हिंदी
CISCEISC (Arts) Class 12
प्रश्न पत्र३७२
पाठ्यपुस्तक उत्तर१६६८१
महत्वपूर्ण प्रश्न एवं उत्तर५८८५
अवधारणा स्पष्टीकरण और वीडियो९५३
समय सारणी१५
पाठ्यक्रम

Overview of Geological Evolution and Structure

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CISCE: Class 12

Definitions: Basic Geological Terminology

  • Geology is the science of the earth's crust.
  • Historical geology is the historical study of the earth which studies the patterns of development of the crust in time and space.
  • Eras are the primary intervals of time, and periods are secondary intervals.
  • Group is the name given to rocks formed during an era.
  • System is the name given to rocks formed during a period.
  • Standard geological eras are the Pre-Cambrian (over 570 million years old), the Paleozoic (245-570 million years old), the Mesozoic (66-245 million years old) and the Cenozoic (66 million years old to the present).
  • Indian geological eras are the Archean (Early Pre-Cambrian), the Purana (Late PreCambrian), the Dravidian (400-570 million years old), and the Aryan (400 million years old to the present).
  • The Pre-Cambrian derives its name from the rock formations of Cambria, Latin name for Wales in the United Kingdom. The periods got their names form places where rock formations of that period were formed. The Pre-Cambrians do not contain fossils of plants and animals. The Paleozoic have the fossils of very early lives, the Mesozoic have middle lives and the Cenozoic recent lives.
CISCE: Class 12

Key Points: Geological History

  • Three main geological units of India: (i) Peninsular Plateau, (ii) Himalayan Mountains, (iii) Indo-Gangetic (Great) Plains between them.
  • Peninsular Plateau (Southern Plateau): Oldest landform; part of the Indian Plate (Indo-Australian Plate). It emerged in the Pre-Cambrian and never submerged again; behaves like a rigid block (often compared to a horst).
  • Features of the Peninsular Plateau: Mostly residual mountains; river valleys are shallow with low gradients; affected mainly by vertical movements and block faulting (not major folding).
  • Himalayas: Much younger than the plateau; they are Young Fold Mountains formed from sediments of the Tethys Sea deposited between Angaraland (north) and Gondwanaland (south).
  • Great Plains formation: A large depression formed due to Himalayan uplift and subsidence of the northern edge of the plateau; rivers from the Himalayas and plateau filled it with alluvium, creating a flat plain. The plains are still forming (e.g., continued growth of the Sundarban Delta).
CISCE: Class 12

Key Points: Difference between the Peninsular Plateaus and the Himalayan Mountain

Basis Peninsular Plateau Himalayan Mountains
1. Age & rock type Very old landmass; mainly hard rocks Young mountains; mainly soft rocks
2. Landform Tableland, often compared to a horst Fold mountain chain formed by tectonic uplift
3. Geological history Mostly remained land since Cambrian (coastal parts submerged briefly) Remained under sea for long time (Cambrian till mountain formation)
4. Structure & stability More stable; mostly residual mountains (e.g., Aravalis) Weak and flexible; shows folding and deformation
5. Rivers & valleys Rivers have shallow valleys and low gradients Rivers are torrential and show youthful features
6. Shape Triangular in shape Arc-shaped mountain system
7. Dominant rock type Mostly igneous rocks Mostly sedimentary rocks
CISCE: Class 12

Key Points: Geological Formations of India

  • Four major geological groups (GSI): India’s rock formations are classified into Archean, Purana, Dravidian, and Aryan in chronological order.
  • Time coverage: Archean + Purana = Pre-Cambrian (first and second half). Dravidian = Cambrian to middle Carboniferous. Aryan = Upper Carboniferous to Pleistocene.
  • Archean System: Oldest rocks (mainly gneiss and granite, no fossils) found widely in the Peninsular India (e.g., TN, AP/Telangana, Karnataka, Odisha, MP, Chhattisgarh, Jharkhand, Rajasthan) and also in parts of the Himalayas.
  • Dharwar System: Earliest sedimentary rocks now mostly in metamorphic form, no fossils; found in Karnataka, MP, Jharkhand, Meghalaya, Rajasthan and also central/northern Himalayas; important for minerals like gold, iron ore, manganese, copper, chromium, uranium, etc.
  • Cuddapah & Vindhyan Systems:
    a. Cuddapah rocks occur in Rajasthan, TN, AP, MP, Chhattisgarh and are rich in iron ore, manganese, slate, marble.
    b. Vindhyan rocks cover large parts of MP, Chhattisgarh, UP, Rajasthan; include limestone, sandstone, shale, slate (building materials) and generally lie above Cuddapah.
  • Gondwana Rocks: Formed due to basin-shaped depressions in the peninsula (Upper Carboniferous); buried plants/animals formed major coal deposits; found mainly in Damodar, Mahanadi, and Godavari valleys; show climatic changes over time.
  • Later formations (Mesozoic to Quaternary):
    Deccan Traps (Mesozoic end): huge lava flows in Maharashtra/Deccan; thin fossil layers between flows show eruption was not continuous; linked with major events like Gondwana breakup and Himalayan uplift.
    i. Tertiary: mostly in Himalayas, and coastal Gujarat, Kerala, TN; contain brown coal, rock salt, gypsum, limestone.
    ii. Quaternary: most recent deposits—Kashmir Ice Age deposits, Indo-Gangetic alluvium, Rajasthan desert, Rann of Kachchh, laterite, regur (black) soils.
CISCE: Class 12

Key Points: Major Geological Formations of India

No. Key revision point (from the table)
1 Geological time is grouped into Cenozoic, Mesozoic, Paleozoic, and Pre-Cambrian; Indian eras shown are mainly Aryan, Dravidian, Purana (Late Pre-Cambrian), and Archean (Early Pre-Cambrian).
2 Cenozoic → Quaternary (Recent/Pleistocene) is the youngest period (shown as less than ~2 million years).
3 In the Peninsula, Quaternary formations include new alluvium, deserts, and laterites.
4 In the Extra-Peninsula, Quaternary is linked with the formation of the Ganga plain and continued Himalayan development (youngest landforms).
5 Mesozoic (about 66–179 million years shown) includes Triassic–Jurassic–Cretaceous; main Peninsular feature is Deccan Traps, along with marine deposits.
6 Paleozoic (around 245–325 million years shown) includes Carboniferous–Permian; Peninsula has Lower Gondwana deposits (coal deposits), while Extra-Peninsula has deposits in Kashmir and Spiti.
7 Pre-Cambrian (about 570 million years shown): Purana gives Vindhyan system; Archean/Purana include very old rocks like Cuddapah, Dharwar, Aravalli, Archean systems and gneiss (oldest foundation rocks).
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