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Translations of Encyclopedia about Geology


Weathering and Erosion

Weathering and erosion are important geological processes, which cause changes on the earth's surface. The term weathering means decay and decomposition of the rocks.

The speed of the decay depends on the type of rock, climate, and the length of time during which the rock is exposed to the atmospheric elements. There are two kinds of weathering, physical and chemical. Physical factors are freezing and melting, and the action of water streams and glaciers. They are the cause of rock decay.

During chemical weathering, the minerals are chemically transformed or entirely decomposed. When the rock weathers, the erosion will carry it away and a new uncovered rock is exposed to weathering. An important factor affecting the intensity and speed of weathering is the soil, in which the mineral is found. For example, if the soil is moist or acid, the chemical weathering will be faster or slower. This means that the principal factors of weathering are climate, type of rock, soil, and time.

When we compare the state of monuments on different continents, we observe that different climatic conditions have different impact on the chemical and physical weathering. In humid and hot climate, the monuments are more weathered, in contrast, in dry and hot climate the monuments are less weathered. Chemical weathering in cold and dry regions is almost negligible. For example, if we erect a limestone monument in the state of Massachussetts, it will almost "melt" within one hundred years. Its soft surface will "flatten," letters carved in the stone will lose the sharpness and depth. In contrast, if we use granite instead of limestone, it will take many centuries before any great weathering occurs.

This means that atmospheric precipitation and temperatures have an important impact on the chemical weathering. Warmer and more humid climate supports the weathering, cooler and drier climate will slow that process down. One of the reasons why the decay is faster under warmer temperatures (for example, in the tropics), is because the climate also supports faster growth of vegetation and bacteria. These produce acids supporting the decay. In addition, a majority of chemical reactions proceedv faster under higher temperatures, which obviously is valid also in the case of chemical weathering.

The term chemical decay means the chemical reaction of minerals found in the rocks when they are exposed to air or water. Some minerals dissolve, others coalesce with water and atmospheric gases, such as carbon monoxide and oxygen, forming new compounds. Weathering then produces the formation of new minerals.

This process is easily observed on the basis of experiments with feldspar, which is the most common mineral on earth. Feldspar is a silicate and is the main component of many kinds of rocks igneous, sedimentary, and metamorphic. Feldspar is found for example in granite, a solid and hard rock, which is held together by quartz and other crystals.

In humid tropical climate, however, granite will decay to such a degree that, if they are weathered, even large pieces may be shattered into individual mineral grains with just one blow. If we look carefully, we will discover that parts of feldspar eroded and are surrounded by a layer consisting of soil minerals. The resulting earth or clay (which is used to make pottery) is creamy in colour and is called kaolin, named by the Chinese mountain Kao-Ling, where it was first discovered. The Chinese used kaolin to produce ceramics long before it reached Europe in 18th century.

In dry climate, feldspar weathers very gradually, which means that what changes feldspar into kaolin is mainly water. Other silicates weather in a similar way.

The majority of iron ores, which are used in the production of iron and, ultimately, steel construction, formed thanks to changes of silicate minerals rich in iron, for example, olivine (ferrous-magnesium silicate). Iron is obtained by dissolving minerals. Iron oxide minerals form as a result of chemical reaction with oxygen from our atmosphere. This is called oxidation. When these minerals weather, they turn reddish brown, the colour of oxidised iron. This may be observed in many kinds of rocks. One example of this colouring is the Ayers Rock in central Australia.

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Caves provide geologists information concerning the underground water and its impact, and they also provide information concerning mineralogy. Many thousands of years ago, caves were used for shelter and food storage, and they also served as places of worship. Later, in the Middle Ages, people started exploring the caves.

At that time, however, the interest was not of scientific nature. People were looking for the bones of the ice-age animals, such as cave bear bones. They were marketed as bones of mythical unicorns having healing properties, and were sold for a lot of money.

Every year, thousands of people visit caves to see interesting dripstone features or prehistoric paintings of animals. It took hundreds of years for the caves to form. They formed either simultaneously with the rocks, for example in the igneous rocks (gas bubbles), coral reefs, or later by the action of water, by grinding sands, or in conjunction with the formation of clefts. Cave ranges in size from a few meters to many kilometres. They may look like gaps, tunnels, or halls. Sea caves may be created on the coast by heavy surf. One of such caves is the Blue Cave in Capri. Solution caves form in gypsum, rock salt and, particularly, in limestone and dolomite. Limestone caves form by means of chemical extraction together with mechanical erosion by water.

Caves form in places where the relatively easy to dissolve limestone deposits are close to the surface, that is to say, in places where sufficient amounts of water, which dissolves the limestone, can seep through. Rain water, enriched by carbon monoxide from the atmosphere, seeps into the soil, collecting on its way sufficient amounts of carbon monoxide produced by plants and bacteria. Water rich in carbon monoxide flows downwards to the level of the underground water (water table). As it travels, it dissolves limestone alongside cracks and clefts. These cavities gradually widen, until there is an entire network of cavities filled with water. When these cavities are full of water, limestone dissolves alongside the entire perimeter, i.e., ground, walls, and ceiling. If the level of the underground water falls, water flows out of the caves and the caves may be explored.

A special feature of speleothem caves are their dripstone formations. These formations occur in caves when they are filled with air, after the water, rich in calcium bicarbonate, deposits the calcite (calcium carbonate). This happens, when carbon monoxide escapes from water by evaporation or higher temperature. Soluble calcium bicarbonate dissolves and insoluble calcite separates.

Every drop of water on the cave ceiling yields a tiny amount of calcite, creating formations that grow from the ceiling downwards and are called stalactites. When water droplets fall on the ground of the cave, it allows the carbon monoxide to escape and simultaneously the water again deposits a tiny amount of calcite on the ground. These calcite deposits grow upwards and are called stalagmites. When a stalactite and a stalagmite join into one formation, they form a columnar feature called stalagnate.

The rate of growth of the speleothems varies greatly. Some may grow only a few centimetres in 200 years, while other will grow 5 to 10 centimetres in one year. Using modern scientific methods we can now determine the age of individual layers, which means we also know the age of the cave.

As the limestone continues to dissolve, the roof of the cave sometimes becomes so thin that it may collapse and create a sinkhole. Gradual weathering will eventually flatten the sinkhole. Sudden collapses may bury under cars and people.

Caves may be repeatedly flooded, because the water level of underground rivers and lakes fluctuates. That makes cave exploration very dangerous.

The occurrence of caves varies greatly from one region of the world to another. In France, as well as in Italy, there are over 10.000 caves, in the United States more than 13.000. In Germany, caves are found mainly in Swabian and Frankian highlands, and some also in Harz Mountains.

Ice caves exist in higher latitudes. The condition for the formation of these caves is that temperature cannot fall below freezing. An example of this type of caves is the "Ice World" in Austria (Salzburg).

The largest system of caves is found in the United States. One of the famous caves is the Mammoth Cave, which includes a series of caves, extending over many kilometres. It is a 5-story, interconnected system. Another large cave are the Carlsbad Caverns in the state of New Mexico. This cave is 200 metres wide, 1200 metres long and 100 metres high.

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Hot and Cold Deserts

Deserts represent probably the calmest regions of the earth. They may be the most inhospitable for humans, but with their blare rocks, sand dunes, and strange fauna and flora, they are also very fascinating. The only sound one can hear there is the wind. It may produce important changes on the earth's surface.

Deserts are found on all continents. They have one thing in common: low precipitation, in general less than 25 millimetres annually.

There are several types of desert. In the hottest regions of the earth, between 30 degree of northern and 30 degrees southern latitude, the air pressure is always the same. Due to strong sun, hot air rises. In higher altitudes, the air cools and expands. The water vapour condenses, but as it descends into the lower, warmer air, it evaporates, so there is no precipitation. Sahara and Kalahari deserts in Africa are good examples.

Deserts form also in middle geographical latitudes, for example deserts in central Asia, or the Mojave desert in the United States. In this case, the moist winds are either captured by mountain ranges or their distance from the ocean is such, that the air loses its moisture before it reaches inland.

In polar regions the deserts form because the cold air can absorb an extremely low amount of moisture and the soil does not retain moisture. Iceland is an example of such a desert.

In principle, the origin of a desert may be traced to the movement of the tectonic plates. The colliding oceanic and continental plates form mountains, which create a rain shadow. Other deserts owe their existence to the continental drift from higher to lower geographical latitudes. For example, one such desert is in Australia. At one time, the interior of Australia was situated in a zone with a lot of precipitation. However, due to gradual, northerly shift into the andean subtropical zone, the Australian interior turned into a desert.

The one force that shapes the surface of a desert is the wind. Its impact is observed when we look at the amounts of sand blown into sand dunes. The shape of the dunes depends on the amount of available sand, but also on the direction, speed, and duration of the wind.

Barchans, or small crescent-shaped dunes, occur individually or in groups. They form when the supply of sand is limited and the wind is not too strong and blows constantly in one direction. The limbs of barchans are turned downwind.

When several barchans coalesce, they form transverse dunes, which are narrow, linear dunes. These form at right angles to the prevailing wind direction.

Other dunes form when the prevailing winds come from two directions. While one wind accumulates the wind, the wind coming from the other direction blows the sand away.

An erg, an extensive sand sea, forms where the wind is strong and there is a sufficient supply of sand. An erg may cover up to 500000 square kilometres. We find an example of such a sand sea in the great Arabic desert Ruba el-Chali.

Very large dunes, reaching the height of up to 400 metres, are called draa. They are hilly, wide-spaced, mutually overlapping formations. They move much slower, a maximum of one metre per year.

Pyramidal dunes and very interesting, mushroom-shaped rocks are another desert formation.

Water also participates in the shaping of a desert. Although precipitation in the desert is very rare, when it does rain, it is a heavy rain, which beats into the rock and the crusty soil. Streams of water carve deep ruts in the parched soil. Water collects there, creating gullies, and carries away masses of rock.

Local temperatures also vary according to the type of desert. In coastal deserts the temperature do not vary greatly, both daily and annually.

In inland deserts, the summer-month temperature reaches 50 degrees Celsius in the shade. During clear nights the temperature drops quickly, very often by 30 degrees Celsius. In winter it is possible for the temperature to drop below freezing in inland deserts that are situated in high altitudes.

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