• The continuous formation and deformation of Earth's landforms result from the ongoing influence of both external and internal forces. These processes, collectively known as geomorphic processes, bring about changes on the Earth's surface.

Earth Movements

• The Earth's crust and surface undergo constant evolution due to a variety of forces, resulting in physical and chemical changes to the geomorphic structure. These large-scale alterations on the Earth's surface are caused by both endogenic and exogenic forces.
• Earth movements are categorized based on the forces that drive them. There are two main classifications: Endogenic forces, which act within the Earth's interior, and Exogenic forces, which operate on the Earth's surface.

Classification of Earth Movements:

Endogenic Movements:

• Result from the works of endogenetic forces.
• Endogenetic forces lead to land upliftment, subsidence, folding, fracturing, and volcanic eruptions.
• Endogenic movements are further classified into two major categories.

(i) Catastrophic Movements:

• These sudden movements are caused by abrupt endogenetic forces originating deep within the Earth.
• Result in sudden and rapid events at and below Earth's surface.
• Termed as destructive movements, examples include volcanic eruptions, earthquakes, and tsunamis.

(ii) Diastrophic Movements:

• Diastrophic movements involve vertical and horizontal movements leading to the formation of primary landforms like mountains, plateaus, plains, lakes, and major faults.
• Vertical movements can be categorized as upward and downward movements.
• Upward movements result in the emergence of continents, either as a whole or coastal land. For instance, the Allah Bund, an elongated ridge of emergent land bordering the Gulf of Kutch.
• Operate slowly, and their effects become observable over thousands and millions of years, termed as constructive movements.
• Diastrophic Movements are further divided into three processes: isostatic, eustatic, and tectonic.

(a) Isostatic Movement:

• According to this concept, the less dense materials of Earth's surface (Sial) float over the denser magma (Sima) in the Earth's interior.
• The core comprises denser materials, while the Earth's surface is composed of lighter material.
• Isostasy maintains a balance between the upstanding and low-lying parts of the Earth's crust, as explained by Airy and Pratt.
• The term "isostasy," derived from the German word "isostasios," was first proposed by American geologist Dutton in 1889 to express the state of balance.

(b) Eustatic Movement:

• Eustatic changes occur when the volume of seawater changes, influenced by factors such as global warming and the melting of ice sheets.
• This results in either a rise in sea level or a fall during ice ages.

(c) Tectonic Movement:

• Tectonic changes are associated with alterations in the level of the land.
• Sub-divided into Epeirogenic and Orogenic forces.
• Epeirogenic Movement: Refers to regional uplift without marked deformation.
• Caused by radial forces operating at right angles to the Earth's surface, resulting in vertical movement like uplift or subsidence.
• Upwarping and downwarping are outcomes of this vertical movement.
• Uplifted areas form continents, while subsided areas form ocean basins.
• Evidence includes raised beaches, elevated wave-cut terraces, and fossiliferous beds above sea level for uplift, and submerged forests and valleys for subsidence.
• Orogenic Movement: Involves mountain-building, causing deformation in the crust and Earth's surface.
• Far more localized, occurring episodically and widely spaced in geological time.
• Orogeny is a horizontal Earth movement, with tangential forces parallel to the Earth's surface.
• Compression and tension forces result in horizontal movements, leading to folding, faulting, and continental drift.
• Compression forces push rock strata against a hard plane, leading to the bending of rock layers and the formation of fold mountains.

Folding:

• Folding is the result of rock strata bending due to compressional forces acting tangentially or horizontally towards a common point or plane from opposite directions.
• This process leads to the crumbling of strata into folds, forming a series of alternating troughs and crests.
• Upfolds are known as anticlines, and downfolds are termed synclines.
• Along the crest of the anticline, a zone of tension and weakness is present.
• Rivers flowing in this area breach the anticline, erode the material, and deepen the valley. Once fully eroded, the ridge is replaced by a valley, known as an anticlinal valley.
• This phenomenon exemplifies what is commonly referred to as inversion or relief topography.
• Between two anticlinal valleys, the synclinal portion stands higher than the valleys, forming a synclinal ridge.
• Approximately 400 million years ago, the great fold mountains of the world formed as a result of folding.
• Fold mountains are created when two or more of Earth's tectonic plates are pushed together.
• Different types of folding include:
  • Monocline: Formed when horizontally laid beds are tilted and then flatten out, resulting in simple flexure.
• Symmetrical Fold A fold is said to be symmetrical, if both the limbs are equal and inclined at the same angle.
• Asymmetrical Fold These types of folds are generated when one limb in a fold structure is steeper than the other.
• Overturned Fold When one limb occupies the normal position, while the other bends more than 90°, it is said to be an overturned fold.
• Isoclinal Fold Isoclinal folds are formed as a result of continued lateral compression upon an overturned fold. Here, both the limbs dip at equal angles in the same direction.
• Recumbent Fold These are fold lying down. These are formed as a result of continuation of pressure. The axial plane and both limbs of a fold lie roughly and horizontally.

• Closed Fold occurs when the angle between the two sides of the fold is acute (less than 90 degrees), and it is formed due to excessive compression.
• Open Fold, on the other hand, is characterized by an obtuse angle between the two sides of the fold (greater than 90 degrees and less than 180 degrees). These folds are typically created through undulating folds caused by compression.
• Fan Fold has a fan-like appearance, consisting of numerous minor anticlines and synclines known as anticlinorium and synclinorium, respectively.
• Anticlinorium is a large anticline with superimposed minor folds, while Synclinorium is a large syncline with superimposed minor folds.
• Plunge Fold forms when the axis of the fold becomes tilted instead of being parallel to the horizontal plane.
• Warp Fold, a short form of Warping, is created by convergent forces, resulting in crustal bending. Warping involves the upward or downward warping (raising) of the crust over a large area.
• Nappe occurs when the pressure exerted on a recumbent fold is strong enough to tear it from its roots and thrust it forward.

Tension

• Tension forces lead to the breaking or fracturing of rock strata, forming rocks and fractures in the crust.
• The upward or downward displacement of rocks from their original position along such fractures is termed faulting.