Layers of the Earth and Plate Tectonics
Think of an egg: a thin shell, a thick white and a hot yolk. The Earth also has a thin outer skin, a thick middle and a very hot centre, and the skin is cracked into big pieces.
The Earth has three main layers: crust, mantle and core. The crust with the upper mantle forms the lithosphere, which is broken into tectonic plates that move slowly over the semi-molten asthenosphere.
Key idea 1: The layers — Crust: 30–40 km under continents, 5–7 km under oceans. Mantle: very thick and hot (about 2900 km). Outer core: fluid layer of iron and nickel. Inner core: solid, hot, spinning metal ball, the densest part.
- Easy: We live on the crust, the outermost layer.
- Medium: Lithosphere = crust + upper mantle; the asthenosphere beneath is semi-molten.
- Exam-type: Oceanic crust is thinner than continental crust.
In the figure: Going inward, the layers get hotter and denser.
Key idea 2: Why plates move — Plate tectonics (W.J. Morgan) says the lithosphere is broken into large and small plates that move a few centimetres a year. The cause is convection currents in the mantle: heat from the core makes hot material rise and cooler material sink, pushing and pulling the plates. Plates are continental, oceanic or mixed. Major plates: Pacific, Eurasian, African, North American, South American, Indo-Australian and Antarctic.
- Easy: Plates move only a few centimetres a year.
- Medium: Convection currents are like hot water rising in a heated pan.
- Exam-type: Plate movement forms mountains, volcanoes, plains, valleys and ocean basins.
Rigid plates move because they float on the semi-molten asthenosphere.
Thinking plates move fast. They move only a few centimetres a year.
Layers in order. Memory clue: CMC = Crust, Mantle, Core (outside to inside).
Spot the difference
| Layer | Feature |
|---|---|
| Crust | Thin, rocky, where we live |
| Mantle | Thick and hot |
| Outer core | Fluid iron and nickel |
| Inner core | Solid, dense metal ball |
Practice questions
- 1Name the three main layers of the Earth.
- 2What is the lithosphere made of?
- 3What allows tectonic plates to move?
- 4Name any three major plates.
- 5What causes plate movement?
- 6Why is the crust thinner under oceans?
- 7How would you explain convection currents using a pan of water?
- 8Why does the chapter say plate tectonics helps people?
Show answers (try them yourself first)
- 1Crust, mantle and core.
- 2The crust and the upper part of the mantle.
- 3The semi-molten asthenosphere beneath them.
- 4Pacific, Eurasian and African (or any others from the list).
- 5Convection currents in the mantle driven by heat from the core.
- 6Oceanic crust is only 5–7 km thick compared with 30–40 km under continents.
- 7Hot water rises and cooler water sinks; similarly hot mantle rises and cooler mantle sinks, moving the plates.
- 8It helps identify earthquake- and volcano-prone regions.
Plate Boundaries, Earthquakes and Volcanoes
Imagine two heavy carpets pushed together on a floor: they wrinkle upward. Pulled apart, a gap opens; slid sideways, they scrape and jerk. Plates behave in these three ways at their edges.
A plate boundary is the edge where tectonic plates meet. There are three types: convergent (plates move towards each other), divergent (plates move apart) and transform (plates slide past each other).
Key idea 1: Three boundaries — Convergent: two continental plates collide to form fold mountains such as the Himalaya; an oceanic plate sinks under a continental plate, causing volcanoes and earthquakes. Divergent: magma rises and forms new crust, for example the Mid-Atlantic Ridge. Transform: no crust is made or destroyed, but earthquakes occur, for example the San Andreas Fault.
- Easy: Convergent means moving towards each other.
- Medium: Divergent boundaries create new crust.
- Exam-type: Transform boundaries mainly cause earthquakes.
In the figure: Convergent: plates collide. Divergent: plates separate. Transform: plates slide past.
Plate Boundaries, Earthquakes and Volcanoes
In the app you can move this figure with your finger.
Open in the appKey idea 2: Earthquakes and volcanoes — Most earthquakes and volcanoes occur along plate boundaries, especially around the Pacific Ocean, the Ring of Fire. India has had major earthquakes, such as Gujarat in 2001. In early India the earthquake was called 'bhūkampa' (shaking of the Earth); Varāhamihira in the Bṛihatsaṁhitā noted signs such as changes in wind, rain, clouds and animal behaviour. The mud volcano at Baratang Island (Andaman and Nicobar) bubbles mud due to underground gases and pressure.
- Easy: The Ring of Fire lies around the Pacific Ocean.
- Medium: Compare a plate map with an earthquake map; the dots follow plate edges.
- Exam-type: Internal forces are earthquakes, volcanic eruptions, folding and faulting.
Boundaries decide where mountains, volcanoes and earthquakes appear.
Thinking earthquakes occur randomly everywhere. They cluster along plate boundaries.
Three motions. Memory clue: C-D-T = Collide, Depart, (Slide) Transform.
Spot the difference
| Boundary | Plate motion | Example |
|---|---|---|
| Convergent | Towards each other | Himalaya |
| Divergent | Apart | Mid-Atlantic Ridge |
| Transform | Slide past | San Andreas Fault |
Practice questions
- 1What is a plate boundary?
- 2What forms when two continental plates collide?
- 3What happens at a divergent boundary?
- 4Name one example of a transform boundary feature.
- 5What is the Ring of Fire?
- 6Which Indian state had a major earthquake in 2001?
- 7A line of volcanoes lies along a coast where an ocean plate sinks. Name the boundary type.
- 8What did the word 'bhūkampa' mean?
Show answers (try them yourself first)
- 1The edge where tectonic plates meet.
- 2Fold mountains such as the Himalaya.
- 3Plates move apart and magma rises to form new crust.
- 4The San Andreas Fault.
- 5The belt of earthquakes and volcanoes around the Pacific Ocean.
- 6Gujarat.
- 7Convergent boundary.
- 8The shaking of the Earth.
Plate Boundaries, Earthquakes and Volcanoes
In the app you can move this figure with your finger.
Open in the appWeathering, Erosion and Agents of Gradation
A boundary wall in a rainy town cracks, then bits fall off and rain washes the pieces into the drain. Cracking is like weathering; washing away is like erosion.
Weathering breaks rocks into smaller pieces where they lie. Erosion wears away material and carries it elsewhere. Agents of gradation are running water, glaciers, wind, waves and groundwater.
Key idea 1: Weathering and erosion — Weathering: physical (temperature changes, frost, wind), chemical (reactions with water, air, acids) and biological (roots, animals, micro-organisms). It also forms soil. Erosion: by water, wind, glaciers and coastal waves; it moves the broken material. Erosion removes farmers' topsoil, washes away land near rivers and coasts, makes mining and construction unsafe and harms tourism and fishing.
- Easy: Weathering breaks; erosion breaks and carries.
- Medium: Roots splitting a rock show biological weathering.
- Exam-type: Erosion lowers crop yields by removing fertile topsoil.
In the figure: Five agents lower high land and fill low land.
Key idea 2: Protecting the land — Contouring digs trenches along the contour lines of a hillside; bunding builds earthen embankments; terracing makes level steps on slopes; check dams slow streams. Old examples include techniques of the Sindhu-Sarasvatī civilisation, the Zabo system in Nagaland and land guidelines in the Arthaśhāstra. Landforms also shaped history: river plains gave rise to early cities, the Himalayas protected India yet let ideas pass through the Khyber, and the Thar encouraged the Silk Route.
- Easy: Terracing turns a slope into steps.
- Medium: Bunding slows run-off and lets water soak in.
- Exam-type: Deforestation increases erosion because roots no longer hold soil.
Weathering = break in place; erosion = break and carry; deposition = drop.
Thinking weathering and erosion are the same. Erosion needs movement.
Three steps. Memory clue: W-E-D = Weather (break), Erode (carry), Deposit (drop).
Spot the difference
| Process | What happens |
|---|---|
| Weathering | Rock breaks in place |
| Erosion | Material worn away and carried |
| Deposition | Material dropped and built up |
Practice questions
- 1Define weathering.
- 2Name the three types of weathering.
- 3How does erosion differ from weathering?
- 4Name two agents of gradation.
- 5How does erosion affect farmers?
- 6Name two soil conservation methods.
- 7A hillside field loses soil each monsoon. Suggest a plan.
- 8Why did mountains and deserts affect trade and history?
Show answers (try them yourself first)
- 1Breaking of rocks into smaller pieces where they lie.
- 2Physical, chemical and biological.
- 3Erosion also carries the broken material away.
- 4Running water and wind (also glaciers, waves, groundwater).
- 5It removes fertile topsoil and lowers crop yield.
- 6Terracing and bunding (also contouring, check dams).
- 7Use terraces or contour bunds and plant trees to hold soil.
- 8Mountains acted as barriers and passes; the Thar encouraged routes such as the Silk Route.
Landforms of Rivers, Waves and Glaciers
A tap running onto a sand heap carves a channel and builds a small fan at the end. A river, the sea and a glacier do the same on a huge scale.
Running water, waves and glaciers cut away land (erosion) and leave it elsewhere (deposition), creating distinct landforms.
Key idea 1: Rivers — Upper course: V-shaped valleys, waterfalls (hard rock over soft rock) and rapids. Middle course: meanders and oxbow lakes, floodplains. Lower course: levees, deltas and alluvial fans. Meanders erode the outer bank and deposit on the inner bank. Deltas are fertile (rice, jute) and good for fishing but can flood; the Sundarbans is an example.
- Easy: A waterfall forms when a river drops over a cliff.
- Medium: An oxbow lake is a cut-off bend.
- Exam-type: A delta forms where a river deposits sediment at its mouth.
In the figure: Landforms change along the river's course.
Key idea 2: Waves and glaciers — Waves make beaches, sand bars, cliffs, wave-cut platforms, caves, arches and stacks (arch collapses leave a stack). Glaciers carve U-shaped valleys, cirques, arêtes, hanging valleys and fjords; glacial deposits are moraines (lateral, medial, terminal). Glaciers are also sources of fresh water for rivers downstream.
- Easy: A beach is built by waves depositing sand.
- Medium: A sea cave becomes an arch, then a stack.
- Exam-type: Terminal moraine marks the furthest advance of a glacier.
Each landform can be both a resource and a risk for people.
Thinking deltas form in the mountains. They form at the river's mouth.
River landforms in order. Memory clue: V-M-D = V-valley, Meander, Delta (upper, middle, lower).
Spot the difference
| Agent | Erosion landform | Deposition landform |
|---|---|---|
| River | V-valley, waterfall | Delta, levee |
| Waves | Cliff, arch, stack | Beach, sand bar |
| Glacier | U-valley, cirque | Moraine |
Practice questions
- 1Name two landforms of the upper course of a river.
- 2What is an oxbow lake?
- 3What is a delta?
- 4How is a sea stack formed?
- 5What is a fjord?
- 6Name the three types of moraine.
- 7Why are deltas good for farming?
- 8Why are glaciers important for people downstream?
Show answers (try them yourself first)
- 1V-shaped valley and waterfall (also rapids).
- 2A lake formed from a cut-off bend of a river.
- 3Land built from sediments where a river meets a sea, ocean or lake.
- 4A cave becomes an arch and when the arch collapses a pillar, the stack, remains.
- 5A deep narrow sea inlet made when the sea floods a glacial valley.
- 6Lateral, medial and terminal.
- 7They have rich alluvial soil and plenty of water.
- 8They store fresh water that feeds rivers.
Wind, Groundwater and Landform Disasters
Strong wind in a dry field lifts dust into your eyes; in a dry place that same wind can shape a hill of sand. Water seeping through rock can slowly make a hidden cave.
Wind builds desert landforms such as dunes. Underground water in limestone makes caves, stalactites, stalagmites and sinkholes (karst). Landslides, avalanches, GLOFs and dust storms are disasters linked to landforms.
Key idea 1: Wind and underground water — Wind makes yardangs, ventifacts, deflation hollows (which may reach water and form an oasis), desert pavements and dunes (barchan, longitudinal, star, parabolic). Dunes are barriers against desertification and give sand for building. Underground water dissolves limestone to form caves, stalactites (hang from the roof), stalagmites (rise from the floor), sinkholes and underground rivers.
- Easy: Barchan dunes are crescent-shaped.
- Medium: Star dunes form where winds come from different directions.
- Exam-type: A sinkhole forms when ground collapses into an underground cavity.
In the figure: Four disasters linked to slopes, snow, glacial lakes and dry land.
Key idea 2: Disasters — Landslides: heavy rain, earthquakes, steep slopes, deforestation, mining and poor drainage. Avalanches: heavy snow, sudden warming, wind and human activity. GLOFs: rapid melting, heavy rain or snow, and collapse of an ice or moraine dam. Dust storms: strong winds, drought, sparse vegetation, deforestation and overgrazing. Planning and mitigation save lives.
- Easy: Heavy rain makes slopes heavier and slippery.
- Medium: GLOF means Glacial Lake Outburst Flood.
- Exam-type: Deforestation increases the risk of landslides and dust storms.
Know your local risks and plan before a disaster happens.
Thinking disasters are only natural. Human actions such as deforestation and unplanned building raise the risk.
Cave pillars. Memory clue: stalaCtites hang from the Ceiling; stalaGmites rise from the Ground.
Spot the difference
| Disaster | Main causes |
|---|---|
| Landslide | Rain, steep slope, deforestation |
| Avalanche | Heavy snow, warming |
| GLOF | Melting, weak dam |
| Dust storm | Wind, drought, bare soil |
Practice questions
- 1What is a dune?
- 2Which dunes are crescent-shaped?
- 3What is a sinkhole?
- 4Difference between stalactite and stalagmite?
- 5Give two causes of landslides.
- 6What does GLOF stand for?
- 7A village has cut its slope forest and built roads without drainage. Which disaster risk rises?
- 8Why do dust storms occur more where vegetation is sparse?
Show answers (try them yourself first)
- 1A hill or ridge of sand formed by wind.
- 2Barchan dunes.
- 3A depression formed when the ground collapses into an underground cavity.
- 4Stalactites hang from the cave roof; stalagmites rise from the floor.
- 5Heavy rain and deforestation (also earthquakes, steep slopes, mining).
- 6Glacial Lake Outburst Flood.
- 7Landslides.
- 8Bare, dry soil is easily lifted by strong winds.