Shaping of the Earth's Surface

The One Thing

The Earth’s surface is continuously reshaped by the interaction of internal forces, such as plate movement, volcanism, folding, faulting, and earthquakes, and external processes, including weathering, erosion, transportation, and deposition. Although natural hazards cannot always be prevented, scientific knowledge, early-warning systems, preparedness, and sustainable land-use planning can substantially reduce their effects.

Who and What

• Crust: The thin, solid outermost layer of the Earth. It may be continental or oceanic; oceanic crust is generally thinner and denser than continental crust.
• Mantle: The thick layer below the crust, composed of very hot, dense material that can move slowly. Convection currents caused by heat inside the Earth help drive mantle and tectonic-plate movement.
• Core: The innermost part of the Earth, mainly composed of iron and nickel. It consists of a liquid outer core and a solid inner core.
• Lithosphere: The rigid outer layer made up of the crust and the uppermost mantle.
• Asthenosphere: The softer, slowly flowing layer below the lithosphere on which tectonic plates move.
• Plate tectonics: The theory that the lithosphere is divided into large plates that move over the asthenosphere.
• Tectonic plate: A large, rigid section of the lithosphere that moves slowly, usually at a rate of a few centimetres per year, and interacts with other plates.
• Convergent boundary: A boundary where two plates move towards each other. This can cause subduction, mountain building, volcanoes, or earthquakes. At convergent boundaries, crust may be destroyed through subduction.
• Divergent boundary: A boundary where two plates move apart. Magma rises, cools, and creates new crust.
• Transform boundary: A boundary where two plates slide horizontally past each other, often causing earthquakes.
• Earthquake: The sudden shaking of the ground caused by the release of energy along faults or tectonic-plate boundaries.
• Focus: The point inside the Earth where an earthquake begins.
• Epicentre: The point on the Earth’s surface directly above the focus. The focus is underground, whereas the epicentre is located at the surface.
• Earthquake magnitude: A measure of the energy released by an earthquake.
• Earthquake intensity: A measure of the observed effects and damage at a particular place.
• Volcano: An opening in the Earth’s crust through which magma, ash, and gases reach the surface.
• Weathering: The breakdown of rocks at or near the Earth’s surface without movement of the broken material.
• Physical weathering: The breaking of rocks into smaller pieces without changing their chemical composition.
• Chemical weathering: The decomposition of rocks when minerals react with water, air, or acids.
• Biological weathering: The breakdown of rocks by plants, animals, and microorganisms.
• Erosion: The wearing away and removal of weathered material by moving water, ice, wind, or sea waves.
• Transportation: The movement of eroded material from one place to another.
• Deposition: The settling or laying down of transported sediment when the transporting agent loses energy.
• River landforms: Features made by running water, including V-shaped valleys, waterfalls, meanders, floodplains, oxbow lakes, levees, and deltas.
• Glacial landforms: Features formed by moving ice, including U-shaped valleys, cirques, moraines, and hanging valleys.
• Wind landforms: Features formed by wind action in dry regions, including sand dunes, mushroom rocks, and loess deposits.
• Coastal landforms: Features created by sea waves, including sea cliffs, caves, arches, stacks, beaches, spits, and bars.
• Groundwater landforms: Features formed when groundwater dissolves and deposits minerals in limestone areas, including caves, stalactites, and stalagmites.
• Flood: The overflow of water onto land that is normally dry, often caused by heavy rainfall, river overflow, or poor drainage.
• Landslide: The rapid movement of rock, soil, and debris down a slope due to gravity.
• Disaster mitigation: Actions taken before, during, and after a hazard to reduce loss of life, property, and the environment.

Causes and Consequences

• Internal heat and Earth structure drive tectonic movement.

The Earth is commonly divided into the crust, mantle, outer core, and inner core. Heat from within the Earth produces convection currents, helping move the mantle and tectonic plates over the asthenosphere.

• Plate movement creates major geological features.

At divergent boundaries, magma rises and cools to form new crust. At convergent boundaries, subduction may destroy crust, while collision can produce mountains, volcanoes, and earthquakes. At transform boundaries, plates slide past one another and frequently generate earthquakes.

• The Himalayas resulted mainly from continental collision.

The Indian Plate collided with the Eurasian Plate, causing mountain building and forming the Himalayas.

• Earthquakes result from the release of stored energy.

Movement along faults or plate boundaries produces an earthquake beginning at the focus. The resulting surface effects are greatest or most directly measured in relation to the epicentre. Magnitude records the energy released, whereas intensity describes the effects and damage at a particular location.

• Internal and external forces have contrasting effects.

Internal forces generally build up or uplift the surface through folding, faulting, volcanism, and earthquakes. External forces generally wear down, transport, and redeposit material.

• Weathering prepares material for erosion.

Weathering occurs in place and may be physical, chemical, or biological. Erosion differs because it involves the removal and movement of the weathered material.

• Different agents produce distinctive landforms.

The main agents of erosion are running water, glaciers, wind, sea waves, and groundwater. The resulting landform depends on the agent of change, the strength and speed of the process, the type of rock, the slope, and the length of time involved.

• Running water changes its dominant role along a river.

A river generally erodes its upper course, transports material in its middle course, and deposits much of its load in its lower course. Running water commonly forms V-shaped valleys, whereas glaciers form U-shaped valleys. A meander is a winding curve in a river; when a meander is cut off, it may form an oxbow lake. A delta forms near a river’s mouth when the river slows and deposits sediment.

• Glacial movement produces characteristic erosional and depositional features.

Moving ice forms U-shaped valleys, cirques, moraines, and hanging valleys.

• Wind forms and deposits material in dry regions.

Wind action produces sand dunes, mushroom rocks, and loess deposits.

• Sea waves both erode and deposit material.

Waves may erode headlands to form caves, arches, and stacks. They may also deposit sand to form beaches and spits; coastal depositional features also include bars. Sea cliffs are another coastal landform produced by wave action.

• Groundwater modifies limestone landscapes.

In limestone regions, carbon dioxide dissolved in water can form a weak acid that dissolves limestone. This process contributes to the formation of caves, stalactites, and stalagmites.

• Natural hazards become disasters when vulnerability is high.

Floods, landslides, earthquakes, and other hazards cause greater losses when people, buildings, and activities are located in vulnerable areas without adequate preparation or safety measures.

• Preparedness reduces disaster-related losses.

Important preparedness measures include hazard mapping, public awareness, emergency kits, evacuation plans, drills, and reliable communication systems.

• Earthquake mitigation combines construction, behaviour, and warnings.

Risk can be reduced by constructing buildings according to safety codes, securing heavy objects, staying away from windows, and using “Drop, Cover, and Hold”.

• Flood mitigation depends on land-use and water-management decisions.

Measures include improving drainage, protecting wetlands, building embankments where suitable, avoiding construction on floodplains, and following warnings.

• Landslide risk can be reduced by stabilising slopes.

Relevant measures include slope stabilisation, vegetation cover, proper drainage, retaining walls, and restrictions on unsafe construction.

• Vegetation limits soil erosion.

Planting trees helps reduce soil erosion by holding soil with roots and slowing surface runoff.

What Gets Asked

• How do the crust, mantle, core, lithosphere, and asthenosphere differ, and how does internal heat drive tectonic-plate movement?
• How do convergent, divergent, and transform boundaries differ in their movements and consequences, including the formation of the Himalayas through collision between the Indian Plate and the Eurasian Plate?
• How should earthquakes be distinguished in terms of focus, epicentre, magnitude, and intensity?
• How do weathering, erosion, transportation, and deposition differ, and how do running water, glaciers, wind, sea waves, and groundwater create contrasting landforms?
• Why do rivers erode in their upper courses, transport in their middle courses, and deposit in their lower courses, producing features such as V-shaped valleys, meanders, oxbow lakes, and deltas?
• How can preparedness, safe construction, early-warning systems, hazard mapping, appropriate land use, and measures such as “Drop, Cover, and Hold” reduce the effects of floods, landslides, and earthquakes?