Endogenic Geomorphic Movements

• Earth’s crust and its surface are constantly evolving (changing) due to various forces emanating from below (endogenic forces) as well as above the surface of the earth (exogenic forces).
• These forces cause physical and chemical changes to the geomorphic structure (earth’s surface).
• Some of these changes are imperceptibly slow (e.g. weathering, folding), some others are gradual (e.g. erosion) while the remaining are quite sudden (earthquakes, volcanic eruptions).
• Geomorphic: relating to the form of the landscape and other natural features of the earth’s surface.
• Geomorphic agents: mobile medium (like running water, moving ice masses or glaciers, wind, waves, currents etc.) which removes, transports and deposits earth materials.
• Geomorphic processes: physical and chemical processes that take place on the earth’s surface (folding, faulting, weathering, erosion, etc.) due to endogenic and exogenic forces.
• Geomorphic movements: large scale physical and chemical changes that take place on the earth’s surface due to geomorphic processes.

Endogenic Geomorphic Movements

• The large-scale movements on the earth’s crust or its surface brought down by the forces emanating from deep below the earth’s surface are called as endogenic geomorphic movements or simply endogenic movements (endo: internal; genic: origin; geo: earth; morphic: form).
• The geomorphic processes that are driven by the forces emanating from deep below the earth’s surface are called endogenic geomorphic processes (folding, faulting, etc.).

The force behind Endogenic Movements

• The ultimate source of energy behind forces that drive endogenic movements is earth’s internal heat.
• Earth’s internal heat is a result of mainly radioactive decay (50% of the earth’s internal heat) and gravitation (causes pressure gradients).
• Differences in temperature and pressure (temperature gradients or geothermal gradients and pressure gradients) among various layers of the earth give rise to density differences and these density differences give rise to conventional currents.
• Convectional currents in the mantle drive the lithospheric plates (crust and upper mantle) and the movement of the lithospheric plates (tectonics) is the cause behind endogenic movements.
• The Earth’s rotation (Coriolis effect) can influence where convection currents travel.
• The destination of convection currents determines the nature and location of the endogenic movements.

Classification of Endogenic movements

• Endogenic movements are divided into diastrophic movements and catastrophic or sudden movements.
• Diastrophism refers to deformation of the Earth’s crust.
• Diastrophic movements are gradual and might stretch for thousands of years.
• On the other hand, sudden movements like earthquakes and volcanic eruptions occur in a very short period.
• Diastrophic movements are further classified into epeirogenic movements (continent forming ? subsidence, upliftment) and orogenic movements (mountain building ? folding, faulting).

Diastrophism

• Diastrophism refers to deformation of the Earth’s crust due to diastrophic movements (deforming movements) such as folding, faulting, warping (bending or twisting of a large area) and fracturing.
• All processes that move, elevate or build up portions of the earth’s crust come under diastrophism. They include:
• orogenic processes involving mountain building through severe folding (crust is severely deformed into folds) and affecting long and narrow belts of the earth’s crust;
• epeirogenic processes involving uplift or warping of large parts of the earth’s crust (simple deformation);
• earthquakes and volcanism involving local relatively minor movements;
• plate tectonics involving horizontal movements of crustal plates.
• The most obvious evidence of diastrophic movement can be seen where sedimentary rocks have been bent, broken or tilted.

Epeirogenic or continent forming movements

• Epeirogenic or continent forming movements are radial movements (act along the radius of the earth).
• Their direction may be towards (subsidence) or away (uplift) from the centre.
• They cause upheavals or depressions of land exhibiting undulations (wavy surface) of long wavelengths and little folding.
• The broad central parts of continents are called cratons and are subject to epeirogeny, hence the name continent forming movements.

Uplift

• Raised beaches, elevated wave-cut terraces, sea caves and fossiliferous beds above sea level are evidence of upliftment.

Uplifted landforms

• In India, raised beaches occur at several places along the Kathiawar, Nellore, and Tirunelveli coasts.
• Several places which were on the sea some centuries ago are now a few miles inland due to upliftment.
• For example, Coringa near the mouth of the Godavari, Kaveripattinam in the Kaveri delta and Korkai on the coast of Tirunelveli, were all flourishing seaports about 1,000 to 2,000 years ago.

Subsidence

• Submerged forests and valleys, as well as buildings, are evidence of subsidence.
• In 1819, a part of the Rann of Kachchh was submerged as a result of an earthquake.
• Presence of peat and lignite beds below the sea level in Tirunelveli and the Sundarbans is an example of subsidence.
• The Andamans and Nicobars have been isolated from the Arakan coast by submergence of the intervening land.

Arakan coast (Highlighted part)

• On the east side of Bombay island, trees have been found embedded in the mud about 4 m below low water mark. A similar submerged forest has also been noticed on the Tirunelveli coast in Tamil Nadu.
• A large part of the Gulf of Mannar and Palk Strait is very shallow and has been submerged in geologically recent times. A part of the former town of Mahabalipuram near Chennai is submerged in the sea.

Orogenic or the mountain-forming movements

• In contrast to epeirogenic movement, the orogenic movement is a more complicated deformation of the Earth’s crust, associated with crustal thickening (due to the convergence of tectonic plates).
• Such plate convergence forms orogenic belts that are characterised by “the folding and faulting of layers of rock, by the intrusion of magma, and by volcanism.
• Orogenic or the mountain-forming movements act tangentially to the earth surface, as in plate tectonics.
• Tension produces fissures (since this type of force acts away from a point in two directions), and compression produces folds (because this type of force acts towards a point from two or more directions).

Fissure and Fold

Sudden Movements

• Sudden geomorphic movements occur mostly at the lithospheric plate margins (tectonic plate margins).
• The plate margins are highly unstable regions due to pressure created by pushing and pulling of magma in the mantle (convectional currents).
• These movements cause considerable deformation over a short period.

Earthquakes

• Earthquakes occur when the surplus accumulated stress in rocks in the earth’s interior due to folding, faulting or other physical changes is relieved through the weak zones over the earth’s surface in the form of kinetic energy (seismic waves).
• Such movements may result in uplift or subsidence in coastal areas.
• An earthquake in Chile (1822) caused a one-metre uplift in coastal areas.
• An earthquake in New Zealand (1885) caused an uplift of up to 3 metres.
• An earthquake in Japan (1891) caused subsidence of up to 6 metres.
• Earthquakes may cause a change in contours, change in river courses, shoreline changes, glacial surges (as in Alaska), landslides, soil creeps, mass wasting etc.

Volcanoes

• Volcanism includes the movement of molten rock (magma) onto or towards the earth’s surface through narrow volcanic vents or fissures.
• A volcano is formed when the molten magma in the earth’s interior escapes through the crust by vents and fissures in the crust, accompanied by steam, gases (hydrogen sulphide, sulphur dioxide, hydrogen chloride, carbon dioxide etc.) and pyroclastic material (cloud of ash, lava fragments carried through the air, and vapour).
• Depending on the chemical composition and viscosity of the lava, a volcano may take various forms.

Fold & Fault in Geology, Fold Mountains and Block Mountains

Fold Mountains

• Fold mountains are formed when sedimentary rock strata in geosynclines are subjected to compressive forces.
• They are the loftiest mountains, and they are generally concentrated along continental margins.

Geosyncline: a large-scale depression in the earth’s crust containing very thick deposits. E.g. Tethys geosyncline.

‘Fold’ in geology

• A fold is an undulating structure (wave-like) that forms when rocks or a part of the earth’s crust is folded (deformed by bending) under compressional stress. The folds are made up of multiple strata (rock layers).
• The folds that are upwardly convex are called as anticlines. The core (centre) of an anticline fold consists of the older strata, and the strata are progressively younger outwards.
• In contrast, the folds that are downwardly convex are called synclines. The core of a syncline fold consists of the younger strata, and the strata are progressively older outwards.

• Limbs: The limbs are the flanks of the fold.
• Hinge line: the where the flanks join together (the line of maximum curvature).
• Axial plane: plane defined by connecting all the hinge lines of stacked folding surfaces (the plane in which hinge lines of various strata lie).

Types of folds

• A symmetrical fold is one in which the axial plane is vertical.
• An asymmetrical fold is one in which the axial plane is inclined.
• An isoclinal fold has limbs that are essentially parallel to each other and thus approximately parallel to the axial plane.
• An overturned fold has a highly inclined axial plane such that the strata on one limb are overturned.
• A recumbent fold has an essentially horizontal axial plane.

Classification of fold mountains

On the basis of period of origin

• On the basis of the period of origin, fold mountains are divided into very old fold mountains, old fold mountains and Alpine fold mountains.

Very Old Fold Mountains

• They are more than 500 million years old.
• They have rounded features (due to denudation).
• They are of low elevation.
• Some of the examples are Laurentian mountains, Algoman mountains, etc.

Old Fold Mountains

• Old fold mountains had their origin before the Tertiary period (tertiary period started 66 million years ago).
• The fold mountain systems belonging to Caledonian and Hercynian mountain-building periods fall in this category.
• The Appalachians in North America and the Ural Mountains in Russia are the examples.
• They are also called thickening relict fold mountains because of lightly rounded features and medium elevation.
• Top layers are worn out due to erosional activity. Example: Aravalli Range in India.
• The Aravalli Range in India is the oldest fold mountain systems in India.
• The range rose in post-Precambrian event called the Aravalli-Delhi orogeny.

Alpine or young fold mountains

• Alpine fold mountains belonging to the Tertiary period (66 million years ago to present) can be grouped under the new fold mountains category since they originated in the Tertiary period.
• Examples are the Rockies, the Andes, the Alps, the Himalayas, etc.

Characteristics

• Rugged relief.
• Imposing height (lofty).
• High conical peaks.

On the basis of the nature of folds

Simple fold mountains

• Simple fold mountains with open folds in which well-developed systems of synclines and anticlines are found, and folds are of wavy patterns.

Complex fold mountains

• Complex fold mountains in which the rock strata are intensely compressed to produce a complex structure of folds.
• In the Himalayas, over folds and recumbent folds are often found detached from their roots and carried a few hundred kilometres away by the tectonic forces. These detached folds are called ‘nappe.’

Characteristics of Fold Mountains

• Fold mountains belong to the group of youngest mountains of the earth.
• The presence of fossils suggests that the sedimentary rocks of these folded mountains were formed after accumulation and consolidation of silts and sediments in a marine environment.
• Fold mountains extend for great lengths whereas their width is considerably small.
• Generally, fold mountains have a concave slope on one side and a convex slope on the other.
• Fold mountains are mostly found along continental margins facing oceans (C-O Convergence).
• Fold mountains are characterized by granite intrusions (formed when magma crystallises and solidifies underground to form intrusions) on a massive scale.
• Recurrent seismicity is a common feature in folded mountain belts.
• High heat flow often finds expression in volcanic activity (Himalayas is an exception, because of C-C convergence).
• These mountains are by far the most widespread and also the most important.
• They also contain rich mineral resources such as tin, copper, gold etc.

Block Mountains

• Block mountains are created because of faulting on a large scale (when large areas or blocks of earth are broken and displaced vertically or horizontally).
• The uplifted blocks are termed as horsts, and the lowered blocks are called graben.
• The Great African Rift Valley (valley floor is graben), The Rhine Valley (graben) and the Vosges mountain (horst) in Europe are examples.
• Block mountains are also called fault-block mountains since they are formed due to faulting as a result of tensile and compressive forces.

There are two basic types of block mountains:

1. Tilted block mountains have one steep side contrasted by a gentle slope on the other side.
2. Lifted block mountains have a flat top and extremely steep slopes.

‘Fault’ in Geology

• When the earth’s crust bends folding occurs, but when it cracks, faulting takes place.
• A fault is a planar fracture (crack) in a volume of earth’s crust, across which there has been significant displacement of a block/blocks of crust.
• The faulted edges are usually very steep, e.g. the Vosges and the Black Forest of the Rhineland.
• Faults occur due to tensile and compressive forces acting on the parts of the crust.

• Large faults within the Earth’s crust result from the action of plate tectonic forces, such as subduction zones or transform faults.
• Energy release associated with rapid movement on active faults is the cause of most earthquakes.
• In an active fault, the pieces of the Earth’s crust along a fault move over time.
• Inactive faults had movement along them at one time, but no longer move.
• The type of motion along a fault depends on the type of fault.

Types of faults

Types of faults

Strike-slip fault

• In a strike-slip fault (also known transcurrent fault), the plane of the fault is usually near vertical, and the blocks move laterally either left or right with very little vertical motion (the displacement of the block is horizontal).

Transform fault

• A special class of strike-slip fault is the transform fault or transform boundary when it forms a plate boundary.
• A transform fault is the only type of strike-slip fault that is classified as a plate boundary.
• Most of these faults are hidden in the deep ocean, where they offset divergent boundaries in short zigzags resulting from seafloor spreading.
• They are less common within the continental lithosphere. The best example is the Dead Sea transform fault.
• The transform boundary ends abruptly and is connected to another transform, a spreading ridge, or a subduction zone.

Dip-slip faults

• Dip-slip faults can be either normal or reverse.
• In a normal fault, the hanging wall (displaced block of crust) moves downward, relative to the footwall (stationary block). In a reverse fault (thrust fault) the hanging wall moves upwards.
• Reverse faults occur due to compressive forces whereas normal faults occur due to tensile forces.
• A downthrown block between two normal faults is a graben.
• An upthrown block between two normal faults is a horst.
• Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary.
• Reverse faults occur in areas where the crust is being shortened such as at a convergent boundary.

Rift Valley system

• Tension causes the central portion to be let down between two adjacent fault blocks forming a graben or rift valley, which will have steep walls.
• The East African Rift Valley system is the best example.
• In general, large-scale block mountains and rift valleys are due to tension rather than compression.

Block Mountains

• Block mountains may originate when the middle block moves downward and becomes a rift valley while the surrounding blocks stand higher as block mountains.

Plateaus

• Sometimes, the surrounding blocks subside leaving the middle block stationary. Such cases are found in high plateau regions.

Oblique-slip faults

• A fault which has a component of dip-slip and a component of strike-slip is termed an oblique-slip fault.
• Nearly all faults have some component of both dip-slip and strike-slip.
• Many disastrous earthquakes are caused along the oblique slip.