro in the depths.

Volcanites weather immediately after their formation, plutonites only when the overlying rock layers have been removed. Because both volcanites and plutonites became rock from cooled magma, both belong to the category of igneous rocks.

Metamorphic rocks

It happens inside the earth: strong pressure and high temperatures cause the components of the rock, the minerals, to react with each other and transform. In this way, new rock is formed. Because the Greek word for transformation is “metamorphosis”, geologists also speak of metamorphic rocks.

A correspondingly high pressure occurs when two earth plates collide and one plate dives under the other. The rock is then squeezed together, as if in a huge press. A common result of such rock metamorphosis is blue slate. Its source rock is basalt or a rock with a similar composition to basalt.

Great heat also causes rocks to transform. For example, in the vicinity of a magma earth, it is baked like in an oven. Marble, for example, is nothing other than limestone that has been heated very strongly in the earth’s interior; during this process, new minerals form and the rock becomes harder. Sandstone also transforms at high temperatures, because its quartz grains then stick together: the original sedimentary rock becomes the harder quartzite.

In contrast to complete melting through volcanism, the rock remains solid during metamorphosis. However, if the temperature continues to rise, the rock eventually becomes liquid magma. When this mass cools down, it becomes magmatic rock again. The cycle of the rock is in full swing.

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.

Cut off from the mainland

For hours yesterday, two tourists were trapped on a rock in the roaring sea. One of the two arches of the rock sculpture “London Bridge” on Australia’s famous “Great Ocean Road” suddenly collapsed. This cut off the visitors’ way back. 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 crunching sound. When they looked around, the arch had already collapsed, cutting off the connection to the shore. Fortunately, no one had just stayed on the first arch, there were no other victims. After five hours of waiting, the couple was happily brought back to shore by a helicopter.

The double arch of “London Bridge” was one of the most famous rock formations on Australia’s south coast. Wind and waves are increasingly eroding this coastline, causing part of the tourist attraction to now collapse. After the collapse, the “London Bridge” was renamed without further ado: It is now called “London Arch”.

On the Great Ocean Road

The surf is wild on Australia’s south coast, along which the famous Great Ocean Road runs. The stormy sea has already claimed many victims here: Over a hundred ships have already been wrecked on the rocky coast. Wind and waves grind up everything in their path here. And that is above all the relatively soft limestone with its bizarre rock colossi: London Bridge was only 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 disintegrates in the raging sea almost like sugar in hot tea. Without pause, the forces of nature gnaw at the coast and reshape it. So if you still want to see the twelve apostles in all their glory, you should hurry.