Altitudinal zonation or altitudinal zoning is a change in natural conditions and landscapes in the mountains as the altitude increases above sea level. Altitudinal belts form strips that are relatively uniform in natural conditions.
Since in the mountains there is less cloudiness and precipitation, more intense solar radiation, lower air pressure and less dust, then per 1 km of ascent the air temperature decreases by an average of 6°C. Adapting to more severe other conditions within the same latitude, plants formed belts of vertical zonation.
Between latitude zones and altitudinal zones there is partial similarity in climatic features, vegetation and soils.
Types of altitudinal zones
At different latitudes, the altitudinal zones are different. All climatic zones can be observed only in large mountain ranges of equatorial and tropical latitudes (Andes, ). And as we approach the poles, warm climate zones disappear. So in the Scandinavian mountains there are only three altitude zones out of a possible seven.
Two groups of altitudinal zone types are most clearly distinguished: coastal and continental. The coastal group is characterized by mountain-forest belts in the lowlands and an alpine belt in the highlands. For the continental group - a desert-steppe belt in the foothills and a mountain-meadow belt in the highlands.
Examples of types of altitudinal zones:
- The coastal type is represented by the mountains of the Western Caucasus. The lowest is the mountain forest belt with sub-belts of broad-leaved and coniferous forests. Above there is an alpine (in the broad sense) belt with sub-belts of subalpine crooked forests and high-grass meadows, actual alpine short-grass meadows and nival.
- An example of the continental type is the mountains of Central Asia: the Ural and Tan Shan with a change of belts from deserts at the foothills to mountain steppes on the slopes, in places with transitions to mountain forests, meadows and high-mountain deserts, above which the nival belt also extends.
mountain-tundra belt in the foreground, mountain-forest belt in the center and nival belt in the background
High altitude zones
Desert-steppe belt- zones with a dry climate, predominantly desert and steppe vegetation. Characteristic of the foothills and lowlands of continental mountain ranges.
As you gain altitude in the desert-steppe belts, the landscapes change from mountain-desert to mountain-semi-desert, and then to mountain-steppe.
Mountain meadow belt- a belt uniting subalpine or alpine belts.
Subalpine belt- a zone in which subalpine meadows alternate with woodlands. Combines open landscapes and crooked forests.
Alpine belt
in the northern Caucasus
Mountain-tundra belt characterized by long, harsh winters and short, cold summers. Average monthly temperatures in this zone are less than +8°. All upper mountain zones are characterized by strong winds that blow through the snow cover in winter and dry out the soil surface in summer. The vegetation is moss-lichen and arctic-alpine shrubland.
Nival belt
in the Taurus Mountains
Small spaces free of snow experience increased frost weathering, which causes the presence of coarse weathering crust (stones, rubble). It is inhabited by lichens and single flowering herbs. Some insects, birds, and isolated species of rodents and predators sometimes enter the nival belt.
Areas of altitudinal zonality or altitudinal zonation characterize natural stratification at different altitudes due to differences in environmental conditions. Temperature, humidity, soil composition and solar radiation are important factors in determining altitudinal zones, which consequently support different species of plants and animals. Altitudinal zonation was first proposed by geographer Alexander von Humboldt, who observed that temperature decreases with increasing altitude. Zoning also occurs in intertidal and marine environments, as well as on shorelines and marshes. Currently, altitudinal zonation is a basic concept in mining research.
Factors
A variety of environmental factors determine the boundaries of altitudinal zones (belts) in mountains: from the direct effects of temperature and precipitation to indirect characteristics of the mountain itself, as well as biological interactions of species. The reason for zoning is complex due to the many possible interactions and overlapping species.
The soil
The nutrient content of soils at different altitudes further complicates the delineation of altitudinal zones. Soils with higher nutrient content, due to higher rates of decomposition or greater weathering of rocks, better support the growth of large trees and vegetation. The height of the best soils depends on the specific mountain. For example, for mountains located in regions, lower elevations show less diversity of terrestrial species due to the thick layer of dead leaf litter covering the forest floor. Acidic, humic soils are common in these areas and exist at higher elevations at the mountain or subalpine level. In another example, weathering is prevented by low temperatures at higher elevations in the Rocky Mountains of the western United States, resulting in thin, coarse soils.
Climate:
Temperature
A decrease in air temperature usually coincides with an increase in altitude, which directly affects the length of the growing season in different zones. For mountains located in deserts, extremely high temperatures also limit the ability of large deciduous or coniferous trees to grow near the base of the mountains. In addition, plants may be particularly sensitive to soil temperature and are able to have specific elevation ranges that support their healthy growth.
Humidity
The humidity of certain zones, including precipitation levels, air humidity and evapotranspiration, changes with increasing altitude and is an important factor in determining altitudinal zones. The most important variable is deposition at different altitudes. As warm, moist air rises up the windward side of a mountain, the air's temperature and ability to hold moisture decreases. Thus, the highest rainfall is expected at mid-elevations, allowing deciduous forests to grow. Above a certain altitude, the rising air becomes too dry and cold, and thus inhibits the growth of trees. Although precipitation may not be a significant factor for some mountains, air humidity or aridity is sometimes more important than climatic conditions that affect altitudinal zones. The overall level of precipitation affects soil moisture.
Flora and fauna
In addition to physical forces, biological forces can also create zoning. For example, a strong competitor may force a weaker competitor to move higher or lower. There is evidence that competing dominant plants can take over preferred sites (i.e. warmer sites or more fertile soils). Two other biological factors are also capable of influencing zonation: grazing and crosstalk, as the abundance of grazing animals and mycorrhizal associations suggest that they significantly influence the distribution of flora.
Solar radiation
Light is another important factor in the growth of trees and other photosynthetic vegetation. The Earth's atmosphere is filled with water vapor, particulate matter and gases that filter the radiation coming from the Sun to the Earth's surface. Consequently, mountain peaks and hills receive much more intense radiation than plains. Along with dry conditions, at higher elevations, shrubs and grasses tend to grow well due to their small leaves and extensive root systems. However, high altitudes also experience frequent cloud cover, which reduces high-intensity radiation.
Physical Features
The physical characteristics and relative location of the mountain itself must also be considered when predicting altitudinal zonation patterns. This factor explains that the zonation of rain forests on lower parts of the mountains may reflect the zonation expected on high mountains, but belts occur at lower elevations.
Other factors
In addition to the factors described above, there are a number of other features that can affect altitudinal zonation. These include: frequency of damage (such as fire or monsoons), wind speed, rock type, topography, proximity to streams or rivers, history of tectonic activity, and latitude.
What are the altitude zones?
The identification of altitudinal zones is complicated by the factors described above, and, therefore, the relative heights of each zone begin and end without reference to a specific height. However, the altitudinal gradient can be divided into five main zones, used by ecologists under different names. In some cases, these levels follow each other with decreasing heights.
Nival belt (glaciers)
This belt of eternal snow and glaciers is the highest altitude zone in the mountains. It is located above the snow line and is covered with snow for most of the year. Vegetation is extremely limited, with only a few species present that grow on silica soils. Below it borders with the Alpine belt. The biotemperature of the nival belt does not exceed 1.5 ° C.
Plants and animals
Small areas where there is no snow are subject to increased frost weathering, which causes the presence of stones and rubble. In such conditions algae, lichens and some flowering plants grow. Some insects and birds can also be found in this area.
Alpine belt
This is a zone that extends between the subalpine belt in the south and the nival zone in the north. The Alpine belt is characterized by a significant degree of solar radiation, negative average annual temperatures, strong winds and stable snow cover. It includes alpine meadows and. The biotemperature of the belt is between 1.5 and 3° C.
Plants and animals
The plants have adapted to the harsh alpine environment and are very hardy, but in some respects the ecosystem is quite fragile. The disappearance of tundra plants leads to weathering of the soil and its restoration can take hundreds of years.
Alpine meadows form where precipitation caused by rock weathering creates sufficiently well-developed soils to support grasses and sedges. Alpines are quite common throughout the world, and the World Wildlife Fund has classified them as.
Animals that are found in the alpine zone can be either permanent inhabitants of this zone (hay farmer, field mouse, marmot) or temporary (argali, chamois antelope).
Subalpine belt
The subalpine zone is a biotic zone (zone of life) located below the alpine belt and the forest boundary. The exact level of the forest boundary varies depending on the local climate. In the tropical regions of Southeast Asia, the tree line can be above 4000 m, while in Scotland it does not exceed 450 m. The biotemperature of the subalpine zone is between 3-6 ° C.
Plants and animals
Trees in the subalpine zone are often stunted and have a twisted shape. Tree seedlings can germinate on the leeward (sheltered) side of rocks and grow protected from the wind. Snow cover protects trees in winter, but unprotected branches from the wind usually collapse. Well-adapted trees can reach ages ranging from several hundred to a thousand years.
A typical subalpine forest includes silver fir (subalpine fir), Engelmann spruce, and other conifer species. The subalpine flora is also characterized by the presence of plants from the grass family, forbs and tall grasses.
Due to difficult climatic conditions and lack of food, the fauna in this zone is not diverse enough. However, in the subalpine zone there are representatives, bears, hares, martens and squirrels, as well as some species of birds.
Mountain belt
The mountain belt is located between the foothill and subalpine zones. The altitude at which one habitat passes into another varies differently in different parts of the globe, especially with latitude. The upper limit of montane forests is often characterized by hardier vegetation species that occur in less dense stands. For example, in the Sierra Nevada, California, the montane forest contains dense tree pines and red fir, while the subalpine zone of the Sierra Nevada contains rare whitebark pines.
The lower limit of a mountain zone may be the "lower timber line" that separates the mountain forest from the drier steppe or desert area.
Mountain forests are different from lowland forests in the same area. The climate of montane forests is colder than lowland climates at the same latitude, so montane forests often contain species typical of high-latitude lowland forests.
Temperate climate
Mountain forests located in temperate climates are usually coniferous or broad-leaved and mixed forests. They are well known in northern Europe, northern United States and southern Canada. The trees, however, are often not identical to those further north: geology and climate give rise to different related species in montane forests.
Mountain forests around the world tend to be richer in species than those in Europe, as major European mountain ranges blocked species migration during the last Ice Age.
Mountain forests are found in the temperate climates of Europe (Alps, Carpathians, Caucasus, etc.), North America (Cascade Mountains, Klamath Mountain Range, Appalachians, etc.), southwestern South America, New Zealand and the Himalayas.
Mediterranean climate
These forests are typically mixed coniferous and broadleaf forests with several conifer species. Pine and juniper are typical trees found in Mediterranean mountain forests. Broadleaf trees are more varied and are often evergreen, such as the evergreen oak.
This type of forest is found in the Mediterranean basin, North Africa, Mexico and the southwestern United States, Iran, Pakistan and Afghanistan.
Subtropical and tropical climate
In the tropics, montane forests may consist of broadleaf forests in addition to conifers. One example of a tropical montane forest is a cloud forest, which gets its moisture from clouds and fog. Cloud forests often have an abundance of mosses covering the ground and vegetation, in which case they are also called mossy forests. Depending on latitude, the lower limit of montane rainforests on large mountains is usually between 1500 and 2500 metres, while the upper limit is between 2400 and 3300 metres.
Foothills
This is the lowest section of the mountains, which clearly varies in climate and is characterized by a wide range of names depending on the surrounding landscape. Such low-lying belts are found in tropical and desert areas.
Tropics
Characterized by deciduous forests in oceanic or temperate continental regions and grasslands in more continental regions. They extend from sea level to approximately 900 m. The vegetation is abundant and dense. This zone is the typical base layer of tropical regions.
Deserts
Characterized by open evergreen oak and other forests, most common in desert areas. There is a limitation of evaporation and soil moisture. Very common in the Southwestern United States.
desert grasslands
Desert grasslands are located below the desert belt and are characterized by varying densities of low-lying vegetation. These areas cannot support tree growth due to extreme aridity. Some desert areas are able to support the growth of trees at the foot of the mountains, and thus do not develop distinct grassland zones in these areas.
Distribution of animals depending on altitudinal zones
Animals also demonstrate zonation depending on altitudinal zones. more clearly defined in the belts because they are usually less mobile than vertebrates. animals often move through high altitude zones depending on the season and the availability of food. Typically, the diversity and abundance of animal species decrease with increasing mountain heights due to harsher environmental conditions. It is difficult to study in detail the distribution of animals depending on altitudinal zones, since representatives of the fauna tend to frequently change their habitats.
Altitudinal zonation and human activity:
Agriculture
Human populations have developed agricultural production strategies to take advantage of the different features of high altitude zones. Altitude, climate and soil fertility determine the crops that can be grown in each zone. Population groups living in the mountainous Andean region of South America took advantage of the distinctive high-altitude conditions to grow a wide variety of crops.
Environmental degradation
Population growth is leading to environmental degradation in high-altitude environments through deforestation and overgrazing. Increasing accessibility to mountainous regions allows more people to travel between belts and use the land for commercial purposes. In addition, improved road access has contributed to environmental degradation.
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1) How do air temperature and atmospheric pressure change with the altitude above sea level?
Air temperature and pressure decrease with altitude.
2) How does the sequence of zones change when going up into the mountains: the same as when moving along the plain - from north to south - or from south to north?
The sequence of zones when ascending the mountains changes in the same way as when moving along the plain from south to north.
Questions in a paragraph
*Determine in which mountains of Russia the altitudinal zones are most fully represented, explain this.
The belts are most fully represented in the Caucasus Mountains, this is explained by their southern position.
* What is altitudinal zonation?
Altitudinal zonality, altitudinal zonality - a natural change in natural conditions, natural zones and landscapes in the mountains as the absolute height (altitude above sea level) increases. Do you think altitudinal zonation is a deviation from the norm or a confirmation of the law of latitudinal zonation?
Questions at the end of the paragraph
1. Why does the change in natural conditions in the mountains occur vertically and manifest itself more sharply than on the plains?
The change in natural zones in the mountains occurs more abruptly than in the plains, since natural conditions change faster.
2. What altitude zones predominate in the Russian mountains? What areas of the world can they be compared to?
The mountains of Russia are dominated by taiga, tundra zones and arctic desert zones. They can be compared to the northern regions of Eurasia and North America.
3. What determines the set of altitude zones?
The set of altitude zones depends on the geographical location of the mountains.
4. If in the north of the Russian Plain there were mountains higher than the Caucasus, would they be richer in the number of altitudinal zones?
The high mountains in the north of the Russian Plain would not be richer in the set of belts of the Caucasus.
5. How do mountains affect human life and health?
With the height of the mountains, individual components of nature and the entire natural complex change. As you rise upward, the air temperature decreases, the amount of precipitation increases (especially on the windward slopes of the mountains), and the air humidity changes. All this affects the characteristics of the soil cover and the organic world. Compared to the plains, the mountains have their own “nature calendars” - the timing of the development of plants, both cultivated and wild. Life in the mountains is subject to the course of natural processes. The way of life of the people, their clothing, and traditional activities are different here.
The “press” of nature in the highlands, that is, on the highest mountain “floors,” is felt by everyone: permanent residents, observers at weather stations, mine workers, and climbers. It is colder here, atmospheric pressure is lower, there is less oxygen, more ultraviolet rays. Even cars feel the specific climate of the sky: the boiling point of water, the proportions of the combustible mixture in engines, and the properties of lubricating oils change with altitude.
FINAL ASSIGNMENTS ON THE TOPIC
1. Prove that a natural area is a natural complex.
Both natural zones and natural complexes have a unity of natural components. When natural conditions change, both natural complexes and natural zones change.
2. Which Russian scientist was the founder of the doctrine of natural zones?
Vasily Vasilievich Dokuchaev
3. Name all the natural zones of Russia. Prove that they are placed regularly.
On the territory of Russia there is a change from north to south of the following natural zones: arctic deserts, tundras, forest-tundras, taiga, mixed and broad-leaved forests, forest-steppes, steppes, semi-deserts. Almost all zones of our country stretch for thousands of kilometers from west to east, and yet throughout their entire length they retain significant common features determined by the prevailing climatic conditions, the degree of moisture, soil types, and the nature of the vegetation cover. Similarities can also be seen in surface waters and modern relief-forming processes.
4. Name the treeless zones of our country. Where are they located? What are their similarities and what are their differences?
Treeless zones are arctic deserts, tundra, steppes, deserts and semi-deserts. Arctic deserts and tundra are located in the Arctic and subarctic zones, in the northern regions. The steppe zone, deserts and semi-deserts are located in the southern regions. Their similarity is the absence of woody vegetation. The differences are that the reason for treelessness in the northern regions is the harsh climate, in the southern regions it is due to insufficient moisture.
5. Which natural zone of our country occupies the largest territory? Find areas within its boundaries that have different natural conditions and think about what explains this.
The taiga zone is the largest natural zone in Russia. In different areas of the vast taiga zone, many natural conditions are different - the overall severity of the climate, the degree of moisture, mountainous or flat terrain, the number of sunny days, and the diversity of soils. Therefore, the coniferous trees that form the taiga are also different, which, in turn, changes the appearance of the taiga in certain areas. Dark coniferous spruce-fir forests predominate in the European part of the zone and in Western Siberia, where they are joined by pine forests. Most of Central and Eastern Siberia is covered with larch forests. Pine forests grow everywhere on sandy and gravelly soils. The forests of the Far Eastern Primorye have a very special character, where on the Sikhote-Alin ridge the usual conifers - spruce and fir - are joined by such southern species as Amur velvet, cork oak, etc.
The zone of mixed and broad-leaved forests has the greatest productivity. It has fertile soils, sufficient moisture, and rich flora and fauna.
8. Determine what natural zone we are talking about if the following grows in it:
A) dwarf birch, dwarf cedar, moss;
B) larch, cedar, birch, aspen, alder. Name the soils and typical animals characteristic of both zones.
A) tundra. Animals – reindeer, arctic fox, goose, goose.
B) mixed forests. Animals – elk, roe deer, hares, foxes, badgers, lynxes, black grouse, partridges.
9. Name the optimal natural conditions necessary for successful farming. In which natural area do you know such conditions exist?
Favorable thermal conditions, sufficient moisture, fertile soils. The thermal regime of the zone of mixed and deciduous forests and the degree of its moisture content are favorable for agriculture. Soddy-podzolic and gray forest soils are characterized by high fertility.
11. Practical work No. 10. Explanation of the principles of identifying large natural regions on the territory of Russia. Compare the map diagram (Fig. 81) with the physical and climatic maps of Russia in the atlas.
What natural boundaries do the boundaries of natural areas coincide with?
The boundaries of natural areas coincide with the boundaries of large landforms.
Do climate indicators influence the drawing of boundaries?
Climatic indicators also influence the drawing of boundaries.
Draw a conclusion about which components of nature are the most important when zoning a territory.
The main components of nature when zoning a territory are relief and climate.
Altitudinal zonality or altitudinal zonality is a natural change in natural conditions and landscapes in the mountains as the absolute height increases. Accompanied by changes in geomorphological, hydrological, soil-forming processes, composition of vegetation and fauna. Many features of altitudinal zonation are determined by the location of the slopes in relation to the cardinal points, dominant air masses and distance from the oceans. The number of belts usually increases in high mountains and as one approaches the equator.
Altitudinal zonality is determined by changes in density, pressure, temperature, moisture and dust content of air with altitude. Atmospheric pressure decreases in the troposphere by 1 mmHg. Art. for every 11-15 m of height. Half of all water vapor is concentrated below 1500 - 2000 m, quickly decreasing with increasing altitude and dust content. For these reasons, the intensity of solar radiation in the mountains increases with height, and the return of long-wave (or thermal) radiation from the surface of mountain slopes into the atmosphere and the influx of counter thermal radiation from the atmosphere decrease. This leads to a decrease in air temperature within the troposphere by an average of 5-6°C for every kilometer of altitude. The conditions for condensation of water vapor are such that the number of clouds, concentrated mainly in the lower layers of the troposphere, increases to a certain height. This leads to the existence of a belt of maximum precipitation and to its decrease at higher altitudes.
The set of altitudinal zones of a mountain system or a specific slope is usually called the spectrum of zones. In each spectrum, the basic landscape is the foothills of the mountains, close to the conditions of the horizontal natural zone in which the given mountain system is located.
There is an analogy in the change of altitudinal zones within the spectrum of a mountainous country, on the one hand, and horizontal geographical zones from low to high latitudes, on the other hand. However, there is no complete identity between them. For example, the tundra of Arctic latitudes is characterized by a polar day and a polar night, and with them a special rhythm of hydroclimatic and soil-biological processes. The high-mountain analogues of the tundra in lower latitudes and alpine meadows lack such features. The high-mountain regions of equatorial latitudes are characterized by special landscapes - paramos (Andes of Ecuador, Kilimanjaro), which have little in common with the belt of alpine meadows.
The most complete altitudinal spectra can be observed in the high mountains of equatorial and tropical latitudes (Andes, Himalayas). Toward the poles, the levels of altitudinal belts decrease, and the lower belts at certain latitudes wedge out. This is especially well expressed on the slopes of meridionally elongated mountain systems (Andes, Cordillera, Urals). At the same time, the altitudinal spectra of the external and internal mountain slopes are often different.
The composition of the altitudinal spectra also changes greatly with distance from the seas inland. Oceanic regions are usually characterized by a predominance of mountain-forest landscapes, while continental regions are characterized by treeless ones.
The composition of altitudinal spectra also depends on many local conditions - features of the geological structure, slope exposure in relation to the sides of the horizon and prevailing winds. For example, in the Tien Shan mountains, the high-altitude belts of mountain forests and forest-steppe are characteristic mainly of the northern, i.e., shady and more humid, slopes of the ridges. The southern slopes of the Tien Shan at the same levels are characterized by mountain steppes.
Altitudinal zones create a variety of impressions and, as a result of the contrast of the zones, their special acuteness when traveling and climbing in the mountains. Within one day, the traveler manages to visit different zones - from the belt of broad-leaved forests to alpine meadows and eternal snow.
In Russia, a particularly complete range of altitudinal zones is observed in the Western Caucasus in the Fisht or Krasnaya Polyana region. Here, on the southern slope of the Main Caucasus Range, rising, for example, from the Mzymta valley (500 m above sea level) to the Pseashkho peak (3256 m), one can observe a change in numerous altitudinal belts. Oak forests, alder forests and subtropical Colchis forests of the foothills give way higher up to beech forests with the participation of hornbeam and chestnut forests. The upper belts of vegetation are formed by dark coniferous fir and spruce forests, light pine forests, and park maple forests. This is followed by crooked forests, subalpine and alpine meadows. The top of the pyramid at altitudes above 3000 m is closed by the subnival and nival-glacial belts.
The altitudinal zones of the territory of the Russian Federation are diverse and closely related to latitudinal zones. With altitude, soil and vegetation cover, climate, geomorphological and hydrological processes are transformed.
Changes in the components of nature provoke a change in natural complexes, in the process of which high-altitude zones are formed. The change in territorial natural complexes depending on altitude is called altitudinal zonation or vertical zonation.
Factors influencing the formation of altitudinal zones
The process of formation of different types of altitudinal zones is influenced by the following factors:
1. Geographical location of the mountain system. The altitude position and number of mountain belts in a particular mountain system depends on the latitude of the territory in which they are located, as well as its position in relation to the nearest oceans and seas. The altitude of the mountain belts of Russia increases in the direction from north to south.
A striking example of this theory is the height of the Ural mountain system, which is located in the northern part of the state. The maximum height of the Ural Mountains is 1100 m, while for the Caucasus Mountains this figure serves as an average height. Each mountain system has a different number of altitude zones.
2. Relief. The distribution of snow cover, the preservation of weathering products, and the level of moisture determine the relief of mountain systems. It is the relief structure of the mountains that influences the formation of natural complexes, in particular vegetation cover.
3. Climate. Climatic conditions are the most important factor due to which the formation of altitudinal zones occurs. With increasing altitude relative to sea level, significant changes occur in the level of solar radiation, temperature conditions, wind strength and direction, and general weather type. The climate affects the flora and fauna of mountain systems, ultimately creating a certain authentic natural complex.
4. Slope exposure. The exposure of mountain slopes plays a significant role in the distribution of moisture, heat, and weathering processes. In the northern parts of mountain systems, the slopes are located much lower than in the southern parts.
History of the formation of the altitudinal zonation of Russia
The formation of altitudinal zones on the modern territory of the Russian Federation originates in the early Pleistocene, during the interglacial period (Valdai and Moscow glaciations). Due to repeated climatic transformations, the boundaries of altitudinal zones shifted several times. Scientists have proven that all modern mountain systems in Russia were originally located approximately 6° above their current position.
The altitudinal zonation of Russia led to the formation of mountain complexes - the Urals and the mountains of the south and east of the state (Caucasus, Altai, Baikal mountain ranges, Sayans). The Ural Mountains have the status of the most ancient mountain system in the world; their formation supposedly began in the Archean period. The mountain systems of the south are much younger, but due to the fact that they are closer to the equator, they significantly predominate in terms of height.