Internal Movements and Their Classification
When a pot of water is kept on a stove the water at the bottom heats and moves up and down. In the same way there is movement inside the earth due to heat, but we do not see it.
Movements taking place inside the earth, mainly in the upper layer of the mantle, are endogenetic (internal) movements. They cause earthquakes, volcanoes, mountains and continents.
Key idea 1 : Why do movements occur? — Radioactive materials in the mantle give out huge energy. This energy moves like waves from one place to another and makes the mantle unstable. So the surface looks calm but is unstable from inside.
Key idea 2 : Classification by speed — Slow movements are continuous and gentle and form mountains and continents. Sudden movements occur like an event, e.g. earthquakes and volcanoes.
Key idea 3 : By direction and landform — By direction movements are horizontal or vertical (up-down). By landform they are continent-building (continents, plateaus, highlands) or mountain-building (folding, faulting).
- Easy : Mountain-forming movement by speed? — Slow movement.
- Medium : Earthquake falls under? — Sudden movement.
- Exam-type : In which layer mainly? — Upper mantle.
In the figure: Classification of internal movements.
Slow = mountains, continents; sudden = earthquakes, volcanoes.
Earthquakes and mountain-building do not occur at the same speed; one is sudden, the other slow.
Memory clue: Speed, direction, landform = three bases. Memory clue: "Speed-Direction-Form".
Spot the difference
| Type | Speed | Example |
|---|---|---|
| Slow movement | Slow and continuous | Mountains, continents |
| Sudden movement | Sudden | Earthquake, volcano |
Practice questions
- 1What are endogenetic movements?
- 2Where do these movements mainly occur?
- 3What creates instability in the mantle?
- 4State the types of movements by speed.
- 5Which are the movements by direction?
- 6State the difference between continent-building and mountain-building movements.
- 7Why is the earth called unstable though the surface looks calm?
- 8Give an example of a sudden natural event you know.
Show answers (try them yourself first)
- 1Movements taking place inside the earth.
- 2In the upper mantle.
- 3Energy of radioactive materials.
- 4Slow and sudden.
- 5Horizontal and vertical.
- 6Continent-building = continents, plateaus; mountain-building = folds, faults.
- 7Because movements go on due to internal energy.
- 8Earthquake or volcano.
Slow Movements: Mountains and Continents
If a thick mat is pushed from both sides towards each other it forms folds. If a ball of dough is pulled by both hands it tears. The same pressure and tension arise in the earth’s crust.
Pressure folds rocks and forms fold mountains. Tension breaks rocks and forms faults, which give block mountains and rift valleys. Continent-building movements raise or lower a large part of the crust.
Key idea 1 : Pressure and fold mountains — Waves moving towards each other fold layers of soft rock. With heavy pressure folds become large and complex and the land is lifted. The Himalayas, Aravalis, Rockies, Andes and Alps are fold mountains.
Key idea 2 : Tension, faults and block mountains — Waves moving apart put tension on rocks and cracks form; these are faults. When the part between two parallel faults is lifted a block mountain forms. Its top is flat and slopes are steep. The Meghalaya plateau and the Black Forest of Europe are examples.
Key idea 3 : Rift valley and continent-building — If the part between two faults sinks a rift valley forms; both its slopes are steep. The Narmada valley, the Great Rift Valley of Africa and the Rhine valley are examples. When a large part of the crust is raised above sea level by continent-building movements a continent or wide plateau forms; if it sinks it becomes sea floor.
- Easy : If pressure comes from both sides? — Folds form.
- Medium : Part between two faults sinks? — Rift valley.
- Exam-type : Flat top and steep slopes? — Block mountain.
In the figure: Landforms from slow movements.
Pressure = folds; tension = faults; middle up = block, middle down = rift.
All mountains do not form the same way; the Himalayas are fold and the Meghalaya plateau is block type.
Memory clue: Push = fold, pull = break. Memory clue: "Pressed it folds, pulled it breaks".
Spot the difference
| Landform | Cause | Example |
|---|---|---|
| Fold mountain | Pressure | Himalayas |
| Block mountain | Tension; middle part up | Black Forest |
| Rift valley | Tension; middle part down | Narmada valley |
Practice questions
- 1How are fold mountains formed?
- 2What is a fault?
- 3State the features of a block mountain.
- 4How is a rift valley formed?
- 5State the difference between fold and block mountains.
- 6State the effects of continent-building movements.
- 7What is the difference in the formation of the Meghalaya plateau and the Himalayas?
- 8Find two examples of rift valleys on a map.
Show answers (try them yourself first)
- 1Layers fold due to pressure from both sides.
- 2A crack in rock formed by tension.
- 3Flat top, steep slopes, no peaks at first.
- 4The part between two faults sinks.
- 5Fold = pressure and folds; block = tension and faults.
- 6Continents, wide plateaus or sea floor.
- 7Himalayas by folding; Meghalaya by faulting and uplift.
- 8Narmada valley and Great Rift Valley.
Earthquakes
When a big truck passes outside, glasses in the cupboard rattle. In an earthquake the ground itself shakes, with far more energy.
When the tension stored inside the earth crosses its limit the energy is released as waves and the ground shakes; this is an earthquake. Its magnitude is measured on the Richter scale.
Key idea 1 : Causes and centre — Moving, colliding or sliding plates, cracks in rocks and volcanic eruptions are causes. The place where energy is released is the focus (hypocentre). The spot on the surface just above the focus is the epicentre, which feels the first shock.
Key idea 2 : Seismic waves — P waves are the fastest, shake back and forth and pass through solid, liquid and gas. S waves are somewhat slower, shake up and down and pass only through solids; they are more destructive than P. L (surface) waves come last, travel along the crust and are the most destructive.
Key idea 3 : Seismograph and effects — A seismograph draws a graph of the waves (seismogram) from which magnitude is known. Effects: ground cracks, landslides, wells drying or new water appearing, land rising or sinking, tsunamis in the sea, avalanches in snow areas, collapsing buildings, broken roads and communication.
- Easy : The wave that comes first? — P wave.
- Medium : Wave that cannot pass through liquid? — S wave.
- Exam-type : Spot on the surface above the focus? — Epicentre.
In the figure: Stages of an earthquake.
A big earthquake struck Nepal on 25 April 2015. Early news (as in the textbook) gave magnitude 7.9 and about 1,500 deaths; later the US Geological Survey recorded 7.8 and the toll rose to about 9,000. (Check with the textbook)
P = first, back-forth; S = second, up-down; L = last, on surface.
Epicentre and focus are not the same; the focus is inside the earth, the epicentre on the surface.
Memory clue: P-S-L come in this order. Memory clue: "First, slower, last".
Spot the difference
| Wave | Motion | Medium |
|---|---|---|
| P wave | Back-forth; fastest | Solid, liquid, gas |
| S wave | Up-down | Only solid |
| L wave | On surface; most destructive | Along the crust |
Practice questions
- 1What is an earthquake?
- 2State the causes of an earthquake.
- 3State the difference between focus and epicentre.
- 4State the difference between P and S waves.
- 5Why are L waves the most destructive?
- 6What does a seismograph do?
- 7State the effects of earthquakes on human life.
- 8Write what safe actions you will take during an earthquake.
Show answers (try them yourself first)
- 1Ground shaking when stored tension energy is released.
- 2Plate movement, collision, cracks, volcanoes.
- 3Focus is inside; epicentre is above it on the surface.
- 4P back-forth and fast; S up-down and only through solids.
- 5Because they travel along the crust and strike buildings directly.
- 6Draws a graph of waves to tell the magnitude.
- 7Collapsing buildings, loss of life, blocked roads, tsunami, landslides.
- 8Go under a strong table or to open ground; avoid lifts.
Volcanoes
If a fizzy drink bottle is shaken and opened, froth shoots out. When hot material from inside the earth finds a way out we get a volcanic eruption.
The throwing out of hot solid, liquid and gaseous material from the mantle onto the surface is a volcanic eruption. Molten magma reaching the surface is called lava.
Key idea 1 : By type of eruption — In a central (conical) volcano magma comes through one pipe-like vent and a cone forms around it; Mt. Fuji of Japan, Mt. Kilimanjaro of Tanzania. In a fissure volcano magma comes out of many cracks and spreads on both sides forming volcanic plateaus; India’s Deccan plateau formed this way.
Key idea 2 : By frequency of eruption — Active volcanoes erupt regularly even now (Fuji, Stromboli). Dormant ones are quiet for long but may wake suddenly (Vesuvius, Katmai). Extinct ones are not likely to erupt again; rain water gathers in their craters to form lakes.
Key idea 3 : Plate boundaries and effects — Most volcanoes and earthquake zones lie near plate boundaries. Where one plate goes under another is a destructive boundary; where new material rises is a constructive boundary. Effects: loss of life and property, ash and gases upset the environment, but ash makes land fertile, minerals are found and new land or islands form.
- Easy : Magma from one pipe-like vent; type? — Central.
- Medium : Deccan plateau formed by? — Fissure type.
- Exam-type : Quiet for long but may wake again; type? — Dormant.
In the figure: Type and frequency of eruption.
Barren Island in the Andamans is India’s only active volcano. On 23 January 2017 a team of the National Institute of Oceanography saw it erupting. The textbook says it woke in February 2017; sources say it has erupted on and off since 1991. (Check with the textbook)
One vent = cone; many cracks = plateau; active, dormant, extinct = frequency.
A dormant volcano is not harmless; it can erupt suddenly again.
Memory clue: Active → dormant → extinct. Memory clue: "Awake, asleep, gone".
Spot the difference
| Type | State | Example |
|---|---|---|
| Active | Regular eruptions | Fuji, Stromboli |
| Dormant | Quiet long, may wake | Vesuvius |
| Extinct | Not likely to erupt | Crater lake |
Practice questions
- 1What is a volcanic eruption?
- 2State the difference between magma and lava.
- 3State the difference between central and fissure volcanoes.
- 4What are active, dormant and extinct volcanoes?
- 5Why are most volcanoes found near plate boundaries?
- 6State the good and bad effects of volcanoes.
- 7How was the Deccan plateau formed?
- 8Name India’s only active volcano and its location.
Show answers (try them yourself first)
- 1Hot material thrown from the mantle onto the surface.
- 2Magma is under the ground; on reaching the surface it is lava.
- 3Central = one vent, cone; fissure = many cracks, plateau.
- 4Active = erupting; dormant = quiet but may wake; extinct = no more eruptions.
- 5Because rocks are weak and movements frequent there.
- 6Bad: loss, imbalance; good: fertile land, minerals.
- 7By lava spreading from fissure eruptions.
- 8Barren Island, Andamans.