The formation of fold mountains has been explained through various theories, with the Geosyncline theory being one of the most significant early attempts. This theory provides insight into how long, narrow depressions filled with sediments eventually transform into towering mountain ranges through processes of sedimentation, subsidence, and compression over geological time periods.
What is Geosyncline?
The long, narrow, and shallow depression characterized by sedimentation and subsidence is defined as the geosyncline.
Geosyncline lies between two stable blocks, and these blocks move toward each other due to the subsidence of the floor of geosyncline caused due to the sedimentation of these blocks. Such blocks are known as Foreland.
Geosyncline theory explains the formation of fold mountains. The concept of geosyncline was given by Hall and Dana, and it was further developed by Steers.
CHARACTERISTICS OF GEOSYNCLINE
According to Hall and Dana following are the characteristics of Geosyncline:-
→ These are the narrow, shallow water bodies characterized by sedimentation and subsidence.
→ The sedimentation occurs due to the sediments that are brought by the rivers originating from the surrounding foreland.
→ The forelands are also known as Kratogen, i.e., they are huge landmasses generally made up of igneous and metamorphic rocks and are also known as shields.
• Russian Shield
• Brazilian Shield
• Peninsular India
The Geosyncline after the formation will experience sedimentation which increases the load, and hence the bed of geosyncline starts to subside. This causes the movement of two rigid landmasses towards each other.
TYPES OF GEOSYNCLINE
Monogeosyncline
These are exceptionally long and narrow-shallow water bodies situated between the continent or along its borders.
Appalachian Geosyncline
Polygeosyncline
Long and narrow water bodies which have more than one phase of mountain building or orogenesis.
Urals and Rockies
Mesogeosyncline
They are very long, narrow mobile ocean basins which are bordered/surrounded by continents from all the sides and have passed through several phases are known as Mesogeosyncline.
Himalayas which came out of Tethys Sea
KOBER'S THEORY OF GEOSYNCLINE
Introduction
Famous German geologist Kober attempted to explain the origin of mountains through his famous Geosyncline theory in his book "Der Bau der Erde" (The Structure of the Earth).
Force Applied
According to Kober, two rigid landmasses move toward each other due to contraction caused by the cooling of the Earth's surface, which caused the horizontal movement of the landmasses.
According to him, the present-day mountains were once mobile zones of water.
There are three stages of mountain building:-
- Lithogenesis
- Orogenesis
- Gliptogenesis
STAGES OF MOUNTAIN BUILDING
Lithogenesis (1st Stage)
This is the first stage in the creation of geosyncline in which the forelands are subjected to continuous erosion, and the eroded sediment is deposited in the geosyncline.
Due to increasing weight of the sediment, the bed of the geosyncline gradually subsides. This combination of sedimentation and subsidence results in the deposition of a huge volume of sediment.
Orogenesis (2nd Stage)
Related to the building of mountains, the two forelands start to move towards each other due to contraction resulting from cooling as well as due to sedimentation.
Due to squeezing of sediment marginal mountains are formed on the sides of geosyncline, termed by Kober as Randketten.
If the intensity of the compressive forces is not strong, the middle portion of geosyncline is unaffected and thus remains unfolded. This unfolded part was termed by Kober as Zwischengebirge or Median Mass.
Example: The formation of Tibetan Plateau
If the forces are greater, then the entire region will be folded.
Gliptogenesis (3rd Stage)
In the 3rd stage, the mountains are characterized initially by gradual rise (during Orogenesis) and then later continuous lowering of the height due to the forces of denudation such as rivers, glaciers, etc.
1. Geographers agree that contraction is not a proper force to explain the formation of very extensive mountains such as Himalayas.
In fact, the plate tectonic theory properly explains that such mountains are formed due to collision of plates due to convection currents coming from the Asthenosphere and not due to cooling and contraction.
2. Studies have shown that only one part of the landmass moves while the other part remains stable which is a contradiction with the geosyncline theory in which both the landmasses move toward each other.
Plate tectonics and paleomagnetism have shown enough evidence of Sea Floor Spreading in the Indian Ocean and northward movement of the Indian Plate causing the formation of the Himalayas.
3. The geosyncline theory satisfactorily explains East-West mountains such as Alps and Himalayas but fails to explain North-South mountains like the Andes and Rockies mountains which are associated with active continental margins and subduction zones (Face the Pacific Ocean and are not near any geosyncline).
CONCLUSION
Kober's theory satisfactorily explains only a few aspects of mountain building but suffers from weaknesses in terms of force and limited application.
Thus, Kober can be given credit for advancing the idea of geosyncline, which is then properly explained in the plate tectonic theory.
While the Geosyncline theory has limitations, it represented an important step in understanding mountain formation and laid the groundwork for more comprehensive theories like Plate Tectonics.
Key Terms to Remember
- Geosyncline: Long, narrow, shallow depression with sedimentation
- Foreland/Kratogen: Stable landmass bordering geosyncline
- Randketten: Marginal mountains formed on sides
- Zwischengebirge: Median mass (unfolded middle portion)
- Lithogenesis: Sedimentation stage
- Orogenesis: Mountain building stage
- Gliptogenesis: Erosion and denudation stage
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