भूगोल Geography

Quick summary

Physical geography is the branch of geography that studies the natural features of the earth. It looks at landforms, climate, water, soils and plants, and how they are spread over the earth's surface. It also studies how these natural elements are connected with each other and with human life.

Key points

• Geography means "description of the earth". The word comes from the Greek words geo (earth) and graphein (to describe). The term is credited to the Greek scholar Eratosthenes.
• Geography has two main branches: physical geography (natural environment) and human geography (human activities).
• Physical geography (प्राकृतिक भूगोल) studies the four "spheres" of the earth: lithosphere (land), atmosphere (air), hydrosphere (water) and biosphere (living things).
• It is a bridge between geography and natural sciences such as geology, meteorology, oceanography, botany and zoology.
• It asks three basic questions: What is found, where is it found, and why is it found there.
• The approach is spatial (study of place and distribution) and systematic (study of one element across the whole earth).
• Physical geography helps in planning agriculture, settlements, dams, roads and disaster management.
• Modern physical geography uses tools like maps, aerial photos, remote sensing, GIS and GPS.

Important terms

• Lithosphere – the solid outer layer of the earth made of rocks and soil.
• Atmosphere – the envelope of gases around the earth.
• Hydrosphere – all water on earth: oceans, rivers, lakes, ice and groundwater.
• Biosphere – the zone where life exists, where land, air and water meet.
• Spatial distribution – how a feature is spread over the earth's surface.
• Environment – all natural and man-made surroundings that affect life.

Key facts

• Alexander von Humboldt and Carl Ritter (both German) are called the founders of modern geography.
• Humboldt's famous work is Kosmos.

5 important questions

Q1. Define physical geography.

Physical geography is the branch of geography that studies the natural features of the earth's surface, such as landforms, climate, water, soils and vegetation, their distribution and their inter-relations.

Q2. Name the four spheres studied in physical geography.

Lithosphere (land), atmosphere (air), hydrosphere (water) and biosphere (living world).

Q3. Explain the nature of physical geography.

• It is a natural science branch of geography.
• It studies the earth as the home of man.
• It is spatial: it explains where things are and why.
• It is integrating: it links the lithosphere, atmosphere, hydrosphere and biosphere.
• It studies both the present state and changes over time (dynamic nature).
• It uses scientific methods: observation, measurement, maps and field work.

Q4. Describe the scope of physical geography.

• Geomorphology: origin and evolution of landforms.
• Climatology: insolation, temperature, pressure, winds, rainfall.
• Oceanography: ocean relief, currents, tides, salinity.
• Biogeography and soil geography: plants, animals and soils.
• Hydrology: rivers, lakes and groundwater.
• Applied fields: natural hazards, environment, resource planning.

Q5. Explain the importance of physical geography for human life.

• Helps understand where farming, settlements and industries are possible.
• Weather and climate study helps agriculture (for example, monsoon forecasts in India).
• Knowledge of landforms helps in building roads, railways and dams.
• Helps in disaster management: floods, earthquakes, cyclones, droughts.
• Helps conserve natural resources and protect the environment.

Exam tip

For "nature and scope" questions, always write a definition first, then the branches in a table or list, and end with the importance. Mention Humboldt and Ritter for extra marks.

Quick summary

This topic explains how the earth and the solar system were formed. Over the last 250 years scientists gave many theories, which are grouped into monistic (one parent body) and dualistic (two stars) theories. Today the modern solar nebula theory, together with the Big Bang theory for the universe, is accepted by most scientists.

Key points

• The earth is about 4.6 billion years old; the universe is about 13.8 billion years old.
• Monistic theories say the solar system came from one parent body (a nebula). Example: Kant and Laplace.
• Dualistic theories say a second star came near the sun and pulled out matter. Example: Chamberlin–Moulton, Jeans–Jeffreys.
• Kant's Gaseous Hypothesis (1755): a slowly rotating cloud of cold gas and dust became hot by collisions and formed the sun and planets.
• Laplace's Nebular Hypothesis (1796): a hot, rotating nebula cooled and contracted. As it spun faster, rings of matter separated from its edge and condensed into planets. The central part became the sun.
• Planetesimal Hypothesis (Chamberlin and Moulton, 1905): a passing star raised tides on the sun; ejected matter cooled into small solid bodies called planetesimals, which joined to form planets.
• Tidal Hypothesis (Jeans 1919, Jeffreys 1929): a large star pulled a cigar-shaped filament out of the sun. The thick middle part gave big planets (Jupiter, Saturn) and the thin ends gave small planets.
• Big Bang Theory (Lemaître, 1927; supported by Hubble's discovery of the expanding universe in 1929): the universe began from a single, extremely hot and dense point and has been expanding ever since.
• Modern Solar Nebula Theory: the sun formed at the centre of a collapsing cloud of gas and dust; the remaining disc formed planetesimals and then planets by accretion.
• After formation the earth went through differentiation – heavy iron sank to form the core and light material rose to form the crust; gases from inside formed the early atmosphere, and rain formed the oceans.

Important terms

• Nebula (तेजोमेघ) – a huge cloud of gas and dust in space.
• Planetesimals – small solid bodies from which planets grew.
• Accretion – the process of small particles joining to form larger bodies.
• Angular momentum – the quantity of rotation of a body; Laplace's theory cannot explain why the sun has very little of it.
• Geoid – the true shape of the earth: a slightly flattened sphere, bulging at the equator.
• Differentiation – separation of earth materials into layers by density.

Key facts

• Equatorial diameter about 12,756 km; polar diameter about 12,714 km.
• Rotation: about 23 hours 56 minutes; revolution: about 365¼ days; axis tilted about 23½° from the perpendicular.

5 important questions

Q1. What is the difference between monistic and dualistic theories?

Monistic theories explain the origin of the solar system from one parent body (a nebula), e.g. Laplace. Dualistic theories need two bodies – the sun and a passing star – e.g. the tidal hypothesis of Jeans.

Q2. What are planetesimals?

Small, cold, solid bodies formed from matter thrown out of the sun. According to Chamberlin and Moulton, they joined together to form the planets.

Q3. Explain Laplace's Nebular Hypothesis and its criticism.

• Hot, gaseous, rotating nebula; cooling causes contraction.
• Faster rotation increases centrifugal force at the equator; rings separate one by one.
• Rings condense into planets; the central mass becomes the sun; satellites form the same way from planets.
• Criticism: rings of gas would scatter, not condense; it cannot explain the small angular momentum of the sun; some satellites move in the opposite direction.
• Merit: simple, explains the same direction of revolution of planets.

Q4. Describe the tidal hypothesis of Jeans and Jeffreys.

• A giant star came close to the sun; its gravity raised a huge tide on the sun.
• A long cigar-shaped filament was pulled out; it broke into pieces that formed planets.
• Explains the size order of planets: small near the sun, big in the middle, small at the end.
• Criticism: such hot matter would spread out in space; chance of a star coming so close is very low; the planets are too far from the sun.

Q5. Explain the Big Bang theory and the modern view of the origin of the earth.

• Universe began about 13.8 billion years ago from a singularity; expansion and cooling formed atoms, then stars and galaxies.
• Evidence: expanding universe (red shift), cosmic background radiation.
• Solar nebula collapsed about 4.6 billion years ago; sun at centre; disc → planetesimals → planets.
• Earth's evolution: heating, differentiation into core, mantle, crust; outgassing formed atmosphere; condensation formed oceans; life changed the atmosphere (oxygen).

Exam tip

Learn the table of scientists and years – one-mark questions are often "Who gave the planetesimal hypothesis?". For long answers, always add the criticism of a theory.

Quick summary

The geological time scale is the calendar of the earth's 4.6-billion-year history. Geologists divide this long time into eons, eras, periods and epochs on the basis of rock layers, fossils and major events such as mass extinctions. It helps us know when mountains, coal, oil and life forms appeared.

Key points

• The largest unit is the eon, then era, period and epoch.
• Precambrian time (about 4,600 to 540 million years ago) covers nearly 88% of earth history. It includes the Hadean, Archaean and Proterozoic eons. Life was simple (bacteria, algae).
• The Phanerozoic eon (540 million years ago to today) is the age of visible life. It has three eras: Palaeozoic (old life), Mesozoic (middle life) and Cenozoic (new life).
• Palaeozoic era (540–252 Ma): Cambrian explosion of life, first fish, first land plants, great coal forests in the Carboniferous period. It ended with the Permian mass extinction, the biggest in history.
• Mesozoic era (252–66 Ma) is the age of reptiles (dinosaurs). Pangaea began to break up. It ended with the extinction at 66 Ma, when dinosaurs died out.
• Cenozoic era (66 Ma–today) is the age of mammals. The Himalayas and Alps rose during this era. The Quaternary period (last 2.6 million years) saw ice ages and the rise of humans.
• The present epoch is the Holocene, which began about 11,700 years ago after the last ice age.
• Relative dating tells which rock is older (law of superposition, fossils). Absolute dating gives age in years using radioactive decay.
• Indian link: the Deccan Traps basalt of Maharashtra was formed by huge lava flows near the end of the Cretaceous (about 66 Ma). India's main coal comes from the Gondwana rocks.

Important terms

• Era (महाकल्प) – a major division of geological time, e.g. Mesozoic.
• Law of superposition – in undisturbed layers, lower layers are older than upper ones.
• Index fossil – a fossil of a species that lived for a short time over a wide area; it helps to date rocks.
• Radiometric dating – finding age from the decay of radioactive elements (uranium, potassium, carbon-14).
• Half-life – time taken for half of a radioactive element to decay (carbon-14: about 5,730 years).
• Mass extinction – sudden loss of a large share of species.

Key facts

• Epochs of the Cenozoic: Palaeocene, Eocene, Oligocene, Miocene, Pliocene, Pleistocene, Holocene.
• Older names still used in books: Tertiary (66–2.6 Ma) and Quaternary (2.6 Ma–today).

5 important questions

Q1. What is the geological time scale?

It is a chart that divides the history of the earth into eons, eras, periods and epochs on the basis of rock strata, fossils and major events.

Q2. Why is the Mesozoic era called the age of reptiles?

Because reptiles, especially dinosaurs, were the dominant land animals during this era (252–66 million years ago).

Q3. Describe the eras of the geological time scale with their main features.

• Precambrian: crust and oceans form; bacteria and algae; oxygen slowly builds up.
• Palaeozoic: Cambrian explosion, fish, amphibians, land plants, coal forests; Permian extinction.
• Mesozoic: dinosaurs, birds, flowering plants; Pangaea breaks; Deccan lava flows; K–Pg extinction.
• Cenozoic: mammals spread; Himalayas rise; Quaternary ice ages; humans appear.
• Draw a simple table showing era, period and events.

Q4. Explain the methods of dating rocks.

• Relative dating: superposition, cross-cutting relations, index fossils (William Smith).
• Absolute dating: radiometric methods – uranium-lead for very old rocks, potassium-argon for volcanic rocks, carbon-14 for remains up to about 50,000 years.
• Combined use builds the time scale.

Q5. State the importance of the geological time scale in geography.

• Explains the age and origin of landforms (old Aravalli vs young Himalaya).
• Helps locate minerals and fuels (coal in Gondwana rocks, oil in Tertiary rocks).
• Explains past climates and evolution of life.
• Useful for understanding soils, e.g. black soil from Cretaceous basalt.

Exam tip

Remember the order of eras with "Pre-Pa-Me-Ce" (Precambrian, Palaeozoic, Mesozoic, Cenozoic) and link one Indian example to each.

Quick summary

The interior of the earth cannot be seen directly. Scientists know about it mainly from the study of earthquake (seismic) waves, volcanic material, meteorites, and changes in temperature, pressure and density with depth. The earth is divided into three main layers: crust, mantle and core.

Key points

• The deepest mines and drill holes go only a few kilometres down, so the interior is studied by indirect sources.
• Direct sources: rocks from mines and drilling, volcanic lava and magma.
• Indirect sources: seismic waves, meteorites, gravity, magnetism, temperature and pressure increase with depth.
• Temperature rises with depth, roughly 1°C for every 32 metres in the upper part of the crust; the rate slows down at greater depth.
• The average density of the earth is about 5.5 g/cm³, while crustal rocks are only about 2.7–3.0 g/cm³. So the inner layers must be much denser.
• P-waves (primary) travel through solids, liquids and gases. S-waves (secondary) travel only through solids.
• S-waves do not pass through the outer core. This shows that the outer core is liquid.
• Between about 105° and 145° from the epicentre, no direct P or S waves are recorded. This is the shadow zone.
• The earth's magnetic field is believed to come from the movement of molten iron in the outer core.

Important terms

• Crust – the thin outer layer; continental crust (SIAL) and oceanic crust (SIMA).
• Mantle – the thick middle layer, from the Moho to about 2,900 km depth.
• Core – the central part, made mainly of nickel and iron (NIFE).
• Discontinuity – a boundary where the speed of seismic waves changes sharply.
• Asthenosphere – the weak, partly molten zone in the upper mantle on which plates move.
• Lithosphere – the crust plus the uppermost solid mantle.

Key facts

• Mohorovičić (Moho) discontinuity – between crust and mantle.
• Gutenberg discontinuity – between mantle and core (about 2,900 km).
• Conrad discontinuity – between SIAL and SIMA in the crust.
• Lehmann discontinuity – between outer and inner core.
• The terms SIAL, SIMA and NIFE were given by Eduard Suess.

5 important questions

Q1. What is the Moho discontinuity?

It is the boundary between the earth's crust and the mantle, where the speed of seismic waves increases suddenly. It was discovered by Andrija Mohorovičić in 1909.

Q2. Why is the outer core believed to be liquid?

Because S-waves, which travel only through solids, do not pass through the outer core.

Q3. Explain the sources of information about the earth's interior.

• Direct: mining and drilling, volcanic eruptions (lava and gases).
• Indirect: temperature, pressure and density increase with depth.
• Meteorites, which are believed to be made of material similar to the earth's interior.
• Gravity and magnetic surveys.
• Most important: behaviour of seismic waves (P, S and surface waves) and shadow zones.

Q4. Describe the internal structure of the earth.

• Crust: thinnest layer; SIAL above SIMA; separated by Conrad discontinuity.
• Moho discontinuity separates crust and mantle.
• Mantle: about 2,900 km thick; upper mantle has the asthenosphere.
• Gutenberg discontinuity separates mantle and core.
• Outer core: liquid; inner core: solid; both mainly iron and nickel.
• Temperature, pressure and density increase towards the centre.

Q5. How do seismic waves help us understand the earth's interior?

• P-waves are fastest and pass through all states of matter.
• S-waves pass only through solids.
• Waves bend (refract) and change speed when they enter layers of different density.
• Absence of S-waves beyond about 105° shows a liquid outer core.
• Shadow zone (about 105°–145°) shows the size of the core.
• Sudden speed changes mark the discontinuities.

Exam tip

Draw a neat labelled cross-section of the earth (crust, mantle, outer core, inner core with depths and the four discontinuities). A diagram can earn full marks in a long answer.

Quick summary

Rocks are natural aggregates of one or more minerals that make up the earth's crust. On the basis of their origin, rocks are divided into igneous, sedimentary and metamorphic rocks. One type can change into another through the rock cycle.

Key points

• A mineral is a natural inorganic substance with a definite chemical composition and internal structure (for example, quartz, feldspar, mica).
• A rock (खडक) is a mixture of minerals. The science of rocks is called petrology.
• Igneous rocks form when hot molten magma or lava cools and becomes solid. They are called primary rocks because all other rocks come from them.
• Intrusive (plutonic) igneous rocks cool slowly inside the earth and have large crystals, e.g. granite.
• Extrusive (volcanic) igneous rocks cool quickly on the surface and have small crystals, e.g. basalt.
• The Deccan Plateau of Maharashtra is made mostly of basalt formed by lava flows (the Deccan Traps). The black regur soil of Maharashtra forms from this basalt.
• Sedimentary rocks form from layers (strata) of sediments deposited by water, wind or ice and later pressed and cemented. They often contain fossils.
• Metamorphic rocks form when existing rocks are changed by great heat and pressure.
• Coal and limestone are sedimentary; marble and slate are metamorphic.

Important terms

• Magma – molten rock inside the earth; called lava when it comes out.
• Strata – layers in sedimentary rocks.
• Fossil – remains or marks of ancient plants and animals preserved in rock.
• Metamorphism – change in a rock's form and minerals due to heat and pressure.
• Rock cycle – the continuous process by which rocks change from one type to another.
• Acidic and basic rocks – igneous rocks rich in silica (acidic, e.g. granite) or poor in silica (basic, e.g. basalt).

Key facts

5 important questions

Q1. Why are igneous rocks called primary rocks?

Because they were the first rocks to form from cooling molten material, and sedimentary and metamorphic rocks are ultimately derived from them.

Q2. What are fossils? In which rocks are they found?

Fossils are preserved remains or impressions of ancient plants and animals. They are mostly found in sedimentary rocks.

Q3. Describe the characteristics and types of sedimentary rocks.

• Formed by deposition of sediments in layers (stratified).
• Usually soft and porous; may contain fossils.
• Mechanically formed: sandstone, shale, conglomerate.
• Organically formed: coal, chalk, some limestone.
• Chemically formed: rock salt, gypsum.
• They cover a large part of the earth's surface but form a small share of the crust by volume.

Q4. Explain metamorphic rocks with examples.

• Formed from existing igneous or sedimentary rocks.
• Agents: heat (thermal metamorphism), pressure (dynamic metamorphism), or both.
• Minerals recrystallise; rocks become harder and often banded (foliated).
• Examples: limestone → marble, sandstone → quartzite, shale → slate, granite → gneiss.
• Marble is used in buildings; slate for roofing.

Q5. Explain the rock cycle with a diagram.

• Magma cools to form igneous rock.
• Weathering and erosion break igneous rock into sediments; deposition and compaction form sedimentary rock.
• Heat and pressure change igneous or sedimentary rocks into metamorphic rocks.
• Metamorphic rocks may melt to form magma again.
• Any rock can be weathered into sediments.
• Diagram: circle with arrows linking magma, igneous, sediment, sedimentary and metamorphic rock.

Exam tip

Always give one example for each type and the "original → metamorphic" pairs. Mentioning the Deccan basalt of Maharashtra shows applied knowledge.

Quick summary

Isostasy is the condition of balance between the high and low parts of the earth's crust. The light crust "floats" on the denser, plastic mantle below, much like ice floats on water. Mountains stand high because they have deep roots of light rock, and when load is added or removed the crust slowly sinks or rises to regain balance.

Key points

• The word isostasy (समस्थायित्व) means "equal standing". It was coined by the American geologist Clarence Dutton in 1889.
• The idea came from the Survey of India in the 1850s under George Everest. A plumb line near the Himalayas was pulled less towards the mountains than expected. This meant the mountains had less mass than their size suggested.
• Sir George Airy (1855): all crustal blocks have the same density but different thickness. Higher mountains have deeper roots that sink into the denser layer – "roots of mountains".
• J. H. Pratt (1855–59): crustal blocks have different densities but end at the same depth, called the level of compensation. High mountains are made of lighter rock; ocean floors of heavier rock.
• Heiskanen later combined both ideas; Joly linked isostasy with heat from radioactivity.
• Modern seismic studies show the crust really is thicker under mountains (about 60–70 km under the Himalayas and Tibet) and thin under oceans (about 5–10 km), which supports Airy.
• Isostatic adjustment: erosion removes load from mountains so they rise slowly; deposition adds load to deltas so they sink.
• Post-glacial rebound: after the ice sheets melted, Scandinavia and the Hudson Bay region of Canada are still rising.
• Perfect isostasy is rare; areas like the Himalayas are still not fully balanced, which is linked to earthquakes.

Important terms

• Isostasy – balance of crustal blocks of different heights on the denser mantle.
• Level of compensation – the depth at which pressure of all overlying columns is equal.
• Root of mountain – thick, light crust extending deep below a mountain (Airy).
• Isostatic rebound – slow rise of crust after a load (ice, rock) is removed.
• Sial and sima – the lighter continental crust (silica + aluminium) and denser oceanic crust/upper mantle (silica + magnesium).

Key facts

5 important questions

Q1. What is isostasy?

Isostasy is the state of balance in which the light crustal blocks of different heights float on the denser, plastic layer below, so that equal mass lies above the level of compensation.

Q2. What is post-glacial rebound? Give an example.

It is the slow rising of land after a heavy ice sheet melts and its load is removed. Example: Scandinavia and the Hudson Bay area.

Q3. Compare the views of Airy and Pratt on isostasy.

• Background: the Himalayan plumb-line problem in the Survey of India.
• Airy: uniform density, varying thickness, deep roots under mountains (floating-wood/iceberg example).
• Pratt: varying density, uniform depth of compensation (bars of copper, iron, lead in mercury).
• Criticism of each; modern seismology supports Airy's roots more, but density does differ too.
• Conclude with Heiskanen's combined view.

Q4. Explain how isostatic adjustment takes place.

• Mantle below the crust behaves like a very thick fluid over long time.
• Removal of load (erosion, melting ice) → crust rises; addition of load (sediments, ice, large reservoirs) → crust sinks.
• Examples: rising Scandinavia; sinking of large deltas like the Ganga–Brahmaputra delta.
• Effects: change in sea level along coasts, raised beaches, earthquakes.

Q5. State the importance of the concept of isostasy in geography.

• Explains why continents stand high and ocean floors low.
• Explains the deep roots of fold mountains and their slow rise.
• Helps in understanding sea-level change, raised beaches and submerged coasts.
• Linked with plate tectonics, earthquakes and mountain building.

Exam tip

Draw two simple diagrams – Airy's blocks with roots and Pratt's blocks with a flat base – they earn easy marks in long answers.

Quick summary

This topic explains how continents and ocean basins were formed and how they move. Alfred Wegener's Continental Drift Theory (1912) said that all continents were once joined as one landmass called Pangaea. Later, the ideas of sea-floor spreading and plate tectonics gave a scientific explanation of this movement.

Key points

• Alfred Wegener, a German meteorologist, proposed the Continental Drift Theory in 1912.
• He said there was one supercontinent, Pangaea, surrounded by one ocean, Panthalassa.
• Pangaea broke into Laurasia (north) and Gondwanaland (south), separated by the Tethys Sea.
• Gondwanaland included India, Africa, South America, Australia and Antarctica.
• Evidence: jig-saw fit of coastlines (South America and Africa), similar rocks and mountains on both sides of the Atlantic, same fossils (e.g. Glossopteris plant, Mesosaurus reptile) on different continents, and traces of old glaciation (tillite) in India and the southern continents.
• Weakness: Wegener could not explain the force that moved the continents. His suggested forces (tidal force, pole-fleeing force) were too weak.
• Arthur Holmes (1930s) suggested convection currents in the mantle as a driving force.
• Harry Hess (early 1960s) proposed sea-floor spreading: new crust forms at mid-ocean ridges and moves away on both sides.
• Plate tectonics (developed in the 1960s, e.g. by McKenzie, Parker and Morgan): the lithosphere is broken into rigid plates that move over the asthenosphere.
• The Indian plate moved north, collided with the Eurasian plate and formed the Himalayas.

Important terms

• Plate – a large rigid slab of lithosphere (continental or oceanic).
• Divergent boundary – plates move apart; new crust forms (constructive), e.g. Mid-Atlantic Ridge.
• Convergent boundary – plates move towards each other; one may sink (destructive), forming trenches or fold mountains.
• Transform boundary – plates slide past each other; crust is neither made nor destroyed (conservative), e.g. San Andreas Fault.
• Subduction – sinking of a denser plate under another into the mantle.
• Palaeomagnetism – the record of the earth's past magnetic field in rocks; bands of normal and reversed magnetism on the sea floor support sea-floor spreading.

Key facts

• Major plates: Pacific, North American, South American, Eurasian, African, Indo-Australian, Antarctic.
• Some minor plates: Nazca, Cocos, Philippine, Arabian, Caribbean, Juan de Fuca.
• Rocks of the ocean floor become older as we move away from mid-ocean ridges.
• Ocean floor rocks are much younger than the oldest continental rocks.
• Most earthquakes and volcanoes occur along plate boundaries, e.g. the Pacific "Ring of Fire".

5 important questions

Q1. What was Pangaea?

Pangaea was the single huge supercontinent which, according to Wegener, existed before the continents drifted apart. It was surrounded by a vast ocean called Panthalassa.

Q2. Define sea-floor spreading.

It is the process by which new oceanic crust forms at mid-ocean ridges from rising magma and spreads away on both sides. It was proposed by Harry Hess.

Q3. Give the evidence in support of Continental Drift Theory.

• Jig-saw fit of the coasts of South America and Africa.
• Similar rocks and mountain belts across the Atlantic.
• Same fossils (Glossopteris, Mesosaurus) on separated continents.
• Old glacial deposits (tillite) in India, Africa, Australia, South America.
• Coal deposits in Antarctica, showing a past warm climate.
• Placer gold deposits in Ghana without a local source, matching rocks in Brazil.

Q4. Explain the theory of plate tectonics.

• Lithosphere is divided into major and minor rigid plates.
• Plates move over the soft asthenosphere, driven mainly by mantle convection.
• Three boundaries: divergent, convergent, transform.
• New crust is made at ridges and destroyed at subduction zones.
• Explains earthquakes, volcanoes, fold mountains and ocean trenches.
• Example: Indian plate collision formed the Himalayas.

Q5. Describe the types of plate boundaries with examples.

• Divergent: plates separate; mid-ocean ridges, rift valleys (Mid-Atlantic Ridge, East African Rift).
• Convergent ocean–continent: subduction, trench and volcanic mountains (Andes).
• Convergent ocean–ocean: island arcs and trenches (Japan, Mariana Trench).
• Convergent continent–continent: fold mountains (Himalayas).
• Transform: plates slide sideways; strong earthquakes (San Andreas Fault).

Exam tip

Write Wegener's theory as: proposer and year → main idea → evidence → criticism. Then connect it to plate tectonics. A world sketch of major plates adds value.

Quick summary

Forces from inside the earth (endogenic forces) slowly bend and break the rocks of the crust. Bending of rock layers produces folds, and breaking with movement produces faults. These two processes create many major landforms such as fold mountains, rift valleys and block mountains.

Key points

• Slow internal movements of the crust are called diastrophism (भूहालचाल). They are of two types: epeirogenic (vertical, continent-building – uplift or subsidence of large areas) and orogenic (horizontal, mountain-building).
• Horizontal forces are compressional (pushing together) or tensional (pulling apart).
• Folds form when compression bends layers of rock that are plastic enough to bend.
• The up-fold is an anticline and the down-fold is a syncline. The sides of a fold are called limbs.
• Types of folds by increasing compression: symmetrical, asymmetrical, monocline, isoclinal, overturned, recumbent, and overthrust fold / nappe (when the fold breaks and is pushed far over other rocks).
• Faults form when rocks are brittle and break, and the blocks move along the break (fault plane).
• Normal fault: caused by tension; the hanging wall moves down.
• Reverse fault: caused by compression; the hanging wall moves up. A low-angle reverse fault is a thrust fault.
• Strike-slip (tear) fault: blocks slide sideways past each other, e.g. San Andreas Fault, USA.
• Rift valley (graben): a block sinks between two parallel faults, e.g. Narmada and Tapi valleys in India, the East African Rift, the Rhine valley.
• Block mountain (horst): a block stands high between two faults, e.g. Vosges and Black Forest in Europe; the Satpura range is often described as a block mountain.

Important terms

• Anticline (अपनती) – upward arch-shaped fold.
• Syncline (अभिनती) – downward trough-shaped fold.
• Fault plane – the surface along which a fault block moves.
• Hanging wall / footwall – the block above / below an inclined fault plane.
• Throw – the vertical displacement along a fault.
• Dip and strike – the angle of slope of a rock layer and the direction of its horizontal line.

Key facts

5 important questions

Q1. Distinguish between an anticline and a syncline.

An anticline is an upfold in which the rock layers bend upwards like an arch; a syncline is a downfold in which they bend downwards like a trough.

Q2. What is a rift valley? Give an example.

A long, narrow valley formed when a block of land sinks between two parallel normal faults. Example: the Narmada valley.

Q3. Describe the types of folds with diagrams.

• Define folding and its cause (compression on plastic rocks).
• Parts: limbs, axis, crest, trough.
• Types: symmetrical, asymmetrical, monocline, isoclinal, overturned, recumbent, overthrust/nappe.
• Examples: Himalayas, Alps (nappes), Rockies.
• Draw one labelled diagram for each type.

Q4. Explain faulting and the landforms produced by it.

• Faulting: breaking of brittle rocks and movement of blocks.
• Types: normal, reverse, thrust, strike-slip.
• Landforms: rift valley, block mountain, fault scarp, tilted block.
• Indian examples: Narmada–Tapi rifts, Satpura; world: East African Rift, Vosges.

Q5. Explain diastrophism and its types.

• Meaning: slow movements of the crust due to internal forces.
• Epeirogenic: vertical, broad uplift or subsidence; raised beaches, submerged coasts.
• Orogenic: horizontal; compression gives folds and thrusts, tension gives faults.
• Results: mountains, plateaus, rift valleys; link with plate tectonics.

Exam tip

Diagrams decide marks here. Practise neat sketches of anticline–syncline, normal and reverse faults, horst and graben.

Quick summary

Mountains are high landmasses with steep slopes and narrow summits. The great fold mountains of the world, such as the Himalayas and the Alps, were formed by the folding of thick sediments laid down in long, narrow seas called geosynclines. Many theories explain mountain building; today plate tectonics gives the most complete explanation.

Key points

• Mountain building is called orogeny (पर्वतनिर्मिती).
• Types of mountains: fold mountains (Himalayas, Alps, Andes), block mountains (Vosges, Satpura), volcanic mountains (Fujiyama, Kilimanjaro) and residual mountains (Aravalli, Nilgiri – worn-down old mountains).
• A geosyncline (भूसन्नती) is a long, narrow, shallow sea which slowly sinks while thick layers of sediment collect in it.
• Kober's Geosynclinal Theory (1921): rigid blocks called forelands (kratogens) lie on both sides of the geosyncline. Their movement towards each other squeezes the sediments into folds and raises mountain ranges (orogen). A middle part may stay unfolded as a median mass (zwischengebirge), e.g. the Tibetan plateau.
• Kober's three stages: lithogenesis (filling with sediments), orogenesis (folding and uplift) and gliptogenesis (erosion of the raised mountains).
• Jeffreys' Thermal Contraction Theory: the earth is cooling and shrinking; the crust wrinkles like the skin of a drying apple.
• Holmes' Convection Current Theory (1928–29): heat from radioactivity sets up convection currents in the mantle; where currents converge, crust is pulled down and compressed into mountains.
• Plate tectonic view: fold mountains form at convergent boundaries. Ocean–continent collision gives the Andes; continent–continent collision gives the Himalayas (Indian plate pushing into the Eurasian plate, closing the Tethys Sea).
• Major periods of mountain building: Caledonian (about 430–400 million years ago; Scottish Highlands, Scandinavia), Hercynian / Variscan (about 340–250 Ma; Urals, Appalachians), Alpine (Tertiary, mainly in the last 65 million years; Himalayas, Alps, Rockies, Andes).
• The Aravalli range is among the oldest fold mountains in India, now a residual range.

Important terms

• Orogeny – the process of mountain building.
• Geosyncline – a long, sinking sea basin filled with thick sediments.
• Foreland (kratogen) – a stable, rigid block bordering a geosyncline.
• Median mass – an unfolded middle block between two ranges.
• Nappe – a large rock sheet folded and pushed far forward over other rocks.
• Young fold mountains – recently formed, high, sharp-peaked ranges like the Himalayas.

Key facts

• Tethys Sea: the geosyncline between Angaraland (north) and Gondwanaland (south) from which the Himalayas rose.

5 important questions

Q1. What is a geosyncline?

A geosyncline is a long, narrow and shallow sea whose floor slowly sinks while thick layers of sediments are deposited in it; these sediments are later folded into mountains.

Q2. Name the stages of mountain building in Kober's theory.

Lithogenesis (deposition of sediments), orogenesis (folding and uplift) and gliptogenesis (gradual erosion of the mountains).

Q3. Explain Kober's geosynclinal theory of mountain building.

• Meaning of geosyncline and forelands.
• Stage 1: erosion of forelands and deposition in the geosyncline; subsidence.
• Stage 2: forelands move together; sediments squeezed into marginal ranges (randketten).
• Median mass between ranges (Tibet, Hungarian basin).
• Stage 3: erosion and lowering of mountains.
• Criticism: forces that move rigid forelands are not explained; cannot explain mountains of all shapes.

Q4. Explain the formation of the Himalayas.

• Tethys geosyncline between Angaraland and Gondwanaland.
• Indian plate moved north after the break-up of Gondwana and collided with the Eurasian plate.
• Sediments of Tethys folded, faulted and thrust up in phases: Greater Himalaya, Lesser Himalaya, Shiwaliks.
• The Himalayas are still rising; frequent earthquakes prove it.

Q5. Classify mountains with examples.

• By origin: fold, block, volcanic, residual (with features and examples).
• By age: old (Aravalli, Appalachians) and young (Himalayas, Alps).
• Importance of mountains: rivers, forests, minerals, climate barrier, tourism, defence.

Exam tip

Mention the Tethys Sea and Kober's three stages in any Himalaya or geosyncline answer – examiners look for these keywords.

Quick summary

Earthquakes and volcanoes are sudden movements of the earth caused by forces inside it (endogenic forces). An earthquake is a shaking of the ground due to release of energy in the crust. A volcano is an opening through which molten rock, gases and ash come out. Both mostly occur along plate boundaries.

Key points

• An earthquake (भूकंप) is the sudden shaking of the earth's surface caused by release of energy, usually along a fault.
• The point inside the earth where the earthquake starts is the focus (hypocentre). The point on the surface directly above it is the epicentre. Damage is greatest near the epicentre.
• Earthquake waves: P-waves (fastest, longitudinal), S-waves (transverse, solids only) and surface waves (L-waves, slowest and most destructive).
• Instrument: seismograph. Magnitude is measured on the Richter scale; intensity (damage) on the Mercalli scale.
• Causes: plate movements, faulting, volcanic activity, and human causes like large reservoirs (reservoir-induced seismicity).
• In Maharashtra, the Latur (Killari) earthquake of 30 September 1993 caused heavy loss of life. The Koyna earthquake of 1967 is often linked to the Koyna dam reservoir.
• A volcano (ज्वालामुखी) is a vent in the crust through which magma, gases and fragments erupt.
• Volcanoes are classified by activity: active, dormant and extinct.
• Volcanic landforms: cones, craters, calderas, lava plateaus; intrusive forms like batholiths, laccoliths, sills and dykes.
• About two-thirds of active volcanoes are in the Pacific Ring of Fire. Barren Island in the Andaman Sea is India's only active volcano.

Important terms

• Fault – a crack in the crust along which rocks have moved.
• Seismology – the science of earthquakes.
• Tsunami – huge sea waves caused by an earthquake under the sea (e.g. the Indian Ocean tsunami of 26 December 2004).
• Crater – the funnel-shaped mouth of a volcano; a very large crater is a caldera.
• Lava – magma that reaches the surface.
• Pyroclasts – solid fragments (ash, lapilli, volcanic bombs) thrown out in an eruption.

Key facts

• The Richter scale was developed by Charles F. Richter in 1935; each whole number step means about ten times more ground motion.
• World belts of earthquakes: Circum-Pacific belt, Mid-Atlantic Ridge belt, Alpine–Himalayan belt.

5 important questions

Q1. Distinguish between focus and epicentre.

Focus is the point inside the earth where the earthquake originates. Epicentre is the point on the earth's surface vertically above the focus.

Q2. What is a dormant volcano?

A volcano that has not erupted for a long time but may erupt again in future, e.g. Mount Vesuvius was thought dormant before its eruption in 79 CE.

Q3. Explain the causes and effects of earthquakes.

• Causes: plate tectonic movements, faulting, volcanic eruptions, reservoir loading, mining.
• Effects: collapse of buildings and bridges, loss of life.
• Landslides in hilly areas; changes in river courses.
• Tsunamis in coastal regions.
• Fires and breakdown of water, power and transport.

Q4. Describe the world distribution of earthquakes and volcanoes.

• Circum-Pacific belt (Ring of Fire): Japan, Philippines, Andes, Rockies; most active.
• Mid-Atlantic Ridge: Iceland and Azores.
• Alpine–Himalayan belt: Mediterranean, Iran, Himalayas.
• East African Rift Valley.
• Link with plate boundaries: divergent, convergent and transform.

Q5. Describe the landforms formed by volcanic activity.

• Extrusive: volcanic cone, crater, caldera, lava plateau (Deccan), volcanic island.
• Intrusive: batholith (large dome-shaped mass), laccolith (dome with flat base), lopolith (saucer-shaped), sill (horizontal sheet), dyke (vertical wall).
• Dykes are common in the Deccan Traps of Maharashtra.
• Hot springs and geysers in volcanic regions.

Exam tip

Learn the focus–epicentre diagram and the intrusive landforms diagram. Indian examples (Latur 1993, Koyna 1967, Barren Island, Deccan Traps) give your answer a strong local touch.