Geosynclines Theory

Geosyncline theory was a primary theory of the Earth until the discovery of seafloor spreading and the advent of plate tectonic theory in the 1960s. The Geosyncline theory explains the formation of tertiary fold mountains such as the Great Himalayas, The Atlas, The Alps, The Rockies and the Andes Cordillera.

According to this theory, sediments from the continents gradually reach the long, narrow, and shallow water depressions through rivers. These water depressions are generally bordered by forelands. Because of the earth's constantly changing nature as a result of land compression, those sediments rise as fold mountains.

Historical background

In the year 1857, the American geologist named James Hall observed that the thickness of deformed Appalachians was more than the undeformed Mississippi valley. He opined that crustal failure and down warping occurred due to the great sediment load in the Mississippi valley. Later, in 1873, an influential American geologist, J.D. Dana, claimed that the down warping must have occurred first, leaving the sediments collected on the trough. He coined the term "Geosyncline" and criticized Hall's idea that the sediments are not dense enough to depress the crust.

Mechanism of the Geosynclinal Orogen Theory

German geologist Kober in his book "Der Bau der Erde" explained the origin of fold mountains on the basis of geosyncline theory and also attempted to elaborate on the formation, geological history, evolution and development of fold mountains. According to Kober, the whole process of mountain building passes through four closely linked stages, as discussed below.

• Geosyncline stage − As a result of the cooling of the earth, the contraction force bends the kratogen (foreland) and creates the geosyncline structure.

• Lithogenesis − The sedimentation of the nearby rivers from the continent flows into the trough and forms sedimentary rocks. Due to the increasing weight of the sediment, the super incumbent load gradually subsides the geosyncline structure through isostatic adjustment. This makes kratogen come closer.

• Orogenesis − Rocks and sediments of geosyncline folded into fold mountain ranges due to the lateral compressional force. Face-to-face movements of hard and rigid continental land masses are the source of lateral compression. In the case of moderate lateral compressional forces between the fold mountain ranges, an uplifted median mass is formed. If the lateral compressional force is intense, the folding and inter-thrusting of rocks are complex, which leads to the formation of marginal range.

• Gliptogenesis − It is a stage of isostatic adjustment with erosion and transportation of materials under the influence of the denudational process from the fold mountain ranges.

Evaluation of Kober’s theory

Even though this theory satisfactorily explains the process of mountain building, it has its own limitations too.

• It is heavily argued that the compressive force powered by the cooling of the earth is not strong enough to build very extensive and massive mountains like the Alps, the Himalayas, the Rockies, and the Andes Cordillera.

• The dynamic nature of the foreland and the stationary nature of the back land has not been reasoned out and it has been proved to be invalid according to Plate Tectonic Theory.

• This theory explains only the formation of the East-West running mountains, not the North-South running mountains.

Hans Stille’s Classification of Geosynclines

Stille’s presented a significant classification of geosynclines, dividing the earth’s crust into two major divisions called cratons and ortho-geosynclines. Cratons are further sub-divided into hochkraton (i.e., stable continental crust) and fiefkraton (i.e., stable oceanic crust). An ortho-geosyncline is a linear geosynclinal belt lying between continental and oceanic terranes, which is further classified into Eugeosynclines (internal volcanic belts) and Miogeosyncline (external non-volcanic belts).

Conclusion

The geosynclinal theory throwed light primarily on vertical motions (down-warp, up-warp) with less focus on horizontal motions due to contraction. The continents were formed and remained stationary for eons to pass by undergoing continuous erosion (destruction) and upheaval (construction).

Frequently Asked Questions

What are Geosynclines?

Geosynclines are the areas of long, narrow, and shallow water depressions bordered by rigid masses and characterized by sedimentation from surrounding areas and subsidence that result in the formation of folded mountains over a period of time.

What are the stages involved in the process of mountain building according to Kober?

According to Kober, the whole process of mountain building passes through four closely linked stages,

• Geosyncline stage

• Lithogenesis

• Orogenesis

• Gliptogenesis

What are Cratons and Ortho-geosynclines?

Hans Stille’s presented a significant classification of geosynclines, dividing the earth’s crust into two major divisions called cratons and ortho-geosynclines. Cratons are further sub-divided into hochkraton (i.e., stable continental crust) and fiefkraton (i.e., stable oceanic crust). An ortho-geosyncline is a linear geosynclinal belt lying between continental and oceanic terranes, which is further classified into Eugeosynclines (internal volcanic belts) and Miogeosyncline (external non-volcanic belts).