The earth’s crust is heterogeneous and consists of different structural elements. The study of the structure of the lithosphere (Earth's hard shell) is one of the tasks that science sets itself. Shields are one such element. It is about them that will be discussed in this information article.
Earth's crust and its structure
The main structural elements of the earth's crust are lithospheric plates, which can be continental or oceanic. These two types differ from each other in structure (transverse section): a granite layer is present in the continental type plates.
Platforms call the most stable (in tectonic terms) parts of the lithospheric plates of the Earth. At the same time, they act as nuclei (basis) for the continents. In addition to them, orogenic (folded) belts — epiplatform and epigeosynclinal — also stand out within lithospheric plates.
The platform is the most stable structure of the earth's crust, which was formed where powerful mountain systems existed hundreds of millions of years ago. Over time, they collapsed, and the surface at this place leveled off. Thus, a powerful and stable structure is formed - the foundation. Subsequently, sedimentary rocks begin to accumulate on it, gradually creating a powerful layer (cover).
All existing platforms on Earth are divided into ancient (in English literature they are often called cratons) and young. Below on the map are the main (ancient) platforms of our planet. They are marked with a reddish tint.
The structure of the earth's crust is studied by science of geography (7th grade). Next, we will examine in more detail the structure of the platform.
Structural elements of the platform (geography, grade 7)
The platform consists of two layers: a crystalline foundation (lies at the bottom) and a sedimentary cover (covers the foundation).
In geological science, there are four structures that make up any platform. Shield, stove, anteclise and syneclise are the main ones. Further we will consider them. Familiarization with these structures is quite enough for the full development of the school course "Geography".
Shields are the exits to the earth's surface of the crystalline basement of the platform. The sizes of such exits can reach 1000 or more kilometers in length. As a rule, shields are characteristic of ancient platform structures.
Plates are vast sections of the platform that are completely covered by sedimentary cover. Very often platforms young in age are covered with such a cover entirely. Therefore, they are also called slabs.
Anteclises and syneclises are already 2nd order structures. Anteclise is called flat elevated sections of plates. A syneclise is a gentle hollow on a slab or, which is less common, within a crystalline shield.
In this article, we will consider the shields of ancient platforms of Eurasia - Siberian and East European. However, before this we dwell in more detail on the question "what is a shield."
Geography: shields are ...
The concept of "shield" is widely used in geological science. For the first time this term was used in Germany by Edward Suess (in 1903).
A shield is an exposure of a crystalline foundation within an ancient platform. Thus, Precambrian rocks, the age of which can reach 3.5-4 billion years, come to the surface of the Earth. They, as a rule, are represented by granites, quartzites, gneisses, which are exposed in fairly vast areas.
Features of the structure of shields
Shields are the main and most stable structures of the continents. As a rule, they are surrounded by belts composed of rocks of Cambrian age. In relief, shields are most often expressed by slightly convex plains or small hills.
Shields are surrounded by more mobile and mobile zones, mountain building processes in which were recorded relatively recently (by geological standards - 100-200 million years ago).
The most famous examples of shields on our planet: Canadian, Ukrainian, Aldan, Baltic. These areas are confined to large deposits of ore minerals (iron ore, copper, manganese, gold, nickel, etc.). Thus, powerful deposits of copper ores and apatites were discovered within the Aldan shield. The largest reserves of ferruginous quartzite in the world were found on the Ukrainian Shield (Kryvyi Rih basin).
The history of the formation and structure of the Siberian platform
The Siberian platform is a large geological area, occupying a huge area in the north-eastern part of Eurasia. This is one of the oldest platforms on the planet, the foundation of which was formed in the Archean. After that, it was covered more than once by the waters of the seas, as a result of which a powerful cover of sedimentary rocks formed here.
The Siberian platform has clear boundaries on the surface of the Earth: the northern one is the southern slopes of the Byrranga mountains, the western one is the Yenisei valley, the southern border runs along the Stanovoi Range, and the eastern one - along the lower Lena River.
The foundation of the Siberian platform is composed of rocks of the Archean and Proterozoic age, which are strongly crumpled into folds. These are gneisses, amphibolites, slates, marble and others. Their age is quite solid: from 2.3 to 3.7 billion years. The sedimentary cover of the platform is composed of rocks of different ages. The northeastern end of the platform is characterized by intrusive rocks that form diamond tubes.
The Siberian platform is unusually rich in various mineral resources. There are large deposits of iron ore, mica, apatite, graphite. Significant reserves of gas and oil, as well as coal, diamonds, copper, nickel ores and gold are confined to the sedimentary cover.
Geological structure of the Aldan shield
The Aldan shield is a ledge of the crystalline basement within the Siberian platform. It is localized in its southeastern part and coincides in relief with the Aldan Highlands and the Stanovoi Range. In the south and west, the shield borders the mountain formation region through a system of deep faults. In the northeast, it is covered by a thick cover of sedimentary deposits of the Cambrian age.
The deposits (floors) of the ancient foundation of the Aldan shield can be traced to the evolution of the earth's crust as a whole. So, in the lowest tier lie gneisses, slates, marble and granulite quartzites. The next floor is filled with sedimentary-volcanogenic rocks zoned metamorphosed. The upper floor is represented by thick sediments of clastic and volcanic rocks, as well as large intrusions.
In different geological epochs, tectonic processes in the Aldan shield intensified many times. This happened in the Paleozoic, Middle Mesozoic, and Cenozoic. This is one of the distinguishing features of this crystal shield.
Many mineral deposits are associated with the territory of the Aldan shield. Thus, significant reserves of iron and copper ores, mica, apatites, kimberlites, coal, gold, and various semiprecious stones have been discovered and explored here.
The history of the formation and structure of the East European platform
The East European platform is one of the largest and most stable platforms of the modern earth's crust. It extends from the Scandinavian Peninsula to the Ural Mountains, occupying almost the whole of Northern and Eastern Europe.
Two powerful outcrops of the crystalline basement stand out in its structure - the Ukrainian and Baltic Shields. Here, in many places, ancient rocks come to the surface - mainly granites and quartzites. In places they form high cliffs, outcrops and very picturesque canyons. In the space between these shields are Belarusian and Voronezh anteclises.
The foundation of the platform is composed of igneous and metamorphic rocks of Precambrian age, which are densely cut by deep tectonic faults. The East European platform formed its foundation in the Late Proterozoic. The platform cover consists of slightly deformed sedimentary and volcanic rocks of different geological age.
Minerals East European Platform
Within the East European Platform, the richest deposits of various minerals have been explored. Some of them are associated with the foundation of this geological structure, while others are associated with its sedimentary cover.
Huge deposits of iron ore (Krivbass, Kursk magnetic anomaly, Kremenchug basin and others), copper, titanium, nickel ores and apatites are confined to the places where the platform foundation surface reaches the surface of the basement. Deposits of natural gas (Volgoural oil and gas province, Dnieper-Donets depression and others), coal and brown coal (Donbass, Moscow region), phosphorites, bauxites and various building materials (limestone, marble, dolomites, etc.) are associated with the sedimentary cover of the platform. .
Geological structure of the Ukrainian shield
The Ukrainian crystalline shield is the protrusion of the foundation of the East European platform on its southwestern outskirts. It stretches a thousand kilometers (within Ukraine and partially Belarus) from the Goryn River in the north to the shores of the Sea of Azov in the south. On the map below it is marked in yellow.
The maximum width of the Ukrainian shield is 250 kilometers. The total surface area is approximately 135 thousand square kilometers.
The Ukrainian shield is composed mainly of igneous and metamorphic rocks of Archean age (these are gneisses, granites, amphibolites, migmatites and others). In many places, these crystalline rocks are exposed, forming beautiful cliffs, rapids, and cascades on lowland rivers.
Minerals of the Ukrainian shield
As you know, ore minerals are confined to the ledges of the foundations of ancient platforms. And the Ukrainian shield here is no exception.
Within this geological structure, large reserves of iron ores (Krivorozhsky basin), uranium ores (Zheltovodsk and Ternovsk deposits), zirconium ores (Volnogorsk deposit), precious and semiprecious stones, building materials (in particular, in Zhytomyr and other regions of Ukraine are mined are discovered top quality). In terms of the total mineral resource potential, the Ukrainian shield has practically no equal both in Europe and in the world.
Minerals of sedimentary type are also found on this shield. Their deposits are confined to insignificant (not more than 50 meters) sections of the cover. First of all, it is brown coal of the Dnieper basin, as well as manganese ores of the Nikopol basin.
Conclusion
The study of the structure of the earth's crust is included in the range of tasks posed by the science of geography. Shields are the structural elements of the ancient platforms of the Earth. These are usually confined to powerful deposits of ore minerals and semiprecious stones.
Aldan shield, as well as Ukrainian, are the largest crystalline protrusions of foundations on the continent of Eurasia. The first of them is located in Russia, within the Siberian platform, and the second - in Ukraine, on the East European platform.