Unit 3: Earth movements - Subjective Questions

Earth movements are the movements or deformations of the Earth's crust caused by internal forces operating within the Earth. They are broadly classified into endogenic movements and produce features such as mountains, plateaus, folds, faults, earthquakes, and volcanoes.

• Tensional forces: Pull crustal blocks apart and may produce normal faults and rift valleys.
• Compressional forces: Push crustal blocks together and cause folding, reverse faulting, and mountain building.
• Shearing forces: Move crustal blocks horizontally in opposite directions and produce strike-slip faults.
• Convection currents: Heat-driven movements in the mantle help move tectonic plates.
• Isostatic forces: Cause vertical adjustments of the crust as it responds to changes in load.

Thus, earth movements are important agents in shaping the relief and structure of the Earth's surface.

Folding is the bending of rock layers due to compressional forces acting within the Earth's crust. It generally occurs in relatively plastic or less-resistant rocks under prolonged pressure.

The major parts of a fold are:

• Limb: The sloping side of a fold.
• Hinge: The zone of maximum curvature in a fold.
• Axial plane: An imaginary plane that divides the fold as symmetrically as possible through the hinge.
• Fold axis: A line joining points of maximum curvature along a folded layer.
• Crest: The highest part of an upfold.
• Trough: The lowest part of a downfold.
• Core: The innermost rock layers exposed near the center of a fold.

Folding commonly produces ridges, valleys, and mountain ranges.

Anticlines and synclines are the two basic types of folds formed by compressional forces.

• An anticline is an arch-shaped or upfolded structure. The limbs dip away from the fold axis, and the oldest rocks are generally found at its core.
• A syncline is a trough-shaped or downfolded structure. The limbs dip toward the fold axis, and the youngest rocks are generally found at its core.

The formation process involves:

1. Horizontal compression acting on sedimentary rock layers.
2. Bending of the layers rather than breaking them.
3. Development of alternating upfolds and downfolds.
4. Erosion of the folded structures, which may create ridges and valleys.

Large anticlines may form mountain ranges, while synclines may form elongated valleys or structural basins.

Folds can be classified according to the inclination of their limbs and axial planes:

• Symmetrical fold: Both limbs have approximately equal inclination, and the axial plane is nearly vertical.
• Asymmetrical fold: The two limbs have different inclinations because one limb is steeper than the other.
• Overturned fold: Both limbs dip in the same direction, but one limb has been tilted beyond the vertical.
• Recumbent fold: The axial plane is nearly horizontal, causing the fold to lie on its side.

These fold types reflect increasing intensity of compression and deformation. Symmetrical folds generally result from moderate pressure, whereas overturned and recumbent folds indicate strong compressional forces and intense crustal deformation.

Folding is closely associated with mountain building, also known as orogeny. It occurs mainly at convergent plate boundaries where two tectonic plates move toward each other.

• Sediments deposited in geosynclines or ocean basins are compressed as plates converge.
• The rock layers bend into anticlines and synclines.
• Continued compression may produce overturned, recumbent, and nappe folds.
• Uplift of the folded crust creates fold mountains.
• Subsequent erosion modifies the original folded relief into ridges, valleys, and passes.

Examples include the Himalayas, formed by the collision of the Indian and Eurasian plates, and the Alps, formed by the convergence of the African and Eurasian plates. Folding therefore converts horizontal sedimentary layers into elevated mountain systems.

Faulting is the fracturing and displacement of rocks along a fracture or zone of weakness in the Earth's crust. It occurs when tectonic stress exceeds the strength of the rocks.

The formation of a fault involves:

1. The accumulation of stress caused by tectonic plate movement.
2. Deformation of the rocks around a zone of weakness.
3. Sudden or gradual rupture when the stress becomes greater than rock strength.
4. Displacement of one rock block relative to another.

Important terms include:

• Fault plane: The surface along which movement occurs.
• Hanging wall: The block above an inclined fault plane.
• Footwall: The block below the fault plane.
• Fault scarp: A step or cliff produced at the surface by fault movement.

Faulting can create rift valleys, block mountains, basins, and escarpments.

Faults are classified according to the direction of movement of the blocks:

• Normal fault: The hanging wall moves downward relative to the footwall. It is caused mainly by tensional forces and commonly occurs in rift zones.
• Reverse fault: The hanging wall moves upward relative to the footwall. It is caused by compression.
• Thrust fault: A reverse fault with a low-angle fault plane. It allows older rocks to be pushed over younger rocks.
• Strike-slip fault: The blocks move horizontally past each other along the fault plane. It is mainly caused by shearing forces.

Normal faults commonly produce rift valleys and fault-block mountains. Reverse and thrust faults are associated with mountain building, while strike-slip faults may generate destructive earthquakes without significant vertical displacement.

Rift valleys and block mountains develop mainly in regions affected by tensional forces.

Formation of a rift valley:

• Tensional forces pull the crust apart.
• Parallel normal faults develop.
• The central block between the faults subsides.
• The depressed block forms a graben, or rift valley.

Formation of a block mountain:

• Faulting produces uplifted blocks between parallel faults.
• The uplifted block is called a horst.
• Its steep sides form fault scarps.
• Erosion may further modify the elevated block.

Examples include the East African Rift Valley and the Black Forest and Vosges block mountains of Europe. These landforms demonstrate the role of vertical displacement in shaping the Earth's surface.

Folding and faulting are both forms of crustal deformation, but they differ in important ways:

• Nature of deformation: Folding bends rock layers, whereas faulting fractures and displaces them.
• Rock behavior: Folding occurs when rocks behave plastically; faulting occurs when rocks behave brittly.
• Main stress: Folding is mainly associated with prolonged compression, while faulting may result from tension, compression, or shearing.
• Continuity of layers: Folded layers remain continuous, but faulted layers are broken and displaced.
• Landforms: Folding creates anticlines, synclines, and fold mountains; faulting creates rift valleys, fault scarps, basins, and block mountains.
• Movement: Folding involves bending, while faulting involves movement along a fracture.

Both processes are caused by tectonic forces and may occur together in active mountain belts.

An earthquake is the sudden shaking of the Earth's surface caused by the rapid release of energy stored in rocks, usually along a fault.

Important terms include:

• Focus or hypocentre: The point inside the Earth where the earthquake begins.
• Epicentre: The point on the Earth's surface directly above the focus.
• Seismic waves: Energy waves that travel outward from the focus.
• Magnitude: A measure of the energy released by an earthquake.
• Intensity: A measure of the observed effects and damage at a particular place.
• Aftershocks: Smaller earthquakes that follow the main shock.
• Foreshocks: Smaller shocks that may precede a major earthquake.

Most earthquakes result from sudden movement along faults, although volcanic activity, landslides, and human activities such as reservoir construction can also trigger them.

The elastic rebound theory explains how earthquakes occur along faults.

1. Tectonic forces slowly build stress in rocks on either side of a fault.
2. The rocks deform elastically and store potential energy.
3. When the accumulated stress exceeds the frictional resistance and strength of the rocks, the fault suddenly slips.
4. The deformed rocks rebound toward a less strained position.
5. The stored energy is released as seismic waves, producing an earthquake.

The theory explains why earthquakes may occur after long periods of quietness. It also shows why the land on either side of a fault may be displaced horizontally or vertically during an earthquake. Repeated elastic rebound along active faults causes a sequence of earthquakes over geological time.

Seismic waves are vibrations generated by an earthquake. They are divided into body waves and surface waves.

• Primary waves or P-waves: These are compressional waves that travel through solids, liquids, and gases. They are the fastest seismic waves and arrive first.
• Secondary waves or S-waves: These are transverse waves that travel only through solids. They are slower than P-waves and arrive second.
• Love waves: These are surface waves that move the ground horizontally from side to side. They can cause severe structural damage.
• Rayleigh waves: These produce a rolling or elliptical movement similar to ocean waves and travel along the surface.

P-waves generally cause less damage than surface waves. The arrival-time difference between P-waves and S-waves helps scientists estimate the distance to an earthquake's epicentre.

Earthquake magnitude and intensity describe different aspects of an earthquake.

• Magnitude measures the energy released at the source. It is determined from seismic records and is commonly expressed using the Richter scale or the moment magnitude scale. The moment magnitude scale is preferred for large earthquakes.
• Intensity measures the effects of an earthquake at a particular location, including ground shaking, structural damage, and human observations. It is commonly described using the Modified Mercalli scale.

Magnitude is a single value for an earthquake, whereas intensity varies from place to place. An earthquake may have high intensity near the epicentre but much lower intensity at distant locations. Soil conditions, depth of focus, building quality, and distance from the epicentre influence observed intensity.

Earthquakes produce both direct and indirect effects.

Effects on the physical environment:

• Ground shaking and surface rupture.
• Uplift or subsidence of land.
• Landslides and rockfalls in mountainous areas.
• Liquefaction of water-saturated sediments.
• Changes in river channels, springs, and coastlines.
• Tsunamis caused by undersea earthquakes or seafloor displacement.

Effects on people and settlements:

• Collapse of buildings, bridges, roads, and dams.
• Deaths, injuries, homelessness, and disruption of essential services.
• Fires caused by broken gas or electricity lines.
• Economic losses and interruption of communication and transport.
• Disease risks due to contaminated water and poor sanitation.

The severity of damage depends on magnitude, depth, distance, duration, population density, construction quality, and preparedness.

A volcano is an opening or vent in the Earth's crust through which magma, gases, ash, and other volcanic materials reach the surface.

The main parts of a volcano are:

• Magma chamber: An underground reservoir containing molten rock.
• Conduit or pipe: A passage through which magma rises toward the surface.
• Vent: The opening through which lava and gases are released.
• Crater: A bowl-shaped depression around the main vent.
• Cone: A mound built by accumulated lava, ash, and other ejecta.
• Lava flows: Streams of molten rock that spread over the surface.
• Secondary vents: Smaller openings on the sides or near the main cone.
• Caldera: A large depression formed when a volcanic summit collapses after a major eruption.

The shape and size of a volcano depend on the type of magma and eruption.

Volcanic eruptions occur when magma and volcanic gases rise through weaknesses in the Earth's crust and reach the surface.

The main causes are:

• Plate convergence: A descending oceanic plate melts or releases water into the mantle, promoting magma formation.
• Plate divergence: Plates move apart, allowing mantle material to rise and melt through decompression.
• Hotspots: Persistent columns of hot mantle material rise beneath a plate and generate magma.
• Magma pressure: Accumulation of magma increases pressure within a magma chamber.
• Gas expansion: Dissolved gases expand as magma rises and pressure decreases.
• Crustal fractures: Faults and cracks provide pathways for magma to reach the surface.

An eruption becomes explosive when magma is viscous and rich in trapped gases. Fluid magma generally produces quieter lava flows.

Volcanoes differ according to their shape, materials, and eruption style:

• Shield volcano: Broad and gently sloping, formed mainly by repeated flows of fluid basaltic lava. Eruptions are usually quiet. Example: Mauna Loa.
• Composite volcano or stratovolcano: Tall and steep-sided, built from alternating layers of lava, ash, and pyroclastic materials. Its eruptions may be explosive because the magma is viscous and gas-rich. Example: Mount Fuji.
• Cinder cone volcano: Small and steep-sided, formed from loose ash, cinders, and volcanic fragments around a vent. It usually develops during relatively short-lived eruptions.

Shield volcanoes have low-viscosity magma, whereas composite volcanoes commonly contain more viscous magma. Cinder cones are usually the simplest and smallest volcanic landforms.

Volcanic landforms are classified according to whether magma cools below or above the Earth's surface.

Intrusive landforms: These form when magma cools and solidifies beneath the surface.

• Batholith: A very large, irregular mass of coarse-grained igneous rock.
• Laccolith: A dome-shaped intrusion that arches overlying strata.
• Sill: A sheet of magma intruded parallel to existing rock layers.
• Dyke: A sheet of magma cutting across existing rock layers.
• Lopolith: A basin-shaped intrusion that may depress overlying strata.

Extrusive landforms: These form when lava and volcanic materials cool at or near the surface.

• Lava plateaus.
• Volcanic cones.
• Ash cones.
• Calderas.
• Lava domes.

Intrusive features are exposed mainly after uplift and erosion remove the overlying rocks.

Volcanic eruptions release solid, liquid, and gaseous materials.

• Lava: Molten rock that reaches the surface. Basaltic lava is fluid, while acidic lava is more viscous.
• Volcanic ash: Fine particles of pulverized rock and glass less than millimetres in diameter.
• Lapilli: Fragments ranging from approximately to millimetres in diameter.
• Volcanic bombs: Large molten or semi-molten fragments that become rounded during flight.
• Blocks: Solid angular fragments of older rock.
• Pumice: A light, porous rock formed when gas-rich lava cools rapidly.
• Scoria: A darker, vesicular volcanic rock.
• Gases: Water vapour, carbon dioxide, sulphur dioxide, hydrogen sulphide, and other gases.

These materials may form lava flows, ash deposits, pyroclastic flows, and volcanic cones.

Volcanic activity has both hazardous and beneficial effects.

Negative effects:

• Loss of life and destruction of settlements.
• Lava flows bury farmland, roads, and buildings.
• Ash fall damages crops, machinery, and buildings and may disrupt air travel.
• Pyroclastic flows and lahars can move rapidly and cause extensive destruction.
• Volcanic gases may pollute the atmosphere and create health problems.
• Eruptions can trigger landslides, earthquakes, and tsunamis.

Positive effects:

• Weathered volcanic ash produces fertile soils.
• Volcanic regions may provide geothermal energy.
• Volcanic rocks supply minerals and building materials.
• New land and islands may be created.
• Hot springs and scenic volcanic landscapes attract tourism.
• Volcanic activity contributes to long-term geological recycling.

The overall impact depends on eruption magnitude, population density, warning systems, and land-use planning.