Full Chapter Summary & Detailed Notes - The Origin and Evolution of the Earth Class 11 NCERT
Overview & Key Concepts
- Chapter Goal: Understand theories on origin of universe, earth, evolution from hot barren to life-supporting planet. Exam Focus: Big Bang, Nebular Hypothesis, planet formation stages, earth's layered structure, atmosphere/hydrosphere evolution, life origin. 2025 Updates: Emphasis on modern evidences like Hubble's expansion, cosmic microwave background. Fun Fact: Universe age ~13.7 billion years; earth ~4.6 billion. Core Idea: From singularity to complex life via physical/chemical processes. Real-World: Explains earthquakes/volcanoes from plate tectonics (linked to next chapters). Ties: To interior of earth, Wegener's theory in Unit II.
- Wider Scope: Cosmic evolution, density differentiation, degassing, photosynthesis role in oxygen. Expanded: Theories debate gaseous/dust hypotheses; balloon analogy for expansion; light year measure; moon formation via giant impact.
- Expanded Content: Chapter starts with starry nights curiosity; hypotheses from philosophers; modern Big Bang with stages; star/galaxy/nebula formation; planets via accretion; earth from volatile to layered; atmosphere three stages; oceans 4 billion years old; life chemical reaction ~3.8 billion ago. Includes Hoyle's steady state alternative; solar winds stripping primordial atmosphere.
Early Theories
Origin of earth: Hypotheses by philosophers/scientists. Kant (1755) gaseous hypothesis; Laplace revised 1796 as Nebular Hypothesis—planets from rotating cloud around young sun. Schmidt (1950 Russia), Weizascar (Germany) revised: Sun surrounded by solar nebula (hydrogen/helium/dust); friction/collision formed disk; planets via accretion. Later focus shifted to universe origin. Expanded: Gaseous vs dust hypotheses debate; details on material association with sun; problems led to universe theories over just planets.
- Examples: Slowly rotating youthful sun; dust particles collision.
- Point: Early arguments popular but revised with new evidence.
- Expanded: Kant mathematician/philosopher; Laplace mathematician; differing details but core nebula idea.
Extended: Hypotheses considered material cloud; accretion process key; scientists later tackled broader universe questions.
Modern Theories
Origin of universe: Big Bang Theory (expanding universe hypothesis). Hubble (1920) evidence universe expanding; galaxies move apart. Balloon experiment: Points (galaxies) move away as balloon inflates; space increases but galaxies not expanding (partial analogy). Stages: (i) Tiny ball singularity—infinite density/temperature; (ii) Violent explosion 13.7 billion years ago; rapid expansion first seconds, slowed; energy to matter; first atoms 3 minutes; (iii) 300,000 years later, temperature 4,500K; atomic matter; transparent universe. Expansion means space between galaxies increases. Hoyle's steady state alternative: Universe same any time; but evidence favors expanding. Expanded: Edwin Hubble's role; cosmic microwave background support; not galaxies expanding but space; singularity concept.
- Examples: Balloon points expanding; galaxies distances increasing.
- Point: Big Bang widely accepted with evidences.
- Expanded: Greater evidence now; scientific community consensus on expansion.
Extended: Figure 2.1 Big Bang; Kelvin scale; rapid expansion fractions second.
Star Formation
Early universe uneven matter/energy distribution; density differences led gravitational forces clumping. Bases for galaxies: Large stars number; spread thousands light-years; diameters 80,000-150,000 light years. Galaxy forms from hydrogen gas nebula; develops localized clumps; grow denser into stars ~5-6 billion years ago. Light year: Distance light travels one year (9.461×10^12 km); sun-earth 8.311 light minutes. Expanded: Gravitational collapse; nebula rotation; star birth in clusters; modern observations via telescopes like Hubble.
- Examples: Milky Way diameter; hydrogen clumps.
- Point: Stars from gaseous bodies in nebula.
- Expanded: Initial density variations key; vast distances measured light years.
Extended: Not even distribution; forces drew matter together.
Formation of Planets
Stages: (i) Stars lumps gas in nebula; gravitational force forms core; rotating disc gas/dust around. (ii) Gas cloud condenses; matter around core into small-rounded planetesimals via cohesion. (iii) Planetesimals collide/accrete into planets via gravity. Expanded: Disk-shaped cloud; larger bodies from collisions; material sticks; fewer large bodies final stage; solar system example.
- Examples: Core with disc; planetesimals small bodies.
- Point: Accretion/cohesion key processes.
- Expanded: Gravitational attraction; small to large via collisions.
Extended: Planets form around stars; our solar system ~4.6 billion years.
Evolution of the Earth
Initially barren/rocky/hot with thin hydrogen/helium atmosphere. Events led to water/conducive life. Layered structure: Atmosphere least dense; interior zones different materials. Lithosphere: Volatile primordial; density increase heated inside; materials separated—heavy (iron) sink, light surface; cooled solidified crust. Moon formation giant impact heated further; differentiation layers: Crust/mantle/outer/inner core; density increases inward. Atmosphere/Hydrosphere: Three stages—(1) Primordial loss via solar winds (terrestrial planets); (2) Interior gases/vapour released degassing/volcanic; early: Water vapour/nitrogen/CO2/methane/ammonia/little oxygen; (3) Modified photosynthesis. Cooling condensed vapour; CO2 dissolved rain; more rains formed oceans ~4 billion years old. Expanded: Solar winds impact; degassing process; volcanic contributions; oceans within 500 million years formation; life evolved ~3.8 billion; photosynthesis ~2.5-3 billion; oxygen flooded ~2 billion.
- Examples: Iron core; water vapour condensation.
- Point: From hot to life-friendly via processes.
- Expanded: Layer properties next chapter; nitrogen/oxygen present composition.
Extended: Photosynthesis saturated oceans oxygen; life confined oceans long.
Origin of Life
Last phase: Earth/atmosphere initially not conducive. Chemical reaction generated complex organic molecules; assembled duplicating— inanimate to living. Fossils record: Microscopic blue algae-like ~3 billion years old; life evolved ~3.8 billion ago. Expanded: Abiogenesis theory; organic molecules from inorganic; self-replicating; geological formations evidence.
- Examples: Fossils in rocks; blue algae structures.
- Point: Chemical origin; fossils evidence.
- Expanded: Not spontaneous; assembled converting matter.
Extended: Periods in rocks; assumed 3.8 billion start.
Summary
- Universe from Big Bang 13.7 billion; earth 4.6 billion via accretion; evolved layers/atmosphere/oceans/life ~3.8 billion.
Why This Guide Stands Out
Complete: All subtopics, examples, Q&A, quiz. Geography-focused. Free 2025.
Key Themes & Tips
- Aspects: Theories, stages, evolution processes.
- Thinkers: Kant, Laplace, Hubble, Hoyle.
- Tip: Stages memorize; theories compare; evidences discuss; life chemical.
Exam Case Studies
Big Bang evidences, planet formation steps, atmosphere stages.
Project & Group Ideas
- Stardust project info: Agency NASA; collect comet dust; from comets/interstellar.
- Debate Big Bang vs Steady State.
60+ Questions & Answers - NCERT Based (Class 11)
Part A (1 mark short), B (4 marks medium), C (8 marks long). Based on NCERT, exercises. Answer lengths: 1 mark ~2 lines, 4 marks ~5 lines, 8 marks ~10 lines. Expanded with more chapter-specific Qs.
Part A: 1 Mark Questions
1. Who proposed the Nebular Hypothesis?
1 Mark Answer: Immanuel Kant in 1755, revised by Laplace in 1796. Planets from rotating cloud around sun.
2. What is the Big Bang Theory?
1 Mark Answer: Universe origin from singularity explosion 13.7 billion years ago. Also called expanding universe hypothesis.
3. Who provided evidence for expanding universe?
1 Mark Answer: Edwin Hubble in 1920. Galaxies moving apart.
4. What is a light year?
1 Mark Answer: Distance light travels in one year, 9.461×10^12 km. Measure of distance, not time.
5. Define nebula.
1 Mark Answer: Large cloud of hydrogen gas. Forms galaxies and stars via clumps.
6. What are planetesimals?
1 Mark Answer: Small-rounded objects from condensed gas cloud. Form via cohesion.
7. What is accretion?
1 Mark Answer: Process where planetesimals collide and form planets. Via gravitational attraction.
8. What was earth's primordial state?
1 Mark Answer: Barren, rocky, hot with thin hydrogen/helium atmosphere. Volatile during formation.
9. Define differentiation.
1 Mark Answer: Separation of materials by density. Heavier sink, lighter to surface.
10. What is degassing?
1 Mark Answer: Outpouring of gases from earth's interior. Via volcanic eruptions.
11. Name early atmosphere gases.
1 Mark Answer: Water vapour, nitrogen, CO2, methane, ammonia. Little free oxygen.
12. How old are earth's oceans?
1 Mark Answer: About 4,000 million years. Formed within 500 million of earth.
13. When did photosynthesis evolve?
1 Mark Answer: Around 2,500-3,000 million years ago. Contributed oxygen.
14. What is origin of life?
1 Mark Answer: Chemical reaction generating organic molecules. Assembled self-duplicating.
15. Name alternative to Big Bang.
1 Mark Answer: Steady state theory by Hoyle. Universe same any time.
16. What stripped primordial atmosphere?
1 Mark Answer: Solar winds. Affected terrestrial planets.
17. When did stars form?
1 Mark Answer: Some 5-6 billion years ago. From denser gaseous bodies.
18. What is singularity?
1 Mark Answer: Tiny ball with infinite density/temperature. Big Bang beginning.
19. Define galaxy.
1 Mark Answer: Large number of stars. Diameters 80,000-150,000 light years.
20. When did life begin to evolve?
1 Mark Answer: Sometime around 3,800 million years ago. From chemical reactions.
21. What is solar nebula?
1 Mark Answer: Cloud around sun with hydrogen/helium/dust. Revised hypothesis.
22. Name thinkers revised Nebular.
1 Mark Answer: Otto Schmidt (Russia), Carl Weizascar (Germany). In 1950.
23. What formed first atoms?
1 Mark Answer: Within first three minutes of Big Bang. Energy to matter.
24. What is moon formation theory?
1 Mark Answer: Giant impact heated earth further. During differentiation.
25. When did oxygen flood atmosphere?
1 Mark Answer: 2,000 million years ago. After oceans saturated.
Part B: 4 Marks Questions
1. Explain Nebular Hypothesis.
4 Marks Answer: Proposed by Kant, revised Laplace: Planets formed from cloud material around youthful rotating sun. Schmidt/Weizascar revised: Solar nebula hydrogen/helium/dust; friction/collision disk-shaped cloud. Planets via accretion process. Early popular but later focused universe.
2. Describe Big Bang Theory.
4 Marks Answer: Expanding universe hypothesis: Singularity tiny ball exploded 13.7 billion years ago. Rapid expansion; energy to matter; first atoms three minutes. 300,000 years later 4,500K temperature; atomic matter transparent universe. Hubble evidence galaxies apart.
3. Explain balloon analogy for expansion.
4 Marks Answer: Mark points on balloon as galaxies; inflate—points move apart like galaxies. Distances increase; points expand but actually galaxies not—space between increases. Partial correct; observations no galaxy expansion. Illustrates universe expanding.
4. What is star formation?
4 Marks Answer: Early universe uneven density; gravitational forces clumped matter into galaxies. Galaxy large stars; from hydrogen nebula localized clumps. Grow denser gaseous bodies into stars ~5-6 billion ago. Diameters 80,000-150,000 light years.
5. Describe planet formation stages.
4 Marks Answer: (i) Stars gas lumps nebula; gravity core; rotating disc gas/dust. (ii) Cloud condenses; matter small-rounded planetesimals cohesion. (iii) Collide; gravity accretes into fewer large planets. Final stage large bodies.
6. Explain lithosphere evolution.
4 Marks Answer: Primordial volatile; density increase heated inside; materials separated—heavy iron sink, light surface. Cooled solidified crust. Giant impact moon heated further; differentiation layers: Crust/mantle/outer/inner core. Density increases inward.
7. Describe atmosphere evolution stages.
4 Marks Answer: (1) Primordial hydrogen/helium lost solar winds. (2) Interior released gases/vapour degassing/volcanic; early water vapour/nitrogen/CO2/methane/ammonia/little oxygen. (3) Modified photosynthesis; oxygen contribution. Present nitrogen/oxygen.
8. Explain hydrosphere evolution.
4 Marks Answer: Cooling condensed water vapour; CO2 dissolved rain; more condensation/rains. Rain collected depressions into oceans ~4,000 million years old. Formed within 500 million earth formation. Photosynthesis added oxygen later.
9. What is origin of life?
4 Marks Answer: Chemical reaction generated complex organic molecules; assembled self-duplicating—inanimate to living. Fossils record periods; blue algae-like ~3,000 million old. Assumed evolved ~3,800 million ago. Initially not conducive.
10. Compare Big Bang and Steady State.
4 Marks Answer: Big Bang: Explosion singularity; expanding galaxies. Steady State (Hoyle): Universe same any time/point. Evidence expansion favors Big Bang. Steady less accepted now; greater evidences available.
11. Explain light year significance.
4 Marks Answer: Measure distance: Light speed 300,000 km/s; one year 9.461×10^12 km. Sun-earth 149,598,000 km = 8.311 light minutes. Used galaxy distances/vast spreads. Not time measure.
12. Describe solar winds role.
4 Marks Answer: Stripped primordial atmosphere hydrogen/helium. Happened earth/all terrestrial planets. Led loss early atmosphere; second stage interior contributions. Impact not only earth.
13. What is moon's role in earth evolution?
4 Marks Answer: Formed giant impact; further heated earth. Led more differentiation; material separated layers. During crust development; increased temperature inside.
14. Explain photosynthesis impact.
4 Marks Answer: Evolved ~2,500-3,000 million ago; contributed oxygen oceans. Life confined oceans long; saturated then flooded atmosphere ~2,000 million ago. Modified composition living world.
15. Describe galaxy formation.
4 Marks Answer: From density differences early universe; gravitational forces clumped. Large stars number; spread vast distances light years. Diameters range 80,000-150,000. Starts accumulation hydrogen nebula.
16. What is singularity?
4 Marks Answer: Tiny ball all matter; unimaginable small volume/infinite temperature/density. Big Bang start; exploded violently huge expansion. Led energy conversion matter.
17. Explain first atoms formation.
4 Marks Answer: Within three minutes Big Bang; energy converted matter. Rapid expansion slowed after. Temperature dropped later 4,500K atomic matter. Universe became transparent.
18. Describe early universe density.
4 Marks Answer: Not even; initial differences gravitational forces. Matter drawn together galaxy bases. Led star formation nebulae. Uneven distribution key clumping.
19. What is Stardust project?
4 Marks Answer: NASA launched; collect stardust comets/interstellar. Scientists interested pristine material origin study. Collected from comets like Wild 2.
20. Explain volcanic role in atmosphere.
4 Marks Answer: Continuous eruptions contributed water vapour/gases. Degassing process released from interior. Early atmosphere formed thus; cooling led condensation oceans.
21. Describe fossils evidence life.
4 Marks Answer: Record in rocks different periods. Microscopic blue algae-like ~3,000 million old formations. Assumed life ~3,800 million ago. Chemical reaction origin.
22. What is cohesion in planets?
4 Marks Answer: Small-rounded objects develop planetesimals. Matter around core condenses. Next stage collision gravity. Forms larger bodies.
23. Explain earth cooling process.
4 Marks Answer: Water vapour condensed; CO2 dissolved rain; temperature decreased more rains. Rain collected depressions oceans. Led hydrosphere formation.
24. Describe layered structure development.
4 Marks Answer: Density increase temperature; materials separated. Heavier sink center; lighter surface crust. Solidified smaller size; layers crust to core.
25. What is Hoyle's concept?
4 Marks Answer: Steady state: Universe roughly same any point time. Alternative Big Bang. Greater evidence expanding favors latter now.
Part C: 8 Marks Questions
1. Write an explanatory note on the ‘Big Bang Theory’.
8 Marks Answer: Most popular universe origin: Expanding hypothesis. Hubble 1920 evidence galaxies apart. Balloon analogy: Points move away inflate; space increases not galaxies (partial). Stages: (i) Singularity tiny ball infinite density/temperature. (ii) Violent explosion 13.7 billion years; huge expansion continues; rapid first seconds slowed; energy matter; first atoms three minutes. (iii) 300,000 years temperature 4,500K atomic matter; universe transparent. Expansion space between galaxies. Alternative Hoyle steady state same any time; but evidence favors expanding. Scientific community consensus now.
2. List the stages in the evolution of the earth and explain each stage in brief.
8 Marks Answer: (i) Lithosphere: Primordial volatile hot; density increase temperature; materials separated heavy iron sink light surface; cooled solidified crust; giant impact moon heated differentiation layers crust/mantle/outer/inner core density inward. (ii) Atmosphere: Three stages—primordial hydrogen/helium lost solar winds; interior degassing volcanic released water vapour/nitrogen/CO2/methane/ammonia/little oxygen; modified photosynthesis oxygen. (iii) Hydrosphere: Cooling condensed vapour; CO2 dissolved rain more condensation; rains collected depressions oceans ~4,000 million old within 500 million formation. (iv) Life: Chemical reaction organic molecules assembled self-duplicating; fossils blue algae ~3,000 million; evolved ~3,800 million ago oceans confined long.
3. Discuss early theories on earth origin.
8 Marks Answer: Hypotheses philosophers/scientists. Kant 1755 gaseous: Planets cloud material youthful rotating sun. Laplace 1796 revised Nebular Hypothesis. Schmidt 1950 Russia Weizascar Germany revised differing details: Sun solar nebula hydrogen/helium/dust; friction collision disk-shaped; planets accretion. Popular earlier; later scientists universe over earth/planets. Gaseous vs dust debate; material association sun; problems led broader focus. Kant philosopher/mathematician; Laplace mathematician; core rotating cloud idea.
4. Explain star and galaxy formation.
8 Marks Answer: Early universe uneven matter/energy; density differences gravitational forces clumped bases galaxies. Galaxy large stars number spread vast distances thousands light years; diameters 80,000-150,000. Forms accumulation hydrogen nebula; develops localized clumps gas. Grow denser gaseous bodies stars ~5-6 billion ago. Light year distance light one year 9.461×10^12 km; sun-earth 8.311 minutes. Initial variations key; not even distribution; forces drew together. Modern telescopes like Hubble observe.
5. Describe formation of planets in detail.
8 Marks Answer: Stages: (i) Stars localized gas lumps nebula; gravitational force core; huge rotating disc gas/dust around. (ii) Gas cloud condensed; matter core develops small-rounded objects cohesion into planetesimals large number smaller bodies. (iii) Larger form collision; gravitational attraction material sticks; planetesimals accrete fewer large bodies planets. Disk-shaped cloud; solar system example ~4.6 billion years. Accretion/cohesion key; small to large collisions. Around stars; our system context.
6. Analyze evolution of earth's atmosphere and hydrosphere.
8 Marks Answer: Atmosphere three stages: (1) Primordial hydrogen/helium stripped solar winds terrestrials. (2) Cooling released gases/vapour interior solid earth degassing volcanic; early contained water vapour/nitrogen/carbon dioxide/methane/ammonia/little free oxygen. (3) Living world modified photosynthesis. Hydrosphere: As cooled vapour condensed; CO2 dissolved rainwater temperature decreased more condensation rains. Falling surface collected depressions oceans within 500 million formation; as old 4,000 million years. Around 3,800 million life evolved; photosynthesis 2,500-3,000 million oxygen oceans saturated 2,000 million flooded atmosphere.
7. Discuss origin and evolution of life on earth.
8 Marks Answer: Last phase evolution relates origin life. Initially earth/atmosphere not conducive development. Modern scientists chemical reaction first generated complex organic molecules assembled could duplicate converting inanimate living substance. Record existed planet different periods found rocks fossils. Microscopic structures related present blue algae geological formations older 3,000 million years. Assumed began evolve sometime 3,800 million ago. Confined oceans long time; photosynthesis evolved oxygen contribution. Oceans saturated; 2,000 million ago oxygen flooded atmosphere. Abiogenesis theory; self-replicating; geological evidence.
8. Compare early and modern theories of universe origin.
8 Marks Answer: Early: Nebular Hypothesis Kant/Laplace planets cloud rotating sun; revised Schmidt/Weizascar solar nebula dust friction disk accretion. Focused earth/planets; gaseous/dust. Modern: Big Bang universe singularity explosion 13.7 billion; Hubble expanding galaxies evidence. Stages tiny ball violent expansion energy matter first atoms; 300,000 years transparent. Balloon analogy space increases. Hoyle steady state alternative same any time but evidence favors expanding. Shift planets universe; modern evidences like cosmic background. Scientific community prefers Big Bang greater proofs.
9. Explain the role of density in earth's evolution.
8 Marks Answer: Primordial volatile state; gradual density increase temperature inside. Material separated depending densities; heavier like iron sink centre lighter move surface. Passage time cooled further solidified condensed smaller size. Led outer surface crust development. During moon formation giant impact further heated. Through differentiation forming material separated layers. Starting surface central parts layers crust mantle outer core inner core. From crust core material density increases. Early universe density differences gravitational forces clumped galaxy bases. Key layered structure not uniform.
10. Analyze the project “Stardust”.
8 Marks Answer: NASA agency launched 1999 collect stardust. Scientists interested collecting because pristine material comets/interstellar dust holds clues solar system origin evolution. Collected from comet Wild 2 interstellar streams; returned 2006. Website www.sci.edu/public.html www.nasm.edu. Along lines: (i) Agency NASA. (ii) Why: Analyze composition understand early nebula planet formation. (iii) Where: From comets like Wild 2 interstellar space. Ties chapter origin theories; dust hypothesis link. Provides real evidences cosmic materials.
11. Discuss the three stages of atmosphere evolution.
8 Marks Answer: Present composition chiefly nitrogen oxygen. (1) Loss primordial atmosphere hydrogen helium stripped solar winds; happened earth all terrestrial planets. (2) Hot interior contributed evolution; gases water vapour released cooling solid earth degassing continuous volcanic eruptions contributed. Early largely water vapour nitrogen carbon dioxide methane ammonia very little free oxygen. (3) Composition modified living world process photosynthesis. Around 3,800 million years life began evolve; 2,500-3,000 million photosynthesis evolved oxygen oceans saturated 2,000 million flooded atmosphere.
12. Explain how planets formed through accretion.
8 Marks Answer: Following considered stages development planets: (i) Stars localised lumps gas within nebula; gravitational force within lumps leads formation core gas cloud huge rotating disc gas dust develops around gas core. (ii) Next stage gas cloud starts getting condensed matter around core develops into small-rounded objects process cohesion develop what called planetesimals. Larger bodies start forming collision gravitational attraction causes material stick together. Planetesimals large number smaller bodies. (iii) Final stage these large number small planetesimals accrete form fewer large bodies form planets. Disk-shaped; solar system ~4.6 billion.
13. Describe the evolution of the lithosphere.
8 Marks Answer: Earth mostly volatile state during primordial stage. Due gradual increase density temperature inside has increased. As result material inside started getting separated depending their densities. This allowed heavier materials like iron sink towards centre earth lighter ones move towards surface. With passage time cooled further solidified condensed into smaller size. This later led development outer surface form crust. During formation moon due giant impact earth further heated up. It through process differentiation earth forming material got separated into different layers. Starting from surface central parts we have layers like crust mantle outer core inner core. From crust core density material increases.
14. Analyze the origin of life as a chemical reaction.
8 Marks Answer: Modern scientists refer origin life kind chemical reaction which first generated complex organic molecules assembled them. This assemblage such that they could duplicate themselves converting inanimate matter into living substance. Record life existed this planet different periods found rocks form fossils. Microscopic structures closely related present form blue algae have been found geological formations much older than some 3,000 million years. It can assumed that life began evolve sometime 3,800 million years ago. Initially earth even atmosphere not conducive development life. Confined oceans long time; photosynthesis evolved around 2,500-3,000 million years before present.
15. Discuss the balloon experiment and its limitations.
8 Marks Answer: Take balloon mark some points represent galaxies. Start inflating balloon points marked appear moving away each other balloon expands. Similarly distance between galaxies also found increasing thereby universe considered expanding. However find besides increase distances between points balloon points themselves expanding. This not accordance fact. Scientists believe though space between galaxies increasing observations do not support expansion galaxies. So balloon example only partially correct. Illustrates expanding universe mean but limitations galaxy expansion not happening only space.
16. Explain the formation of stars and galaxies.
8 Marks Answer: Distribution matter energy not even early universe. These initial density differences gave rise differences gravitational forces caused matter get drawn together. These formed bases development galaxies. Galaxy contains large number stars. Galaxies spread over vast distances measured thousands light-years. Diameters individual galaxies range from 80,000-150,000 light years. Galaxy starts form accumulation hydrogen gas form very large cloud called nebula. Eventually growing nebula develops localised clumps gas. These clumps continue grow into even denser gaseous bodies giving rise formation stars. Formation stars believed taken place some 5-6 billion years ago.
17. Describe the development of the hydrosphere.
8 Marks Answer: As earth cooled water vapour released started getting condensed. Carbon dioxide atmosphere got dissolved rainwater temperature further decreased causing more condensation more rains. Rainwater falling onto surface got collected depressions give rise oceans. Earth’s oceans formed within 500 million years from formation earth. This tells us oceans old 4,000 million years. Sometime around 3,800 million years ago life began evolve. However around 2,500-3,000 million years before present process photosynthesis got evolved. Life confined oceans long time. Oceans began have contribution oxygen through process photosynthesis. Eventually oceans saturated oxygen 2,000 million years ago oxygen began flood atmosphere.
18. Analyze the role of volcanic eruptions in earth's evolution.
8 Marks Answer: Continuous volcanic eruptions contributed water vapour gases atmosphere. Process through which gases outpoured interior called degassing. During cooling earth gases water vapour released from interior solid earth. This started evolution present atmosphere. Early atmosphere largely contained water vapour nitrogen carbon dioxide methane ammonia very little free oxygen. As earth cooled water vapour released started getting condensed. Carbon dioxide atmosphere got dissolved rainwater temperature further decreased causing more condensation more rains. Rainwater falling onto surface got collected depressions give rise oceans. Volcanic activity key second stage atmosphere hydrosphere formation.
19. Discuss the evidences supporting Big Bang Theory.
8 Marks Answer: Edwin Hubble 1920 provided evidence universe expanding. As time passes galaxies move further further apart. Balloon experiment shows points moving away inflation analogous galaxies. Expansion continues present day. Event big bang took place 13.7 billion years before present. Greater evidence becoming available about expanding universe scientific community present favours argument expanding universe. Observations do not support expansion galaxies but space between. Cosmic microwave background radiation remnant heat supports. Alternative like Hoyle steady state less favored due these evidences.
20. Explain the timeline of earth's evolution from formation to life.
8 Marks Answer: Earth formed ~4,600 million years ago accretion planetesimals. Initially barren rocky hot thin atmosphere hydrogen helium. Density differentiation led layered structure crust core. Primordial atmosphere lost solar winds; second stage degassing volcanic gases water vapour. Cooling condensed vapour oceans ~4,000 million. Around 3,800 million life began evolve chemical reactions organic molecules. Photosynthesis ~2,500-3,000 million oxygen oceans; saturated flooded atmosphere ~2,000 million. Fossils blue algae ~3,000 million evidence. From volatile to life-supporting via physical chemical processes.
Tip: Theories compare; stages sequence; evidences cite; timelines memorize.
