As you know, the earth's crust is quite heterogeneous in its structure. Some areas are still subject to the influence of endogenous processes, while others have long been in absolute peace. But do not forget that tectonic movements will constantly change the Earth's surface, and especially the most vulnerable parts of the crust - geosynclines. These areas are highly mobile and have little power, unlike platforms. What are geosynclines? Let's take a closer look at this term in terms of geography.
Geosynclines in geography: definition and general characteristics
What is a geosyncline in geography? The definition will look like this: a large, elongated area that has been subjected to deformation and subsidence for quite a long time, as a result of which a rather impressive layer of rocks of sedimentary and volcanic origin has accumulated in it. These are very plastic and mobile sections of the earth's crust, which throughout the entire tectoniccycle undergo significant changes.
Types of geosynclines
Depending on the tectonic conditions of formation and structure of the sedimentary layer, two types of geosynclines are distinguished. An evolving sequence of tectonic events leads to deformation of the surface of these areas and the formation of both positive and negative landforms:
Miogeosyncline. This form is usually formed on a shallow shelf, in those places where the earth's crust is the thinnest and most vulnerable. Under the influence of heavy loads, it does not break, but bends, all thanks to the plastic structure of the constituent rocks. In the place of the deflection, a depression is formed, which, like a funnel, attracts sedimentary material. An increase in the mass of sedimentary deposits leads to a further decrease in the level of the depression, and this, in turn, provokes the accumulation of huge layers of sediments, which lie on top of each other in layers. The composition of the deposits is quite typical. These are mainly sand, silt, carbonate sediments and silts. Gradually, after millions of years and under the influence of critical pressure, all these deposits are transformed into sedimentary rocks: shale, limestone, sandstone
Eugeosyncline. Quite often, the tectonic conditions in which sediments usually accumulate are sharply disturbed. Most often this occurs in places of convergent (towards each other) moving plates. So, the oceanic plate can approach the continental one, and all this happens at the very base of the continental slope. In these places, the boundary usually lies between the shelf and moredeep part of the ocean. If a sharp fluctuation of the earth's crust occurs within this zone, then subduction (lowering) of the oceanic plate under the continental one will occur, and this will lead to the formation of a deep-water trench. Like miogeosynclines, they are not confined to the shelf zone and can be located anywhere on the ocean floor. But mostly these are island arcs, archipelagos with active volcanoes, continental coasts with increased seismic activity. In the trenches, there is also an intensive accumulation of sediments, but unlike miogeosingkinals, they are of endogenous origin (formed as a result of volcanic activity). The few sedimentary and clastic deposits are of a very rough texture and interspersed with layers of bas alts that have erupted as a result of underwater eruptions. Constant subduction drags these deposits into the very depths of the mantle, where, under the influence of enormous temperatures and pressure, they metamorphose into amphibolites and gneisses
Internal structure of movable belts
The structure of the geosyncline is extremely complex. After all, it is a cunning plexus of absolutely heterogeneous structural elements. Everything is intertwined together: island arcs, sections of the ocean floor, parts of the coast of marginal seas, fragments of continents and oceanic uplifts. But three components can be clearly distinguished:
- Edge deflection. It is located at the junction of folded areas and platforms.
- Periphery zone. Formed as a result of amalgamationoceanic plateaus, island arcs and submarine ridges.
- Zone of orogeny. Places where mountain building processes are constantly taking place, mainly due to the collision of continental and oceanic blocks.
A bit of geology: the rocks that make up geosynclinal regions
In a simple sense, geosynclines are huge troughs that are filled with all sorts of rocks. It should be noted that the constituent material has a very heterogeneous structure. In geosynclinal deposits there are powerful bodies of igneous, sedimentary and even metamorphic rocks. Gradually, all of them are involved in ongoing folding processes and mountain building. Most common geosynclinal formations:
- volcanogenic siliceous;
- flash;
- greenstone;
- clay-shale;
- mollas (mainly oceanic);
Also often the presence of intrusions - atypical inclusions in the bulk of the rocks. Most often, these are granite and ophiolite formations.
Evolution of geosynclines: main stages of development
And now consider the evolution of geosynclines and the stages of their development. In one tectonic cycle, 4 stages pass:
- The first stage. At the very beginning, the geosyncline is a shallow trough with single relief formations. Then there is a further lowering of the earth's crust, and the depression is filled with sedimentary material, which is brought by the riverbeds andcurrents. The structure of the geosyncline is also gradually becoming more complex.
- Second stage. The area begins to be divided into deflections and uplifts, the relief becomes much more complicated. Under the weight of sedimentary strata, crustal fractures and displacements may appear.
- Third stage. The deflection is replaced by an uplift. The amount of accumulated material is so great that a positive landform begins to form from the geosyncline.
- Fourth stage. Exogenous processes are replaced by endogenous ones. At the final stage, tectonic processes in the earth's crust play an important role. They provoke the transformation of the constituent rocks and turn the geosyncline into a fold-block area.
Geosynclinal regions of our planet
As we remember, geosynclines are areas that are constantly in motion and undergo deformation. These factors significantly influenced the distribution of zones over the Earth's surface. Usually they are located between ancient platforms or between the mainland and the oceanic crust. Marginal seas, trenches, island arcs, and archipelagos are most common in these zones. The length of geosynclinal zones can stretch for tens and even hundreds of thousands of kilometers, bending around the Earth step in arcs and belts.
Outdated geological theory
The modern theory of plate tectonics was long preceded by the hypothesis of geosynclines. It received its wide development at the end of the 19th century and was relevant until the 60s of the 20th century. Even at that distant time, scientists were able to determine that deepsubsidence of the earth's crust are the basis for active mountain building processes. It was believed that the reason lies in the activation of the endogenous forces of the Earth, which launched a new cycle under the pressure of the accumulated sedimentary material. Later it turned out that everything depends on the tectonic movement of the plates, and the hypothesis is outdated.
Principal differences between geosynclines and platforms
It is believed that geosynclines are the most active parts of the Earth's crust. They are more unstable and mobile, unlike platforms, which, in turn, are relatively stable. Geosynclines are located on the periphery of tectonic plates, in places of their frequent collision, and therefore occupy thinner and more vulnerable parts of the earth's crust. Platforms, on the contrary, are located in the central and more stable part of the mainland, where the thickness of the crust is maximum.
Geosynclinal belts of the Earth
According to the theory of geosynclines, in the last 1.6 billion years of the development of our Earth, five main mobile belts have formed on the planet:
Pacific. The belt circles around the ocean of the same name and separates its bed from the continental platforms of Asia, North and South America, Antarctica and Australia
- Mediterranean. It connects with the first in the waters of the Malay Archipelago, and then extends all the way to Gibr altar, crossing the southern part of Eurasia and Northwest Africa.
- Ural-Mongolian. The arc goes around the Siberian platform and separates it fromEast European Plain in the west and Sino-Korean in the south.
- Atlantic. Encircles the coasts of the continents located in the northern part of the ocean.
- Arctic. Extends along the Eurasian and North American coasts of the Arctic Ocean.
It is noteworthy that these areas coincide with the places of the highest volcanic activity, as well as a large concentration of mountains and deep-sea trenches in these territories.
