मराठी
Tamil Nadu Board of Secondary EducationHSC Science Class 12
पाठ्यपुस्तक उत्तरे१०९५२
संकल्पना स्पष्टीकरण२०२
अभ्यासक्रम

Geological Time Scale

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Estimated time: 9 minutes
CISCE: Class 12

Introduction

  • The geological time scale represents the entire history of Earth and correlates geological events (changes in rocks and climate) with biological events (origin, diversification, and extinction of organisms).
  • Earth is about 4.5–4.6 billion years old; its age and history are understood using rock strata, fossils, and radiometric dating techniques.
  • Earth’s crust is layered; deeper layers are older and contain older fossils, while upper layers are younger and contain more recent fossils.
CISCE: Class 12

Divisions of Geological Time

Era Period / Epoch Plant Life Animal Life

Cenozoic

(Age of mammals)

 Holocene (30,000 yrs) Herbaceous plants dominant Modern animals: rise of civilised man
Pleistocene (1 mya) — Primitive man; extinction of some great mammals
Pliocene (17 mya) Spread of herbaceous dicots Emergence of man; modern mammals
Miocene (30 mya) Forests decrease First man-like apes
Oligocene (40 mya) Woody angiosperms rise Extinction of archaic mammals
Eocene (60 mya) Angiospermic forests spread Placental mammals diversify
Palaeocene (80 mya) — Rise of first primates

Mesozoic

(Age of reptiles)

Cretaceous (110 mya)

-Upper

 Angiosperms dominate; gymnosperms decline Extinction of giant dinosaurs & toothed birds; rise of modern birds
Middle Cretaceous Rapid increase in angiosperms —
Lower Cretaceous Cycads, conifers, angiosperms develop —
Jurassic (170 mya) Origin of angiosperms; leptosporangiate ferns rise Rise of toothed birds; reptiles dominant

Palaeozoic

(Age of amphibians, fish, and invertebrates)

 Triassic (200 mya) Seed ferns extinct; cycads, conifers, Ginkgoales increase First dinosaurs & egg-laying mammals; primitive amphibians extinct
Permian (240 mya) Primitive gymnosperms & arborescent pteridophytes extinct Marine invertebrates extinct; land vertebrates, modern insects, ammonites rise
Carboniferous (280 mya) Lycopods dominant; origin of bryophytes Origin of reptiles & winged insects
Devonian (360 mya) Origin of land vascular plants; algae abundant Origin of amphibians; fishes abundant
Silurian (375 mya) Origin of land plants; algae dominant Origin of jawed fishes & wingless insects
Ordovician (420 mya) Marine & green algae dominant Origin of jawless fishes
Cambrian (550 mya) Origin of algae All invertebrate phyla established; trilobites dominant

Proterozoic (1500 mya)

(Era of early life)

 Pre-Cambrian Blue-green algae & bacteria Origin of simple marine invertebrates

Archaeozoic (3800 mya)

(Era of dawn life)

 — Simple unicellular organisms Origin of life; no recognisable fossils
Maharashtra State Board: Class 12
CISCE: Class 12

Key Points: Geological Time Scale

  • The geological timescale represents the entire history of the Earth and helps correlate geological and biological events in proper sequence.
  • The Earth’s crust is made of layered rocks, and radioactive dating techniques estimate the age of the Earth to be about 4.5 billion years.
  • Earth’s history is divided into five major eras: Archaeozoic, Proterozoic, Palaeozoic, Mesozoic, and Cenozoic.
  • Each era is further divided into periods and epochs, marked by changes in climate, plant life, and animal life.
  • The timescale shows the gradual evolution of life, from simple unicellular organisms to complex plants, animals, and humans.
  • Major events such as the origin of life, the rise and extinction of dinosaurs, and the evolution of mammals and humans are recorded in the geological time scale.
  • Scientists believe Earth is currently entering a sixth mass extinction (Anthropocene), mainly due to human activities causing rapid loss of biodiversity.
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