Chapter Notes: Shaping of the Earth's Surface
Introduction
The surface of the Earth is dynamic. Over millions of years
it has been shaped and reshaped by forces originating both inside the planet
and at its surface. Mountains rise, rivers carve valleys, glaciers sculpt
U-shaped troughs, and coastlines advance or retreat. Some changes are extremely
slow, measured in millions of years; others - earthquakes, landslides, floods -
occur suddenly and can cause widespread destruction. Studying these processes
helps us understand how the present landscape formed, why some regions are
earthquake- or flood-prone, and how to reduce risk to lives and infrastructure.
This chapter introduces the Earth's internal structure, the
theory of plate tectonics, processes of weathering and erosion,
the main agents of gradation that wear down and build up the surface, major
landforms, and selected natural hazards (earthquakes, landslides, avalanches, glacial-lake
outburst floods and duststorms) with their causes, effects
and practical mitigation measures.
The Interior of the Earth
Understanding the interior of the Earth is essential because
many surface features and events (volcanoes, earthquakes, mountain
building) originate from processes within. The Earth is roughly spherical
with an average radius of about 6,371 km and a diameter of approximately 12,756
km at the equator. Its interior is conventionally described in layers based on
composition and physical properties.
The Layers of the Earth
The Crust
The crust is the outermost rigid layer where we live. It is
the thinnest layer and constitutes less than 1% of the Earth's volume.
Types of crust
- Continental
crust: Forms the landmasses. It is thicker (about 35-70 km under
mountains), composed mainly of lighter silica- and aluminium-rich rocks
and is often referred to by the shorthand SIAL (Si = silica, Al =
alumina).
- Oceanic
crust: Forms the ocean floors. It is thinner (about 5-10 km), denser,
rich in silica and magnesium minerals and commonly referred to as SIMA (Si
= silica, Ma = magnesium).
The boundary between the crust and the mantle is called
the Mohoroviฤiฤ Discontinuity or the Moho.
The Mantle
The mantle lies beneath the crust and extends down to about
2,900 km depth. It is the thickest layer and is composed of dense silicate
rocks rich in iron and magnesium.
Upper and lower mantle
The upper mantle includes a relatively rigid layer that,
together with the crust, forms the lithosphere. Below this rigid zone lies the
partly molten, ductile asthenosphere on which tectonic plates
move. The lower mantle extends below the asthenosphere to the core and, because
of the very high pressure, behaves more solidly despite high temperatures.
The Core
The core extends from about 2,900 km to the centre (about
6,371 km). It is rich in iron and nickel and makes up about 32% of Earth's
mass.
Outer and inner core
The outer core is liquid; movement of molten iron here
generates the Earth's magnetic field. The inner core is solid despite
temperatures of roughly 5,000-6,000 °C because of extremely high pressures that
prevent melting.
Theory of Plate Tectonics
What is the Theory of Plate Tectonics?
The theory of plate tectonics explains that the Earth's
outer rigid shell, the lithosphere, is broken into a number of large and small
fragments called tectonic plates. These plates move slowly over the ductile
asthenosphere at rates of a few centimetres per year. Plate motions account for
the distribution of earthquakes, volcanoes, mountain
ranges and ocean basins.
Continental Drift: Wegener's Contribution
In 1912 German scientist Alfred Wegener proposed
the Continental Drift hypothesis. He noted how coastlines - especially those of
South America and Africa - fit together like a jigsaw. He suggested that all
continents were once joined as a single supercontinent named Pangaea,
surrounded by a single ocean called Panthalassa. Over roughly the
last 200 million years, Pangaea broke apart and the pieces drifted to their
present positions.
Evidence Wegener cited includes:
- Complementary
coastlines of continents (e.g., South America and Africa).
- Similar
fossils across now-separated continents (for example, the plant Glossopteris found
in India, Africa, South America, Australia and Antarctica).
- Matching
rock sequences and geological structures across continents.
- Evidence
of past glaciation in now-tropical regions, indicating different past
latitudinal positions.
Wegener's ideas were initially debated but were later
incorporated and explained by the plate tectonics theory, which provides the
driving mechanisms (mantle convection, sea-floor spreading) he lacked.
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The Tectonic Plates
The lithosphere is broken into about 15 major and several
smaller plates. Major plates include:
- The
North American Plate
- The
South American Plate
- The
Eurasian Plate
- The
African Plate
- The
Indo-Australian Plate
- The
Pacific Plate
- The
Antarctic Plate
Smaller plates include the Arabian, Caribbean, Philippine,
Juan de Fuca, and Cocos plates. The Indian subcontinent lies on the
Indo-Australian Plate, which has collided with the Eurasian Plate to form
the Himalayas.
Why Plates Move
Heat from the Earth's interior (core and mantle) and
radioactive decay generates convection currents in the mantle. Hotter, less
dense material rises, spreads out beneath the lithosphere, cools, becomes
denser and sinks back. These slow circular motions in the mantle create forces
that drive the movement of tectonic plates at rates typically between 1 and 10
centimetres per year.
Types of Plate Boundaries
Most geological activity (earthquakes, volcanoes,
mountain-building) occurs at plate boundaries. There are three main types:
- Convergent
(collision) boundaries: Plates move towards each other.
- Divergent
(constructive) boundaries: Plates move away from each other.
- Transform
(conservative) boundaries: Plates slide past each other horizontally.
Importance of Plate Tectonics
The theory explains the distribution of mountain ranges,
ocean trenches and volcanoes, the locations of earthquakes, the match of fossil
and rock records across continents, and the evolution of ocean basins and
climate over geological time. Plate tectonics provides the framework used by
geologists and planners to identify hazard zones and to guide safer land-use
and infrastructure decisions.
Weathering and Erosion
Gradation: Levelling the Earth's Surface
Gradation is the collective term for the processes that
level the Earth's surface. It involves:
- Degradation -
the wearing down of highlands by weathering and erosion;
- Aggradation -
the building up of lowlands by deposition of eroded material.
Over geological time, degradation and aggradation together
reduce relief and redistribute material, tending toward a more level surface.
What is Weathering?
Weathering breaks rocks in place into smaller fragments and
changes their chemical composition. Weathering alone does not transport
material; it prepares material to be moved by erosion.
Physical (Mechanical) Weathering
Breaks rock into smaller pieces without changing the rock's
chemical composition. Important processes include thermal expansion,
frost action, pressure release, and salt crystallisation.
Chemical Weathering
Involves chemical alteration of minerals. Chemical
weathering is strong in warm, humid climates. Key processes are oxidation,
carbonation, hydrolysis, and hydration.
Biological Weathering
Caused by living organisms: plant roots widen cracks,
burrowing animals disturb and mix soils, and organisms produce acids that
chemically break down rock surfaces.
What is Erosion?
Erosion is the process that picks up and carries away
material produced by weathering. Agents of erosion include flowing water,
glaciers, wind, waves and underground water.
Agents of Gradation
The principal agents that perform erosion, transport and
deposition are rivers, sea waves and currents, wind, glaciers and underground
water. Each agent creates characteristic landforms.
Rivers
Rivers are among the most effective and widespread agents of
gradation. They erode, transport and deposit sediments and create distinctive
landforms along their course.
Typical river landforms by course:
- Upper
course: predominance of vertical erosion produces V-shaped valleys,
gorges, waterfalls and rapids.
- Middle
course: lateral erosion and deposition form wide meanders; cut-offs
can leave oxbow lakes.
- Lower
course: deposition dominates, forming floodplains, levees and deltas.
Sea Waves and Currents
The sea shapes coastlines by erosion, transport and
deposition. Wave energy erodes headlands and forms cliffs, caves, arches and
stacks; deposition forms beaches, sandbars and spits when wave energy falls.
Wind
Wind is a major agent in arid and semi-arid regions.
Processes include deflation and abrasion. Typical landforms include sand dunes,
yardangs, and loess deposits.
Glaciers
Glaciers are powerful erosive agents in high mountains and
polar regions. They erode by plucking and abrasion, producing U-shaped valleys,
cirques, arรชtes, horns and fjords.
Underground Water (Karst Processes)
In areas underlain by soluble rocks (especially limestone),
slightly acidic groundwater dissolves rock to form caves, sinkholes and
underground drainage systems. This landscape is known as Karst
topography.
Major Landforms and their Formation
The interaction of endogenic forces (plate tectonics,
volcanism) and exogenic processes (weathering, erosion and deposition) produces
the major landforms: mountains, plateaus and plains.
Mountains
Mountains are high relief landforms usually rising more than
about 600 metres above surrounding areas. Types of mountains include:
- Fold
mountains: formed when two continental plates converge and compress
rock layers into folds.
- Block
mountains: formed by vertical movement along faults; uplifted blocks
are horsts, down-thrown blocks are grabens.
- Volcanic
mountains: formed by accumulation of lava and pyroclastic material at
volcanic vents.
Plateaus
A plateau is an elevated flat or gently undulating landform
rising sharply from one or more sides. Types include:
- Volcanic
(lava) plateaus: produced by large flows of fluid lava.
- Tectonic
plateaus: uplifted by tectonic forces.
- Dissected
plateaus: plateaus that have been deeply cut by rivers into rugged
terrain.
Plains
Plains are broad, relatively flat areas, often at low
elevations. They are frequently fertile and densely settled. Types include:
- Alluvial
plains: formed by long-term deposition of river sediments.
- Structural
plains: formed by gentle tilting of rock layers.
- Erosional
plains: formed by prolonged erosion of highlands down to near sea
level.
Natural Disasters
Many natural disasters are closely linked to the geological
processes covered in earlier parts of this chapter. Understanding their causes
and impacts aids preparedness and mitigation.
Earthquakes
Definition: An earthquake is the sudden shaking of
the Earth's surface caused by rapid release of energy in the crust.
- Focus
or hypocenter: the point inside the Earth where the earthquake begins.
- Epicentre:
the point on the surface directly above the focus; usually where shaking
is strongest.
- Seismic
waves: energy waves emitted from the focus.
- Seismograph:
instrument that records seismic waves.
- Richter
scale: a logarithmic scale to express earthquake magnitude.
- Mercalli
intensity scale: describes the effects of shaking at particular
locations.
Causes: Most earthquakes result from sudden slip on faults
due to accumulated stress along plate boundaries.
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Also read: Detailed Chapter Notes - Climate |
Landslides
Definition: A landslide is the downslope movement of
rock, earth or debris under gravity.
- Causes:
Natural triggers include heavy rainfall saturating soils, steep slopes,
earthquakes, and undercutting by rivers or waves.
- Effects:
Loss of life, burial of settlements, destruction of roads, bridges and
farmland.
- Mitigation:
Afforestation and vegetative cover to bind soil; engineered retaining
structures and terracing.
Avalanches
Definition: An avalanche is a rapid flow of snow, ice
and debris down a steep slope.
- Causes:
Rapid snow accumulation, sudden temperature rises, wind-deposited
cornices, earthquakes or human triggers.
- Effects:
Avalanches can bury people, damage infrastructure and block mountain
passes.
- Mitigation:
Avalanche sheds, snow fences, controlled blasting to release unstable snow
safely.
Glacial Lake Outburst Floods (GLOFs)
Definition: A GLOF occurs when a lake dammed by
glacial ice or a moraine fails suddenly.
- Causes:
Climate warming increases glacier melt, overtopping and erosion of moraine
dams.
- Relevance
to India: The Himalayan region contains many glacial lakes.
- Effects:
Sudden release of water combined with debris can destroy villages, roads,
bridges and hydropower installations.
Duststorms
Definition: Duststorms occur when strong winds pick
up large amounts of loose dust and sand.
- Causes:
Strong winds over dry, unvegetated land, thermal instability, and land
degradation.
- Where
in India: Duststorms commonly occur in the hot, dry pre-monsoon
months.
- Effects:
Reduced visibility causing road and air accidents, damage to crops,
respiratory illnesses.
