Chapter 3 — Our Changing Earth

What the topic covers
"Overview: Our Changing Earth" introduces how the shape and surface of the Earth are not fixed but change continuously due to internal and external processes. Changes may be very slow (millions of years) or sudden and rapid (earthquakes, volcanic eruptions). The chapter explains the structure of the Earth, major processes that change landforms, and the evidence and consequences of these changes.

Structure of the Earth (simple cross-section)

• Crust: the outer solid layer. Continental crust ~35 km (can be 5–70 km), oceanic crust ~5–10 km.
• Upper mantle and lower mantle: extends to ~2900 km depth; composed of semi-solid rock that can slowly flow.
• Outer core: molten, mainly iron and nickel, thickness ~2200 km.
• Inner core: solid, mainly iron and nickel, radius ~1220 km.
• Average density of Earth ≈ 5.5 g/cm3 (crust ~2.7 g/cm3, mantle ~4.5 g/cm3, core ~11 g/cm3).

Internal (endogenic) processes
These originate from Earth’s interior and include plate tectonics (movement of lithospheric plates), volcanic activity, mountain building (orogeny), folding and faulting, and earthquakes. Plate movements are driven by mantle convection and cause continents to drift, oceans to open or close, and mountain ranges to form.

Evidence for continental drift / plate tectonics

• Jigsaw fit of continents (for example, west coast of Africa and east coast of South America).
• Similarity of fossils and rock types across continents separated by oceans.
• Matching geological structures and past climatic evidence (e.g., glacial deposits).

External (exogenic) processes
These are caused by surface agents powered by the Sun and gravity: weathering (mechanical and chemical), mass wasting, erosion and transportation by rivers, glaciers, wind and sea waves, and deposition. These processes wear down mountains, form valleys, create deltas and alluvial plains, and reshape coastlines.

Timescales and effects
Some processes (volcanic eruption, earthquake, landslide) change the landscape in seconds to days. Others (mountain building, erosion creating valleys, formation of alluvial plains) take thousands to millions of years. Both types affect human life — creating hazards (earthquakes, floods, volcanic ash) and providing resources (soil, minerals).

How we study these changes
Methods include observation of landforms and rocks, fossil studies, measurement of earthquakes (seismology), mapping of ocean floors (sonar), GPS measurements of plate motions, and satellite remote sensing to monitor changes over time.

Key points to remember

• The Earth is layered and dynamic; interior heat drives plate motions.
• Internal forces build up landforms (mountains, volcanoes); external forces wear them down (erosion, weathering).
• Evidence from geology, fossils and matching coastlines supports continental drift and plate tectonics.
• Understanding these processes helps us prepare for natural hazards and manage resources.

What are internal forces?
Internal forces (endogenic forces) originate inside the Earth. They are driven by heat from the Earth's interior and act on the crust and mantle to change the shape of Earth’s surface. These forces cause folding, faulting, earthquakes, and volcanic activity.

Structure relevant to plate tectonics
The outer solid layer is the lithosphere (crust + uppermost mantle). It rests on a weaker, slowly flowing layer called the asthenosphere. The lithosphere is broken into large and small plates called tectonic plates.

Plate tectonics theory (basic idea)
The lithospheric plates move relative to each other over the asthenosphere. Their motion and interaction explain most internal processes and landforms. Plate motion is driven by mantle convection, slab-pull (sinking cold plates), and ridge-push (higher mid-ocean ridges pushing plates away).

Types of plate boundaries and typical features

• Divergent (constructive) – Plates move apart. Sea-floor spreading at mid-ocean ridges forms new crust. Example features: mid-ocean ridges, rift valleys (e.g., Mid‑Atlantic Ridge, East African Rift).
• Convergent (destructive) – Plates move toward each other. If an oceanic plate meets a continental plate, the denser oceanic plate subducts producing deep-sea trenches, volcanic arcs, earthquakes (e.g., Andes, Peru‑Chile Trench). If two continental plates collide, they form fold mountains (e.g., Himalayas).
• Transform (conservative) – Plates slide past each other horizontally. These produce large earthquakes along strike‑slip faults (e.g., San Andreas Fault).

Consequences of plate interactions

• Earthquakes: sudden release of energy along faults where plates lock and then slip.
• Volcanism: magma rises where plates diverge or where subduction melts material.
• Mountain building: folding and uplift at convergent boundaries.
• Ocean trench and island arc formation at subduction zones.

Rates and scale
Plates move very slowly — typically a few centimetres per year (about the speed fingernails grow). Over millions of years these small rates produce large changes in continents and oceans.

Class 7 level summary
Internal forces reshape the Earth from within. Plate tectonics is the unifying theory explaining how plates move and interact to produce most major landforms and many natural hazards. Understanding plate boundaries helps explain the location of mountains, volcanoes, trenches, and earthquakes.

Definition: Folding is the bending of layered rocks (usually sedimentary) caused by compressive forces in the Earth's crust. When horizontal rock layers are pushed together, they may bend into wave‑like structures called folds.

How it happens: Rocks at shallow depths generally behave brittlely, but under high temperature and pressure deep in the crust they behave plastically and bend rather than break. Tectonic forces (collision or lateral compression of plates) squeeze rock layers, producing anticlines (upward-arching folds) and synclines (downward trough-like folds).

Main parts of a fold: crest (highest point of an anticline), trough (lowest point of a syncline), limbs (sides of the fold), axial plane (imaginary plane dividing the fold), hinge (line of maximum curvature).

Types of folds (by geometry): anticline, syncline, monocline (single bend), recumbent fold (almost horizontal axial plane), overturned/overfold (one limb tilted beyond vertical), isoclinal (parallel limbs).

Factors that control folding: nature of rock (brittle vs ductile), temperature and pressure, rate and duration of compressive forces, thickness and layering of sediments.

Effects and significance: Fold mountains (e.g., Himalayas) form where large-scale folding uplifts crustal material. Folds create structural traps for oil and gas, control groundwater flow, and influence landforms and drainage patterns.

Time scale: Folding is a slow geological process that commonly takes millions of years.

What is a fault? A fault is a fracture or break in the Earth's crust along which blocks of rock move relative to each other. Faulting occurs when stress (push, pull or shear) on rocks exceeds their strength, causing sudden or slow movement. Faults are common near plate boundaries but can occur inside plates too.

Key terms

• Fault plane: the surface along which movement occurs.
• Hanging wall: the block above the fault plane.
• Footwall: the block below the fault plane.
• Throw (T): vertical component of displacement.
• Heave (H): horizontal component of displacement.
• Slip (S): the total movement along the fault plane.
• Dip (θ): angle at which the fault plane inclines from the horizontal.

Types of faults

• Normal fault – hanging wall moves down relative to footwall. Caused by tensional (pulling) stress. Produces features like rift valleys and grabens.
• Reverse (thrust) fault – hanging wall moves up relative to footwall. Caused by compressional (pushing) stress. Thrust faults have a low dip and are important in mountain building.
• Strike-slip (transform) fault – movement is mainly horizontal, parallel to the fault. Caused by shear stress. Can be right-lateral (dextral) or left-lateral (sinistral).
• Oblique-slip fault – a combination of vertical and horizontal movement.

How faults affect the landscape

• Earthquakes: sudden slip on a fault releases energy as seismic waves.
• Fault scarps: steep steps or cliffs formed where one block is offset vertically.
• Rift valleys and basins (graben) and uplifted blocks (horst) form from repeated normal faulting.
• Mountain thrusting and folding often accompany reverse/thrust faults.

Why faults form
Faults form because of tectonic plate movements and the build-up of stress in the crust. When rocks deform elastically they store energy; when they break, that energy is released and blocks slip along faults.

Simple geometry (useful to visualise)
If S is the slip along the fault plane, θ the dip angle, then the vertical (throw) and horizontal (heave) components are related to S and θ (see formulas below). This geometry helps students understand how much vertical or horizontal offset a fault produces.

What is an earthquake? An earthquake is the sudden shaking of the Earth's surface caused by the rapid release of energy in the Earth's crust. This energy release produces seismic waves that travel through the ground.

Where and why do earthquakes occur? Most earthquakes occur along plate boundaries where tectonic plates interact: they may collide (convergent), move apart (divergent) or slide past each other (transform). Stress builds up in rocks until it is released by a sudden slip along a fault. Earthquakes can also be caused by volcanic activity or, less commonly, by human actions (large reservoirs, mining or fluid injection).

Key terms

• Focus (hypocentre): The point inside the Earth where the earthquake starts.
• Epicentre: The point on the surface directly above the focus.
• Seismic waves: Body waves (P-waves and S-waves) travel through Earth; surface waves (Love and Rayleigh) travel along the surface and usually cause most damage.
• Magnitude: A measure of the energy released at the source (e.g., Richter magnitude).
• Intensity: A measure of the shaking at a place (varies by location) often described by the Mercalli scale.

Effects of earthquakes include ground shaking, surface rupture, landslides, subsidence, fires (from broken gas lines), and tsunamis if undersea. Damage depends on magnitude, depth, distance from epicentre, local geology, and building quality.

Measuring earthquakes — Seismographs record ground motion. Magnitude scales (like the Richter scale) quantify energy released; intensity scales describe local effects. Seismologists use arrival times of P and S waves at different stations to locate the epicentre.

Preparedness and mitigation — Keeping buildings earthquake-resistant (flexible frames, base isolation, reinforced foundations), following safety drills (Drop, Cover, Hold On), having emergency kits and clear evacuation routes, and zoning laws (avoid weak ground) reduce loss of life and damage.

What is a volcano?
A volcano is an opening in the Earth's crust through which molten rock (magma), gases and ash escape to the surface. When magma reaches the surface it is called lava.

How volcanoes form
Volcanoes form where magma from the mantle rises through the crust. This commonly happens at plate boundaries and at hot spots:

• Subduction zones (convergent boundaries): one plate sinks beneath another, melting rock forms magma (e.g., the Ring of Fire).
• Rift zones (divergent boundaries): plates move apart and magma rises to fill the gap (e.g., mid-ocean ridges, East African Rift).
• Hot spots: a fixed plume of hot mantle rises under a plate, forming volcanoes away from plate boundaries (e.g., Hawaii).

Parts of a volcano

• Magma chamber: reservoir of molten rock beneath the volcano.
• Conduit (pipe): passage through which magma moves upward.
• Vent: the surface opening; may be a single central vent or many vents.
• Crater: bowl-shaped depression around the vent.
• Caldera: a large, collapsed crater formed after very large eruptions.
• Flank: the sides of the volcano; cinder cones or parasitic vents can form on flanks.

Types of volcanoes

• Shield volcanoes: broad, gently sloping cones built by low-viscosity basaltic lava (e.g., Mauna Loa, Kīlauea - Hawaii).
• Stratovolcanoes (composite): steep, layered volcanoes with alternating lava flows and ash; often explosive (e.g., Mount Fuji, Mount St. Helens, Mount Vesuvius).
• Cinder cones: small steep cones of volcanic ash and cinders, built during single short eruptions.
• Lava domes: formed by slow, viscous lava that piles up near the vent.

Types of eruptions and materials
Eruptions range from effusive (lava flows) to explosive (ash, volcanic bombs, pyroclastic flows). Main materials: lava, ash, lapilli, volcanic bombs, gases (water vapor, CO2, SO2).

Effects of volcanic activity

• Destructive: loss of life, destruction of buildings, ash fall disrupting air travel and agriculture, pyroclastic flows, lava flows.
• Constructive / beneficial: formation of new land (islands), very fertile soils (volcanic soils good for farming), geothermal energy, important minerals.

Warning signs of eruptions
Increased earthquake activity, gas emissions (sulphur dioxide rise), ground deformation (swelling), changes in hot spring temperatures and water chemistry.

Safety measures
Evacuation plans, monitoring by volcanologists (seismographs, gas sensors, GPS for deformation), public education, exclusion zones around active vents.

Interesting facts
Large volcanic eruptions can affect global climate (e.g., the 1991 Mount Pinatubo eruption cooled global temperatures for a year or more due to injection of sulfate aerosols into the stratosphere).

What is weathering?

Weathering is the natural process by which rocks are broken down into smaller particles (grains, sand, soil) or chemically altered in place near Earth’s surface. Unlike erosion, which moves the broken material, weathering acts in situ (where the rock originally is).

Types of weathering

• Physical (mechanical) weathering: Breaks rocks into smaller pieces without changing their chemical composition. Major processes include:
• Frost action (freeze–thaw): Water enters cracks, freezes, expands and widens the crack.
• Thermal expansion: Repeated heating and cooling causes outer layers to peel (exfoliation).
• Abrasion: Particles carried by wind, water or ice grind rock surfaces.
• Salt-crystal growth: Salts crystallize in pores and widen them.
• Biological/plant root action: Roots grow into cracks and split rocks; animals can burrow and break rock.
• Chemical weathering: Alters the minerals in rock by chemical reactions, producing new minerals and soluble salts. Main processes:
• Carbonation: CO2 + H2O → H2CO3 (carbonic acid); reacts with calcium carbonate in limestone to form soluble bicarbonate.
• Hydrolysis: Water reacts with minerals (e.g., feldspar → clay minerals + dissolved ions).
• Oxidation: Oxygen reacts with iron-bearing minerals producing oxides (rust).
• Solution (dissolution): Some minerals (e.g., halite, gypsum) dissolve directly in water.
• Acid reaction: Acid rain (sulfuric/nitric acids) increases chemical attack on rocks and monuments.

Factors affecting weathering

• Climate: Temperature and moisture — chemical weathering is fastest in warm, wet climates; physical weathering (freeze–thaw) is effective in cold regions with frequent freeze cycles.
• Rock type and structure: Hardness, mineral composition, presence of joints and fractures — more joints = faster weathering.
• Surface area: Smaller pieces or more fractured rocks weather faster because of greater exposed surface.
• Vegetation and organisms: Roots, lichens and microbes enhance both mechanical and chemical weathering.
• Time: Longer exposure leads to more intense weathering.
• Human activity: Mining, deforestation and pollution (acid rain) speed up weathering.

Importance of weathering

• Produces soil essential for plant growth and agriculture.
• Changes landscapes (formation of caves, gullies, rounded hilltops).
• Supplies sediments for rivers, beaches and deltas.
• Can damage buildings, monuments, roads and other structures.

Difference from erosion: Weathering breaks down rock in place; erosion transports the weathered material away by water, wind, ice or gravity.

Tip for students: When you see potholes, rusted metal, a limestone cave, or tree roots lifting pavement, you are seeing results of weathering.

Overview: Erosion, transportation and deposition are three linked processes by which the surface of the Earth is worn away, material is moved and then laid down. They shape rivers, coasts, deserts and mountain landscapes.

Erosion is the wearing away and removal of rock and soil by natural agents: running water, waves, wind and glaciers. Erosion involves breaking materials loose (weathering helps) and carrying them away. Stronger flow/force and steeper slopes increase erosion.

Agents of erosion (main ones):

• Water (rivers, rain runoff) — cuts valleys, creates gorges and waterfalls.
• Waves (coastlines) — erode cliffs, form sea caves and stacks.
• Wind — erodes and shapes rocks in deserts, forms ventifacts and deflation hollows.
• Glaciers — scour rock, produce U-shaped valleys and cirques.

Transportation is the carrying of eroded material (sediment) by the same agents. Types of transported load in rivers:

• Dissolved load — minerals dissolved in water.
• Suspended load — fine soil, silt and clay carried within the water.
• Bed load — larger particles (sand, pebbles) rolled, slid or bounced along the bed.

Deposition happens when the transporting agent loses energy (slows down or melts) and can no longer carry the material. Coarser particles are deposited first, finer particles travel farther. Deposition builds new landforms.

Important landforms produced:

• By rivers: V-shaped valleys, waterfalls and rapids (erosion); meanders and oxbow lakes (migration and deposition); floodplains and levees; deltas where rivers meet the sea.
• By glaciers: U-shaped valleys, moraines, drumlins (deposition by ice).
• By wind: sand dunes, loess deposits.
• By waves: beaches (deposition), sea cliffs and stacks (erosion).

Why these processes matter: They continually reshape Earth’s surface, create fertile soils (from deposited silt), but can also cause problems such as soil loss, coastal retreat and siltation of reservoirs. Human activities (deforestation, overgrazing, construction) can increase erosion and change patterns of deposition.

Prevention and management (short): Afforestation, terracing, check-dams, controlled grazing, and coastal protection works reduce erosion and manage sedimentation.

What is mass wasting?
Mass wasting (also called mass movement) is the downhill movement of rock, soil and debris under the direct influence of gravity. It does not require a transporting agent like a river or glacier — gravity itself pulls material down slopes. Mass wasting is one of the main surface processes that shape Earth’s landscapes.

Main types of mass wasting

• Fall (rockfall): Rapid free-fall of rock from a steep cliff; results in accumulations of broken rock (talus or scree) at the base.
• Slide: A mass of material moves as one or several coherent blocks along a surface. Two common kinds are translational slides (move along a flat plane) and rotational slides or slumps (move along a curved surface and leave a rounded scarp).
• Flow: Material behaves like a viscous fluid. Examples: debris flows, mudflows, and earthflows. Often very fast and destructive when water content is high.
• Creep: Extremely slow downslope movement of soil and regolith, often measurable by tilted trees, walls, or posts.
• Avalanche: Rapid flow of snow (sometimes mixed with rock and ice) down steep slopes.

Causes (factors that increase the likelihood of mass wasting)

• Gravity — always acting downward and providing the driving force.
• Slope angle — steeper slopes increase the downslope force.
• Water — reduces internal friction and cohesion, increases weight and pore pressure; heavy rain often triggers flows and slides.
• Vegetation loss — roots bind soil; when removed (by deforestation or fire) slopes become unstable.
• Earthquakes — ground shaking can dislodge material and trigger landslides.
• Human activities — excavation, mining, construction, and overload at the top of slopes can destabilize them.

Landforms produced by mass wasting and related surface processes

• Talus (scree) slope: Cone- or apron-shaped piles of angular rock fragments at the base of cliffs from repeated rockfalls.
• Scarp and slump blocks: A steep exposed face (scar) where a slide or slump occurred; slump blocks may remain tilted downhill.
• Debris/Alluvial fan: At the mouth of a steep channel a debris flow deposits a fan-shaped accumulation of coarse sediment.
• Hummocky terrain: Irregular mounds and hollows formed by large landslides or debris avalanches.
• Creep features: Bent trees, tilted fences, terracettes (small steps) on slopes.

Why mass wasting matters
Mass wasting reshapes slopes, fills valleys with debris, alters drainage, can block rivers creating lakes, and poses risks to people, roads and buildings. Understanding causes helps reduce risk and plan safer land use.

Prevention and mitigation (short-term & long-term)

• Improve drainage (prevent water build-up in slopes).
• Plant vegetation and manage forests to stabilize soil.
• Terracing and benching of slopes to reduce steepness.
• Retaining walls, rock bolts, and nets to hold material in place.
• Land‑use planning: avoid building on known unstable slopes and maintain buffer zones.

Simple classroom demonstration: Use a tray filled with dry sand at different tilt angles to show angle of repose, then add a little water to show how moisture changes behaviour (from stable pile to flowing slump).

What is Mountain Building (Orogeny)?

Orogeny (from Greek 'oros' = mountain) is the process by which mountains are formed. Mountain building is the result of movements and interactions of the Earth's lithospheric plates and the deformation of the crust over long periods (millions of years). These processes include folding, faulting, volcanic activity and uplift followed by erosion.

Major Causes

• Plate tectonics: Convergent plate boundaries (collision or subduction) are the main sites of mountain formation. When two continental plates collide they form large fold mountains; when an oceanic plate subducts beneath a continental plate, it produces volcanic mountain chains.
• Folding: Horizontal compressive forces bend rock layers into anticlines (upward folds) and synclines (downward folds).
• Faulting: Rocks break and blocks move along faults. Vertical movements produce block (fault-block) mountains — horsts (uplifted) and grabens (down-dropped).
• Volcanism: Molten rock (magma) rising to the surface builds volcanic mountains.
• Isostatic uplift & erosion: Removal of surface weight by erosion or melting of ice can cause the crust to rise (isostasy); erosion then shapes the mountains over time.

Types of Mountains (by origin)

• Fold Mountains: Formed mainly by compression and folding of sedimentary rocks (e.g., Himalayas, Alps, Andes).
• Fault-block Mountains: Formed when blocks of crust are uplifted or dropped between faults (e.g., Horst and Graben systems like parts of the Basin and Range in the USA; East African Rift examples).
• Volcanic Mountains: Built by volcanic activity (e.g., Mount Fuji, Mount Kilimanjaro, Mount St. Helens).
• Residual (Erosional) Mountains: Remaining high areas left after long-term erosion of surrounding rocks (e.g., parts of the Deccan Traps plateau margins).

Stages of Mountain Building

• Initial compression or uplift (tectonic forces act).
• Deformation (folding, faulting) and volcanic activity as rocks respond.
• Uplift reaches high elevation; mountain range matures.
• Erosion and weathering slowly wear mountains down; rivers and glaciers reshape valleys.

Why Mountains Keep Changing

Mountain ranges are dynamic. Collision zones like the Himalaya are still rising because the Indian plate continues to move northward into the Eurasian plate. Earthquakes, volcanic eruptions and continuous erosion all change mountain shapes and heights through time.

Key Terms

• Anticline: Upward fold in layered rock.
• Syncline: Downward fold.
• Horst: Uplifted block between two normal faults.
• Graben: Down-dropped block between two faults.
• Subduction: One plate sinking beneath another into the mantle.

Simple summary: Mountain building is caused mainly by plate movements that fold, fault and uplift the Earth's crust and sometimes by volcanic activity. Over millions of years these processes create, raise and reshape mountain ranges.

This glossary covers the main terms from Chapter 'Our Changing Earth' (Class 7 Geography). Definitions are simple, with how each term connects to real events and processes that change Earth’s surface.

• Earth’s layers: Crust (thin outer layer), Mantle (thick, semi-solid layer below the crust), Core (inner metallic part: outer core-liquid, inner core-solid). Movements in the mantle drive plate motion.
• Tectonic plate: A large rigid slab of lithosphere (crust + uppermost mantle) that moves over the asthenosphere. Plates interact at boundaries to produce earthquakes, volcanoes and mountains.
• Plate boundaries:
  • Divergent: plates move apart (mid-ocean ridges, rift valleys).
  • Convergent: plates move toward each other (subduction zones, fold mountains).
  • Transform: plates slide past one another (strike-slip faults).
• Continental drift: Idea that continents have moved over geological time (Wegener). Modern theory: plate tectonics explains how and why.
• Seafloor spreading: New oceanic crust is formed at mid-ocean ridges and moves away, pushing plates apart.
• Fault: A fracture in the crust along which movement has occurred. Types: normal (extension), reverse/thrust (compression), strike-slip (horizontal).
• Earthquake: Sudden shaking of the ground caused by rapid release of energy along faults or by volcanic activity.
• Focus (hypocenter): The point inside Earth where an earthquake starts.
• Epicenter: The point on Earth's surface directly above the focus.
• Seismic waves: Energy waves produced by earthquakes. P-waves (primary, compressional, fastest), S-waves (secondary, shear, slower), Surface waves (travel along surface, cause most damage).
• Magnitude: A measure of energy released by an earthquake (logarithmic scale — e.g., Richter or moment magnitude). A 1-unit increase means ~10× amplitude and ~32× energy.
• Intensity: Degree of shaking at a place; depends on magnitude, distance, local geology (measured by scales such as Modified Mercalli).
• Volcano: A vent in Earth’s crust through which magma, ash and gases erupt. When magma reaches surface it is called lava.
• Types of volcanoes: Shield (broad, gentle slopes; low-viscosity lava), Stratovolcano/Composite (steep, explosive eruptions), Cinder cone (small, steep, ash-dominated).
• Subduction: When one plate sinks beneath another into the mantle; creates deep ocean trenches, volcanic arcs and powerful earthquakes.
• Fold: Bend in layered rocks produced by compressive forces. Repeated folding produces fold mountains (e.g., Himalaya).
• Rift valley: A valley formed where the crust is pulled apart (e.g., East African Rift).

Short practical notes: Focus vs epicenter helps locate where an earthquake originated; plate boundary type predicts hazards (convergent → big quakes & volcanoes; transform → strong shallow quakes; divergent → moderate quakes, volcanic ridges).