Endogenetic MovementsClass 9 Geography Chapter 2 Notes – Maharashtra Board

Summary: Endogenetic Movements

Movements inside the Earth mostly happen in the upper mantle. Radioactive materials there release huge energy, which moves around and makes the mantle unstable. These internal movements are grouped by speed (slow or sudden), direction (horizontal or vertical) and the landforms they make.

Slow movements build landforms over millions of years. Horizontal forces pushing together (compression) fold soft rock layers into fold mountains such as the Himalayas, Alps and Andes. Forces pulling apart (tension) crack rocks into faults, producing block mountains like the Black Forest and rift valleys like the Narmada valley.

Continent-building (epeirogenic) movements act up or down, towards or away from the Earth's centre. They lift large parts of the crust above sea level to form continents and plateaus, or sink land below the sea.

Earthquakes are sudden movements. Stress builds up and is released at the focus; the epicentre is the point on the surface just above it. The energy travels as P, S and L (surface) waves, recorded by a seismograph, and the size is given on the Richter scale.

Volcanoes throw out lava, ash and gases from the mantle. Central (conical) eruptions build cone mountains like Mt Fuji; fissure eruptions build lava plateaus like the Deccan. Volcanoes are active, dormant or extinct, and most earthquakes and volcanoes lie along plate boundaries.

Key points to remember

• Endogenetic = internal movements, mainly in the upper mantle, driven by heat from radioactive materials
• Classified by velocity (slow/sudden), direction (horizontal/vertical) and landforms (continent-building/mountain-building)
• Compression (forces moving towards each other) → folds → fold mountains
• Tension (forces moving apart) → faults → block mountains and rift valleys
• Fold mountains: Himalayas, Aravalis, Rockies, Andes, Alps
• Block mountain: Black Forest (Europe); Meghalaya Plateau formed in a similar way
• Rift valleys: Narmada (India), Great Rift Valley (Africa), Rhine (Europe)
• Focus (hypocentre) = where energy is released; epicentre = surface point directly above it
• P waves fastest, travel through solids, liquids and gases; S waves only through solids; L waves along the surface, most destructive
• Earthquake size is measured on the Richter scale; recorded by a seismograph
• Central type → cone (Mt Fuji, Kilimanjaro); fissure type → lava plateau (Deccan Plateau)
• Barren Island in the Andamans is India's only active volcano

Endogenetic Movements – Notes

1. Why the inside of the Earth moves

• The book opens with the 2015 Nepal earthquake: newspapers reported magnitude 7.9, epicentre at Lamjung (about 80 km north-west of Kathmandu), focus only 11 km deep, and many aftershocks felt as far as India, Bangladesh and Pakistan.
• Such events happen because the Earth's interior is not stable. Radioactive substances in the mantle give out great heat energy, which flows from place to place and sets the upper mantle moving.
• The notebook activity shows this: a gentle push shifts the stack slightly, a hard push topples objects, and pulling one notebook out quickly makes the top ones fall — just as crust above a moving layer is disturbed.
• Internal movements are classified in three ways: by velocity — slow (continuous, like mountain and continent building) and sudden (events, like earthquakes and volcanoes); by direction — horizontal and up–down (vertical); by landforms — continent-building and mountain-building (folding and faulting).
• The crust is broken into large plates. The seven major ones are the Indo-Australian, African, Eurasian, North American, South American, Pacific and Antarctic plates. They move in different directions because of compression and tension below.

2. Mountain-building: folding and faulting

• Mountain-building movements are also called orogenic movements. They act mainly in a horizontal direction.
• Compression: when two forces push towards each other, like hands squeezing a paper strip from both ends, the strip bends up and down. In the same way, soft rock layers bend into folds.
• When the pressure is very great, folds become large and complicated, the land is pushed up and fold mountains form — the Himalayas, Aravalis, Rockies, Andes and Alps.
• Tension: when forces pull in opposite directions, like pulling a paper strip apart, rocks crack. These cracks, along which the rocks slip, are called faults. Hard rocks can also fault under compression, because they break instead of bending.
• Block mountain (horst): land between two parallel faults is pushed up, forming a raised block with a flat top and steep sides and, at first, no peaks. Example: the Black Forest in Europe. The Meghalaya Plateau was formed in a similar way.
• Rift valley (graben): land between two parallel faults sinks, forming a long, deep valley with steep walls on both sides. Examples: the Narmada valley in India, the Great Rift Valley of Africa, the Rhine valley in Europe.

3. Continent-building movements

• Continent-building movements are also called epeirogenic movements. They act vertically — pushing land upward, away from the core, or letting it sink towards it.
• They are slow but affect very large areas, lifting or lowering a huge part of the crust as a whole.
• When a large part of the crust is lifted above mean sea level, continents form. Wide plateaus can also be raised in this way.
• When land that was part of a continent sinks below sea level, it becomes part of the sea floor.

4. Earthquakes and seismic waves

• Earth + quake (trembling): an earthquake is a sudden shaking of the crust. Stress keeps building in the rocks; when it goes beyond what they can bear, the stored energy is suddenly released as waves.
• Causes: plates moving, plates colliding, one plate sliding under another, rocks fracturing under tension, and volcanic eruptions.
• Focus (hypocentre): the point inside the Earth where the energy is released. Epicentre: the point on the surface vertically above the focus; the waves reach it first and it is shaken first and usually hardest.
• P (primary) waves arrive first and are fastest. Rock particles move back and forth in the same direction as the wave (push–pull). They pass through solids, liquids and gases, but bend when entering a liquid. They make buildings sway forward and backward.
• S (secondary) waves arrive next and are slower. Particles move up and down, across the direction in which the wave travels. They pass only through solids and are stopped by liquids. They are more damaging than P waves. (The book says the particles move 'in the direction of energy transfer'; in fact S-wave particles move at right angles to it.)
• L (surface) waves form after P and S waves reach the epicentre and travel along the crust around the Earth. They are the slowest but the most destructive.
• A seismograph records the waves as a graph called a seismogram; from it the magnitude is worked out on the Richter scale. Modern instruments record even very tiny tremors.
• Effects: cracks in the ground, landslides, wells drying up or filling, land rising or sinking, tsunamis on coasts, avalanches in snowy areas, collapsed buildings, and broken roads and communication lines.

5. Volcanoes and plate boundaries

• A volcanic eruption throws hot solid, liquid and gas material from the mantle onto the surface: ash, steam, poisonous and burnable gases and molten rock. Molten rock inside is magma; once it reaches the surface it is called lava.
• Central (conical) type: magma rises through one pipe-like vent and piles up around its mouth, building a cone. Examples: Mt Fuji (Japan), Mt Kilimanjaro (Tanzania). Barren Island is becoming cone-shaped for the same reason.
• Fissure type: lava pours out quietly through many long cracks and spreads in sheets on both sides, building a lava plateau. The Deccan Plateau, with its layered basalt seen in the Sahyadris, formed this way.
• By how often they erupt: active — erupt regularly even now (Fuji, Stromboli in the Mediterranean); dormant — quiet for long but may wake up (Vesuvius in Italy, Katmai in Alaska, Barren Island earlier); extinct — not erupted for a very long time and not expected to (Kilimanjaro, as the book classes it).
• Barren Island (Andaman Islands) is India's only active volcano; it began erupting again in February 2017, mainly giving out dust, smoke, mud and some lava.
• On the world map, earthquake zones and volcanoes line up along plate boundaries — for example, the west coasts of North and South America (Rockies, Andes), the Himalayan–Alpine belt and the East African rift.
• Where one plate slides under another and crust is used up, the edge is a consuming (subduction) boundary. Where new material rises, as along the Mid-Atlantic Ridge, it is a constructive boundary.
• Volcanoes kill people and destroy homes, cause tsunamis from undersea eruptions, dust and gases that disturb the climate; but also fertile soil from ash, minerals brought near the surface, new land or islands, and crater lakes in dead volcanoes.

Internal movements at a glance

Key terms

Endogenetic Movements – Flashcards

Read the question, answer it in your head, then tap the card to check. 20 cards.

Endogenetic Movements – MCQ Quiz

15 questions. Tap an option — you will see at once if it is right and why.

How to study Endogenetic Movements

⏱ About 45 minutes

1. Learn the forces (15 min)
   Understand compression and tension and the landforms they make: folds, faults, block mountains, rift valleys and continents.
2. Earthquakes and volcanoes (15 min)
   Learn focus, epicentre, P/S/L waves, central and fissure eruptions and active/dormant/extinct volcanoes.
3. Check yourself (15 min)
   Answer the 'Try it yourself' questions, take the quiz and play 🎮 Play.