Endogenic Forces – Folding, Faulting, Earthquakes And Volcanism

Endogenic Forces – An Overview

 Endogenic forces are the forces that are created inside the earth and are responsible for creation of some relief features along with some phenomenon like folding, faulting, earthquakes and volcanism. Thus these forces are also known as internal forces. These forces are an integral part of the overall earth movements . The forces or movements responsible for the formation of relief features and changes occurring in them are known as earth movements or Geomorphic Processes.  These forces are divided into two broad categories.

• Endogenic forces which cause land upliftment, subsidence, folding, faulting, earthquakes, volcanism etc.
• Exoganic forces which cause destruction of relief features through their Weathering, Erosional and Depositional activities.

Endogenetic Forces (Movements) 

     The forces coming from within the earth and causing horizontal and vertical movements are known as Endogenetic forces. It is these movements which lead to land upliftment and subsidence, folding and faulting, earthquakes and volcanism, etc. Endogenic forces are responsible for giving birth to major relief features such as mountains, plateaus, plains , valleys , etc. These endogenic forces fall into two major categories, on the basis of intensity:

1. Sudden movements: Sudden movement  results in sudden and rapid events such as earthquakes and volcanic eruptions. It must be noted that these events are the result of a long-period of preparation deep within the earth. Only their effects in the form of earthquakes and volcanic eruptions are experienced as sudden events. These are also termed as constructive movements as they produce certain relief features such as volcanic mountains and lava plateaus ( e.g. Deccan )

2. Orogenetic movements: These  movements are caused by the endogenic forces working in a horizontal manner and they involve folding, bending, faulting , and thrusting. These endogenic forces are also known as tangential forces. They are of two types :

• Compressional movement (convergent movement): When Orogenetic or horizontal forces operate face to face. They cause folding of the rock strata on the earth.
• Tensional movement (Divergent Movement): When horizontal forces operate in opposite directions. They produce cracks, fractures and faults in the crustal parts of the earth.

FOLDING

Folding is a process in which the rocks bend instead of breaking when they are subjected to forces generated by tectonic movements. Also folding occurs due to horizontal compression of rock layers by internal forces of the earth. Accordingly this can create a variety of land forms as the surfaces of the folded rocks are eroded in due course of time. Folds are the outcome of slow deformation of rocks. Folding takes place in deep underground where the rocks are under pressure and temperatures are higher. Folded rocks are common in mountain ranges like the Alps, Himalayas and the Scottish Highlands. Up-folds are called anticlines. As shown in the above diagram folds are of various types namely – symmetrical, asymmetrical, overfold, recumbent fold etc. Some other types of folds are :

• Pipe Fold.
• Zigzag Fold.
• Spiral Folds.
• Half-Lock Folds.
• Diaper Folds.
• Drop Folds.
• Inert Folds.

Formation

Fold mountains are often associated with continental crust. They are created at convergent plate boundaries, sometimes called continental collision zones or compression zones. Convergent plate boundaries are ideal sites of collisions, where tectonic plates collide with each other. Thus most fold mountains are found on the edge or former edge of continental plate boundaries. Rocks on the edge of continental crust are often weaker and less stable than rocks found in the continental interior. This can make them more susceptible to folding and warping. Most fold mountains are composed primarily of sedimentary rock and metamorphic rock formed under high pressure and relatively low temperatures. Many fold mountains are also formed where an underlying layer of ductile minerals, such as salt, is present.

FAULTS

When the tensional force is moderate, the crustal rock develop only crack (Fractures) but when intense tensional force work the rock beds are dislocated and displaced also resulting into formation of faults. Thus faults are those fractures in the rock body along which there has been an observable amount of displacement.

Types of  faults

• Normal Faults:  The faults having displacement of both the rock blocks in opposite directions are called  normal faults. Movement of rocks takes place vertically, so that one side is raised or up thrown. In the case of normal faults, there occurs extension of the faulted area.
• Reverse faults (Thrust fault): On account of extreme compression, along with the tensional force , rocks snap and one block of fractured rock is pushed over the underlying block. Fractured rock blocks move towards each other in the reverse faults. There is thus shortening of the crust in these faults.
• Lateral or strike-slip fault: This fault is formed when the rock blocks are displaced horizontally along the fault plane due to horizontal movement. They are commonly produced where one tectonic plate slides past another at a transform fault boundary.

Landforms related to faulting

Rift valley: A linear depression or trough created by the sinking of the intermediate crustal rocks between two or more parallel faults is known as a rift valley. The East African rift valley  system and Rhine rift valley are  famous examples of these morphological features. Dead Sea , the second most saline lake in the world after Lake Van, is situated in a rift valley. Narmada and Tapti rivers are believed to be flowing in a rift valley.

Ramp valley: When both the side block of rocks are raised and the middle portion remains standstill , the resultant trough is known as a Ramp valley. Brahmaputra valley is regarded as a Ramp valley. The best illustration of a ramp valley system is the Turfan Depression, the second lowest place on Earth (154 meters below sea level), which lies within the Tien Shan of western China and along the northern margin of the Gobi.

Block mountain: Also known as fault block mountains, these mountains are  the result of faulting caused by tensile and compressive forces. They represent the upstanding parts of the ground between two faults or on either side of a rift valley. Noted examples are Vosges and Black Forest mountains bordering the faults Rhine rift valley, Wasatch range in USA and Sierra nevada mountains of California (considered to be the most extensive block mountain of the world).

EARTHQUAKES – Endogenic Forces

         Earthquakes are vibrations of the earth caused by ruptures and sudden movements of rocks that have been strained beyond their  elastic limits.

         These Earthquakes can be considered as a form of energy of wave motion transmitted through the surface layer of the earth. The place of origin of earthquakes inside the earth is known as Focus or Hypocentre.

Epicenter: It Is the point on the earth surface vertically above the focus where shock waves reach the first . It is the most affected area.

Seismograph: It is an instrument which records the waves generated by an earthquake.

Isoseismal line: The line joining places which experience equal seismic intensity.

Causes of the earthquakes

Volcanic Eruptions: All volcanic eruptions create vibrations that are manifested in the form of earthquakes.

Faulting : Create strains in the underlying rocks which beyond a certain point fracture creating earthquakes.

Plate Tectonics : Plate boundary interactions give raise to earthquakes

Anthropogenic factors : i.e. man made factors like mining, construction, dams, nuclear explosions etc. can trigger earthquakes.

Distribution of earthquakes

Major earthquakes zones are:

• The circum-pacific zone:  This is a convergent plate boundary zone where most widespread  intense earthquakes occur along the subduction zone. About 66 % of the total earthquakes of the world are recorded in this zone.
• The Mid-Atlantic zone:  This zone, characterized by the divergent plate boundary, spreads over the Mid-Atlantic ridges and several islands near it. The main cause of earthquakes is the sea floor spreading and the associated volcanism.
• Mid-Continental zone: This zone extends along the Alpine mountain system of Europe, through Asia minor to the Himalayan mountain system including the mountains of China and Myanmar. This zone is also located in the convergent boundary of several plates.
• Intraplate of Earthquakes: Exceptionally these earthquakes occur not along the plate boundaries but in the continental platforms. The earthquakes occurring in peninsular India is a noted example of this class. These are mostly associated with the old faulted zone or incomplete rifting.

VOLCANISM – Endogenic Forces

The terms volcano and volcanism connote different things in the earth sciences. A volcano is an opening in the crust of the earth, connected by a conduit to an underlying magma chamber, from which molten lava, volcanic gases steam and Pyroclastic material are ejected. While volcanism (vulcanicity) includes  all the processes and mechanisms related to the origin of magma, gases, vapour etc., their ascent and appearance on the earth’s surface in various forms.

Causes of Volcanism

• In the increasing depth in the earth’s interior the temperature increases gradually at the rate of 10 C per 32 metres ( the rate varies greater depths ). This is caused mainly by the disintegration of radioactive elements inside the earth.
• Molten magma forms because  of lowering of melting point in the earth’s interior caused by reduction in the pressure due to splitting of plated and their movement in opposite direction.
• Gasses and vapor are formed as water, reaching the hot underground through percolation of surface water.
• Movement and splitting of the major and minor plates. Thus volcanic eruptions are closely associated with the movement of plates. Most of the active fissure volcanoes are found along the oceanic ridges representing the divergent boundary while explosive type of volcanoes are found in the zone of convergent plates.

Lava and magma: Molten rock materials are called magma when they are below the earth’s surface whereas when they are at earth’s surface   they are called lava.

Types of volcanoes

On the basis of mode of eruption

Central eruption of type of explosive type.

Fissure eruption or quiet type.

Central eruption or explosive type: It occurs  through a central pipe and small opening by breaking and blowing off crustal surface due to explosive magma and gases. This type is future  sub-divided into:

• Hawaiian Eruption: It erupts quietly due to less  viscous and extremely fluid lava. Boll  of lavas glide in the air like glowing hairs, called  Pele’s Hair by Hawaiian People.
• Strambolian Type : Moderately explosive due to lesser fluidity of basic lava than that of Hawaiian.
• Vulcanian Type : It erupts with great intensity & force due to high viscosity of lava.
• Pelean Type: Most violent type due to extremely viscous and pasty lava. Its salient characteristic is the formation of Nuee Ardente ( glowing cloud)
• Visuvious Type: There occurs extremely violent explusion of magma due to enormous amount of explosive gases
• The most destructive type of eruption is called Plinian Type.

Fissure Eruption or Quiet Type of Volcanoes:  it occurs along a long fracture, or fault and fissure through  slow upwelling of magma. It leads to formation of thick lava plateaus, Deccan plateau, Columbia plateau (USA), Antrim plateau (North Ireland) etc.

Types of volcanoes on the basis of periodicity

• Active Volcanoes: Are those which continuously eject lavas , gases and fragmental materials. About 100 % of world’s active volcanoes are situated along the perimeter of the pacific ocean. Noted examples  are Mona Loa in Hawaii Island (USA), Etna and Stramboli in the Mediterranean Sea, Pinatubo volcano of Philippines, etc. Stramboli is known as the light house of the Mediterranean.
• Dormant   Volcanoes: Are those which have been quiescent for a  long time but in which there is possibility of further eruption. Visuvius mountain of Italy, Mt. Fujiyama of Japan and Mt. Krakatoa of Indonesia are famous examples.
• Extinct Volcanoes: These are volcanoes in which the eruption has completely stopped and is not likely to  occur. Mt. Popa of Myanmar, Mt. Kilimanjaro of  Africa, Mt. Demvand and Koh-sultan of Iran are some of the examples of  Extinct Volcanoes.

Types Of Volcanoes Based On Shapes

Whenever their is magma eruption at the surface as lava, it can form different types of volcano depending on:

• the viscosity of the magma
• the amount of gas present in the magma
• the chemical composition of the magma
• the way in which the magma reaches the surface

Volcanoes can also be classified on the basis of shapes. The most well-known types of volcanoes are cinder cones, composite volcanoes (stratovolcanoes), and shield volcanoes.

Shield volcanoes: Have broad rounded shapes and are the largest. The largest of all volcanoes on Earth is Mauna Loa. It covers a substantial part of the Island of Hawai‘i and has a diameter of nearly 200 km. The summit of Mauna Loa is presently 4,169 m above sea level, but this represents only a small part of the volcano. It rises up from the ocean floor at a depth of approximately 5,000 m. Most shield volcanoes are formed from fluid, basaltic lava flows and produce low viscosity, runny lava, that spreads far from the source and forms a volcano with gentle slopes.

Composite Volcanoes (stratovolcanoes): They have slopes that get steeper near the top of the volcano. A classic composite volcano is conical with a concave shape that is steeper near the top. Cotopaxi is a classic example of Composite Volcano as it consists of layers of lava alternating with layers of tephra (blocks, bombs, lapilli, and ash). Composite volcanoes are active over long periods (tens to hundreds of thousands of years), and erupt periodically. They tend to form relatively quickly and do not last very long. If volcanic activity ceases, it might erode away within a few tens of thousands of years. This is largely because of the presence of pyroclastic eruptive material, which is not strong. Mt. Fuji is another example of this but it expels Basaltic lava.

Cinder Cones (Spatter Cones): Technically known as scoria cones, Cinder cones are the most common type of volcano in the world. They have straight sides and are typically less than 200 m high. They can be eroded away easily, and relatively quickly because cinder cones are made up almost exclusively of loose fragments. Thus they have very little strength. Mt. Capulin, is a classic example of this type.

Spatial Distribution of Volcanoes (Endogenic Forces)

• Circum-Pacific Belt (Ring of Fire): This is the volcanic zone of the convergent oceanic plate boundary and includes the volcanoes of the eastern and western coastal areas. It begins from Erebus mountain of Antarctica and runs northward through Andes and Rockies and then to volcanoes of island arcs ( Sakhalin , Kamchatka , Japan , Philippines etc.) Ojas del Salado , the highest active volcanic mountain of the world and Mt. Cotopaxi are situated in this zone which includes more than 60 % of the world’s volcanoes.

• Mid–Atlantic Zone:  This zone represents  the divergent  boundary or splitting zone of plates located along the mid-Atlantic ridges. Volcanoes of this area are mainly of fissure eruption type. Iceland is the most active volcanic area and is located on the mid-Atlantic ridges. St. Helena, Azores island, etc., are other examples.
•  Mid-Continental Zone: This Is the volcanic zone of convergent continental plate boundary that includes the volcanoes of Alpine mountain chains , the Mediterranean Sea and the fault zone of Eastern Africa. The Himalayas form the most striking exception for none is to be found there.
• Intraplate Volcanoes: Contrary to the general trend, some volcanoes are also found in the inner parts of continental or oceanic plates, Hawaiian volcanic chain, Reunion island, Graben, etc., which lie beyond  convergent or divergent boundary. This volcanism is the result of the activity of Hot spots in the mantle. Hotspots are the mantle plumes of hot material whose location is stationary.

Volcanic Topography

• When magma solidifies below the earth’s surface then intrusive volcanic topography such as Batholith, Phacolith, Sill, Dyke etc., are formed.
• Outpouring of lava from fissures leads to the formation of lava plateaus such as Deccan, Columbia, Parana plateaus etc.
• Crater & Caldera: A funnel-shaped depression formed at the mouth of volcanic vents is called a crater. The enlarged form of a crater is generally called caldera.
• Geysers and Hot Spring: Geysers are a special type of hot spring from which a column of hot water and steam is explosively discharged at intervals. The difference between a hot spring and a geyser is that there is continuous spouting of hot water from the farmer while there is intermittent spouting of matter from the latter. Yellowstone National Park in the USA is famous for geysers.

• Fumaroles: These are vents through which there is ambition of gasses, smokes and water, mostly at intervals. Fumaroles are found in groups near  Katmai Volcano of Alaska which is known as a valley of ten  thousand smokes.
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See Also

Interior Structure Of The Earth

Rock Cycle – Diagram and Explanation

World Climate Zones

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