Geosynclines & Cycle of Erosion – UGC NET Geography – Notes

Geosynclines

A geosyncline may be defined as “a thick, rapidly accumulating body or sediment formed within a long, narrow, subsiding belt of the sea which is usually parallel to a plate margin”. (Oxford Dictionary of Geography)

Or we may say a geosyncline is a “very large linear depression or down-warping of the earth’s crust, filled (especially in the central zone) with a deep layer of sediments derived from the land masses on each side and deposited on the floor of the depression at approximately the same rate as it slowly, continuously subsided during a long period of geological time”. (Penguin Dictionary of Geography)

Evolution of the Concept of Geosynclines

The concept of geosynclines came into existence in 1859. Based upon his research on the stratigraphy and structure of the northern Appalachians, James Hall discovered that the folded Palaeozoic sediments belonging to mountain ranges are shallow-water type of marine origin having a thickness of 12 km. James Hall also found that the thickness was ten to twenty times greater in comparison to the unfolded rock strata of corresponding ages found in the interior lowlands towards the west.

The deposition of massive sequence of shale, sandstone and limestone suggests that the underlying floor of older rocks subsided by a similar amount. The mountain formation was preceded by prolonged periods of down-warping during which the process of sediment accumulation maintained a balance with the subsidence of the crust. Dana (1873) called such elongated belts of subsidence and sedimentation ‘geosynclines’.

H. Stille further categorised geosynclines into miogeosynclines and eugeosynclines. Eugeosynclines are characterised by intermittent volcanic activity during the process of sedimentation, whereas miogeosynclines have low volcanic activity.

The two classes are found side by side separated by a geanticline in the middle. Miogeosynclines are now considered to be former continental margins like those fringing the Atlantic Ocean and eugeosynclines represent the inverted and deformed equivalents of ocean basins of smaller magnitude such as the marginal basins of the western part of the Pacific, the Sea of Japan and the Sea of Okhotsk.

Schuchert categorised geosynclines on the basis of size, location and evolutionary history.

The three categories according to him are as follows:

(i) Monogeosynclines are exceptionally long and narrow tracts. Such geosynclines are situated either within a continent or along the littoral areas. They are called ‘mono’ since they pass through only one cycle of sedimentation and mountain-building. An example is the Appalachian geosyncline which was folded from the Ordovician to the Permian period.

(ii) Polygeosynclines are broader than monogeosynclines. These geosynclines had a longer period of existence than the monogeosynclines. They passed, through more than one phase of orogenesis. The Rockies and Ural geosynclines are examples of polygeosynclines. Such mountain ranges exhibit complex parallel anticlines called geanticlines.

(iii) Mesogeosynclines are surrounded by continents on all sides. They have greater depth and a long and complex geological history.

E. Haug defined geosynclines as deep water regions of considerable length but relatively narrow in width. Haug drew palaeogeographical maps of the world to prove that the present-day fold mountains originated from massive geosynclines of the past. Haug postulated five major landmasses belonging to the Mesozoic Era, namely (i) North Atlantic Mass (ii) Sino-Siberian Mass (iii) Africa- Brazil Mass (iv) Australia-India Madagascar Mass and (v) Pacific Mass. He identified four geosynclines located between these rigid masses: (i) Rockies geosyncline (ii) Ural geosyncline (iii) Tethys geosyncline and (iv) Circum-Pacific geosyncline. According to Haug, the transgressional and regressional phases of seas have a direct impact on the littoral margins of the geosynclines.

The finer sediments are deposited centrally in the geosynclines whereas the coarser sediments are deposited in marginal areas where depth of water is shallow All the geosynclines do not have the same cycle of sedimentation, subsidence, compression and folding of sediments. Haug’s theory is criticised because of its confusing ideas.

The palaeo geographical map by Haug shows land areas disproportionately larger than oceanic areas or geosynclines. Critics raise questions about the existence of such a huge landmass after the Mesozoic Era. Haug’s idea of deep geosynclines isalso not acceptable because of the evidence of marine fossils found in Fold Mountains. Marine organisms from which the fossils are derived are found only in shallow waters. According to J.W. Evans, the form and shape of geosynclines change according to the changes which occur in the environment.

According to Evans, (i) geosynclines may be placed between two landmasses, e.g., Tethys geosyncline between Laurasia and Gondwanaland; (ii) geosynclines may be found in front of a mountain or a plateau, for example, after the origin of the Himalayas there was a long trench in front of the Himalayas which was later filled with sediments leading to the formation of the vast Indo-Gangetic plains; (iii) geosynclines are found along the continental margins; (iv) geosynclines may exist in front of a river mouth.

According to Arthur Holmes, earth movements rather than sedimentation cause subsidence of geosynclines through a long and gradual process, e.g., the deposition of sediments up to 12,160 metres in the Appalachian geosyncline could be possible during a period of 300,000,000 years. Holmes identifies four types.

(i) Geosyncline Formed by Magmatic Migration:

Holmes considers earth crust to be made of three layers:

(a) External layer of granodiorite (10-12 km thick);

(b) Intermediate amphibolite (20-25 km thick);

(c) intermediate amphibolite (20-25 km thick);

(d) Eclogite and some peridotite. The migration of magma from the intermediate layer to the surrounding areas causes subsidence of the upper layers, leading to the formation of a geosyncline.

(ii) Geosynclines Formed by Metamorphosis:

The lowermost rock layers are metamorphosed due to compression caused by convergence of convective currents. Thus the density of rocks increases resulting

in geosyncline formation. Holmes believes that the Caribbean Sea, the western part of the Mediterranean Sea and the Banda Sea were formed by this process.

(iii) Geosynclines Formed by Compression:

Subsidence may occur in the earth crust due to compression. Such a compressional activity occurs because of converging convective currents. Examples are the Persian Gulf and the Indo-Gangetic trough.

(iv) Geosynclines Formed due to Thinner Sialic Layer:

When a column of rising convectional currents diverges after reaching the bottom layer of the crust, two possibilities arise, (a) the sial is stretched apart owing to tensile forces. This causes thinning of sialic layers and the formation of geosynclines. (b) The continental mass may be broken apart to form geosynclines. Examples are found in the former Ural geosyncline.

Dustar identified three types of geosynclines in his classification mainly on the basis of structure of mountain ranges, (i) Inter-continental geosynclines are located between two land masses. (Schuchert’s monogeosyncline coincides with this type.) (ii) Circum-continental geosynclines are located on the borders of continents; (iii) Circum-oceanic geosynclines are found along the littoral areas of oceans. Such geosynclines are also called special type of geosynclines or unique geosynclines.