The Hypothesis of Evans - Evans, J.W. calls geosyncline as sedimentation subsidence. He believes that the forms of geosynclines continue changing. The birth of geosynclines can take place
(i) in the sea close to mountainous cost,
(ii) at the mouth of larger rivers,
(iii) in interior sea between continental blocks,
(iv) close to the mountain or plateau.
In order to maintain its previous shape, the geosynclines begin to subside. Later on, the sediment due to subsidence comes into contact with hot Sima. The Sima is heavier than the upper Sial. Hence Sial.
The Hypothesis of Holmes - Many causes have been stated for the birth of geosynclines by Holmes. A. He has divided the geosynclines on this basis.
(i) Geosyncline of the First Order - According to Holmes the earth's crust consists of three layers. The layer 10 to 12 km., deep is called Granodiorite. It is also known as outer layer. The middle layer is called amphibolite and is about 20-25 km. deep. The innermost layer is called eclogite.
(ii) Geosynclines of Second Order - Due to tension on the outer sial layer, the layer became thin and gave birth to geosyncline. When this tension was produced by the shifting away of the continental blocks, the birth of Tethys geosyncline took place. When the two--blocks separated, the tension so produced-created the Ural geosyncline. The causes of their birth is the displacement of convection currents in the substratum.
(iii) Geosynclines of Third Order - The geosynclines of this order can be further sub-divided into two sub-groups.
Due to the metamorphosis of the sediment in the lower layer, the density of the matter increases and consequently uses subsidence. The medium mass is the birth place of geosynclines of this order. For example, the Western Mediterranean, the Banda Sea, and the Caribbean Sea are examples of this type of geosynclines.
The geosynclines of this type are situated along the mountains and are produced by those compressional forces which have created those mountains. Among the geosynclines of this type, the Indo-Gangetic plain and Persian Gulf are good examples.
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