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Translations of Encyclopedia about Geology

 

Rocks, Mountains, and Valleys

The earth's surface is the result of a combination of many different changes. Forces causing these changes on the earth's surface are at work both on the exterior as well as the interior of the earth.

Convective flow, movement of the plates, volcanoes, earthquakes are some of the interior forces. Water in the lakes, rivers, and oceans, wind, and moving ice are the exterior forces.

All these forces leave visible marks all over the surface of the earth and are the cause of important changes. Mountain ranges form when layers of rock fold as a consequence of a tectonic plates collision.

Mountain ranges quite clearly differ from their lower-situated surroundings by their foothills. In accordance with the height and shape, we distinguish between highlands, where the difference between the summit and the foot is between 500 and 1000 metres and their shape is generally rounded, and high mountain ranges, where the difference of altitude is over 1000 metres. High mountains are often steep and jagged, and in many cases they reflect the changes wrought by glaciers.

From the geological point of view, we classify mountain ranges into volcanic, and in accordance with the type of formation, into fault-block fold, and fault-fold mountains. Residual mountains remain while minerals in the surrounding rocks are being eroded, and then they are carried away by wind or water.

Similar formations are the rocks in the shape of a mushroom, occurring in the desert. Winds close to the surface blow sharp particles of sand against its lower part, leaving it in the end standing on a sort of thin stem. The sand may be deposited in the form of dunes. Dunes are subjected to constant changes as well, because the landscape lacks protective vegetation and the impact of the wind is especially intense.

Caves form when water containing acids flows through limestone and dissolves it. When such a cave collapses, it may create large ravines. We often find on the earth's surface long cavities between jutting rocks. They were formed by flowing water or moving glacier.

However, all forms of the earth's surface are subjected in particular to weathering and erosion. How much these forces will affect everything depends on the climate, type of rock, and the degree of chemical decomposition.

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Origins of the Mountains (Orogenesis)

We may describe the formation of mountains as a process during which a part of the earth's crust is moulded. It involves horizontal and vertical shifting of the layers of rock into different shapes, and it also involves volcanic activity.

Hypotheses concerning the question of how mountain ranges on the earth originated existed already in antiquity. However, real scientific theories have been in existence only for about 150 years.

Exponents of the so-called contraction theory (Prof. Eduard Suess, geologist, 1831-1914), thought that the earth was originally a molten sphere. During the cooling period it contracted and its surface contracted. The solidified earth's crust shifted, joining and overlapping. According to this theory, parts of the crust cracked.

Another group of geologists proposed the theory of expansion, among them the English physicist Dirac (1937). They believed that the earth continued to expand, because the gravity in the universe was decreasing. That led to the cracks in the earth's crust. By additional expansion, the earth's body laterally straightened. This, they believed, caused the folding, that is to say, the formation of mountain ranges.

These theories could explain the formation of trenches, but not the distribution of mountain ranges or the mountain-forming processes. We know today, that the earth cannot contract, because new rock of the earth's crust is continually forming underneath the oceans, when molten magma flows from the earth's mantle to the earth's surface. These expansive massive motions, which are taking place in the earth's mantle, transfer its force of flow to the earth's crust. It is almost certain that the forces resulting from the earth's rotation (centrifugal force, inertia) also intervene in this process.

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Types of Mountain Ranges

According to the kind of origin, we distinguish several types of mountain ranges. There are volcanic mountain ranges, different folded mountain ranges (the type depends on the tectonic formation), and then mountain ranges that were shaped by erosion.

Most of the volcanic mountain ranges form where the tectonic plates collide or pass each other. This is where lava flows up to the earth's surface. Simultaneously, other rocks are pushed to the surface. Layers of ash and lava rise through the crater from the depths to the surface and accumulate, forming a volcanic cone.

An example of this formation is the Devil's Tower in Wyoming. This mountain formed from the neck of a former volcano. Cracks, which are noticeable on the outside, occurred when the ash eroded and the contracting masses of lava cooled down. When magma intrudes upwards through the cracked volcano, it leads to the formation of flatter layers of rock. It often results in the formation of parallel corridors.

When two plates of the earth's crust collide, the rock is compressed. This mountain-forming process is called orogenesis.

Folded mountain ranges form in places where the oceanic crust slides underneath a continent, and the continental crust, which is less dense, folds. These folds may be as small as a millimetre, but they may also form gaps and folds the size of a mountain. Examples are the Appalachians in North America and the Pyrenees in Spain. The process of folding occurs only under extreme pressure and temperature, which exist in the depths of the earth. Folded mountain ranges can never form on the earth's surface, since the properties of the rocks in the interior of the earth change due to those extreme conditions.

Due to great pressure and high temperatures, the rocks turn elastic and mouldable, which means they can move. The material in the earth's interior, called magma, intrudes into cracks and depressions and forms layers. Sediments that were originally layered horizontally may be pushed by lateral pressure upwards (anticlines), or curved downwards (synclines). In addition, the solidified upper earth's crust breaks into fragments, which may divide the rock horizontally, transversally, as well as vertically.

Folded mountain ranges may also form when two continental plates collide. Himalayas are an example of this formation. Here, thick masses of sediments and oceanic crust accumulated when the Indo-Australian plate collided with the Euro-Asian plate.

Mountain ranges are also formed by tectonic processes. However, tectonic uplift is more exposed to the elements, which accelerates rock erosion.

When the mountain-forming process slows down or stops completely due to changes in the plate tectonics process, the rock decays faster, flattening the overall relief of the mountain range. The taller the mountain, the faster the decaying process of the rock, because the atmosphere, the wind, or the temperature changes have an available surface exposed to the weathering. Freezing and melting weaken the rock structure. River streams carry rock fragments from the mountains to the valleys and new rock is ready for the erosion. In cold zones, glaciers grind the surface of the mountains. Here, too, the till is carried away by water and ice.

The Appalachians and the German Middle Mountain Range are examples of mountain ranges shaped by erosion. When the process creating these mountains ended, the erosion took over and formed relatively low mountain ranges, when we compare them to the younger Alps.

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