Geosyncline and isostasy

A. GEOSYNCLINES

Geosynclines are geological features that played a significant role in the formation of continents and ocean basins. They are characterized by long, narrow, and shallow depressions filled with sediments and surrounded by mobile zones of water.

I. Meaning and Concept of Geosynclines

1. What are Geosynclines?

Geosynclines refer to the geological history of continents and ocean basins. They represent ancient nuclei of the present continents, which are surrounded by mobile zones of water. Geosynclines are water depressions characterized by sedimentation and subsidence.

2. Geological History of Continents and Ocean Basins

The concept of geosynclines denotes that the globe was characterized by rigid masses representing ancient continental nuclei, surrounded by mobile zones of water. These mobile zones of water are known as geosynclines.

II. General Characteristics of Geosynclines

Geosynclines possess several general characteristics that define their nature and behavior:

1. Long, Narrow, and Shallow Depression: Geosynclines are elongated in shape, narrow in width, and relatively shallow in terms of depth. They represent low depressions in the Earth’s surface.
2. Gradual Sedimentation and Subsidence: Geosynclines undergo gradual sedimentation, with layers of sediments accumulating over time. Additionally, these areas experience subsidence, leading to further depression.
3. Mobile Zones of Water: Geosynclines are characterized by the presence of water bodies, which are mobile in nature.
4. Bordered by Forelands: Geosynclines are bordered by two rigid masses known as forelands, which are the ancient nuclei of present continents.
5. Changing Nature, Patterns, Locations, Shape, Dimensions, and Extent: Geosynclines undergo changes due to Earth movements, resulting in alterations in their nature, patterns, locations, shape, dimensions, and extent.

III. Evolution of the Concept of Geosynclines

1. James Hall and Dana: The concept of geosynclines was initially proposed by James Hall and elaborated upon by Dana. According to Dana, geosynclines are long, narrow, and shallow beds of the sea that sink gradually.
2. Other Concepts:
   • a. Concept of E. Haug: E. Haug proposed the concept that geosynclines are deeper water bodies existing as mobile zones of water on rigid masses. He identified five rigid masses, including the North Atlantic Mass, Sino-Siberian Mass, Africa-Brazil Mass, Australia-India Madagascar Mass, and Pacific Mass. Between these rigid masses, four geosynclines were identified, namely the Rockies geosyncline, Ural geosyncline, Tethys geosyncline, and Circum-Pacific geosyncline.
   • b. Concept of Arthur Holmes: Arthur Holmes identified four major types of geosynclines and their formation mechanisms. These types include geosynclines formed by the migration of magma, metamorphism of rocks, compression resulting in subsidence, and thinning of the sialic layer due to tensile forces exerted by diverging convective currents.
   • c. Concept of J.W. Evans: J.W. Evans proposed that geosynclines undergo gradual sedimentation and change with changing environmental conditions. They can be narrow or wide and occur in various alternative situations, such as between two landmasses (e.g., Tethys geosyncline between Laurasia and Gondwanaland), in front of a mountain or plateau (e.g., Indo-Gangetic plains), along the margins of continents, or in front of a river mouth.
   • d. Concept of Schuchert: Schuchert categorized geosynclines into three types. Monogeosynclines are exceptionally long and narrow with shallow water tracts and undergo only one sedimentation cycle and mountain-building phase. Polygeosynclines are long and wide water bodies that experience more than one phase of orogenesis. Mesogeosynclines are very long and narrow mobile ocean basins that go through several geosynclinal phases of sedimentation, subsidence, and folding.

IV. Stages of Geosynclines

The formation and evolution of geosynclines occur through three stages:

1. Lithogenesis: This stage involves the creation of a geosyncline through sedimentation and the subsidence of geosynclinal beds.
2. Orogenesis: Orogenesis refers to the squeezing and folding of geosynclinal sediments, leading to the formation of mountain ranges.
3. Gliptogenesis: Gliptogenesis is the gradual rise of the geosyncline, followed by denudation processes that lower the height of mountains.

In conclusion, geosynclines are geological features characterized by long, narrow, and shallow depressions filled with sediments. They have played a crucial role in the geological history of continents and ocean basins, and their concept has evolved over time through the contributions of various scientists. Understanding the general characteristics and stages of geosynclines helps elucidate the complex processes involved in the formation of mountain ranges and the Earth’s dynamic nature.

B. ISOSTASY

Isostasy is a geological concept that explains the mechanical stability between the upper standing parts of the Earth’s crust and the low-lying basins on a rotating Earth. It was first proposed by Dutton and has since played a significant role in understanding the distribution of landmasses and the formation of various relief features.

I. Meaning and Concept of Isostasy

1. First Proposed by Dutton: Isostasy was first proposed by Dutton, who observed discrepancies during mountain expeditions in the Andes. He noticed that the gravitational attraction of the mountains was much smaller than expected based on their mass.
2. Mechanical Stability: Isostasy refers to the mechanical stability between the upper standing parts of the Earth’s crust, such as mountains, plateaus, and plains, and the low-lying basins. It accounts for the equilibrium of these landforms on a rotating Earth.
3. Discovery of the Concept: The concept of isostasy was further solidified through the geodetic survey of the Indo-Gangetic plains. This survey revealed discrepancies in the distribution of mass beneath the Earth’s surface.

II. The Concepts of Airy and Pratt

1. The Concept of Sir George Airy: Airy’s concept of isostasy is related to the principle of flotation. According to Airy, the upstanding crustal parts, such as landmasses, are made of lighter materials. To maintain equilibrium, major portions of these columns are submerged in greater depths of lighter materials. The crustal parts “float” in the denser magma of the substratum, similar to how Sial floats in Sima. For example, the Himalayas are floating in denser magma.

• Uniform Density with Varying Thickness: Airy proposed that the density of rocks remains uniform but their thickness or length varies from place to place.

1. The Concept of Archdeacon Pratt: Pratt’s concept of isostasy is related to the law of compensation. He suggested that there is a level of compensation above which there is variation in the density of different columns of land. However, below this level, there is no change in density.

• Uniform Depth with Varying Density: Pratt’s concept differs from Airy’s in that density doesn’t change within one column but varies from one column to another above the level of compensation.

III. The Concept of Arthur Holmes

Arthur Holmes proposed the concept of equal weight along the level of equal pressure. According to Holmes, higher columns stand because there is lighter material below them for a greater depth. Conversely, smaller columns have lighter material below them for a lesser depth. The total weight of each column along the level of equal pressure is obtained by summing up the product of densities and thickness.

IV. Conclusion

In conclusion, isostasy is a fundamental concept that helps in studying different relief features of varying magnitudes standing on the Earth’s surface. It explains the mechanical equilibrium between landmasses and basins and sheds light on the distribution of mountains, plateaus, plains, lakes, seas, and oceans. By understanding isostasy, geologists can unravel the complex mechanisms behind the formation and stability of various landforms on our dynamic planet.