Classification of Rocks

  1. Igneous Rocks
  • Formation
  • Through cooling and solidification of molten rock (magma)
  • Magma, after reaching the surface is called as Lava
  • Properties
  • Igneous rocks are crystalline in texture
  • Types
  • Based on chemical composition
  1. Acidic
  • High proportion of silica
  • less dense and lighter in colour
  1. Basic
  • Higher proportions of basic oxides
  • denser and darker in colour
  • Based on their origin
  1. Intrusive rocks or Plutonic rocks
  • Formed by cooling of molten rocks from the volcano
  • Solidifies rapidly under the atmosphere
  • Types of formation
  • Lava flows
  • Lava sheets
  • Lava plateaus Deccan plateau
  • Types
  • Basalt
  • Rhyolite
  1. Sedimentary Rocks
  • Formation
  • Formed through sedimentation process
  • Transformation of sediments into rocks by the process of compaction and cementation is called lithification
  • Properties
  • They are non-crystalline and layered in structure
  • there are both hard and soft forms of sedimentary rocks
  • fossils are formed in this rock
  • Types
  1. Clastic rocks
  • rocks formed by the accumulation of older rocks
  • composed of silicates minerals and rock fragments
  • Types
  • Sandstones
  • Shale
  1. Chemical rocks
  • rocks formed by chemical precipitation of mineral constituents in a solution
  • Types
  • Gypsum
  • Nitrates
  • Barites
  1. Organic or Biochemical rocks
  • formed by the accumulation of remains of organic matter
  • formed under high pressure
  • Types
  • Coal
  • Limestone
  • Chalk
  1. Metamorphic rocks
  • Formation
  • Igneous and Sedimentary rocks transform into metamorphic rocks under high temperature and pressure
  • Important metamorphic rocks
  • clay – slate
  • coal – graphite
  • Limestone – marble
  • granite – gneiss
  • sandstone – quartzite
  • shale – schist
  • Types
  1. Foliated rocks
  2. Non-foliated rocks



  • Orogenysis refers to the forces and events leading to a large structural deformation of the Earth’s lithosphere due to tectonic plates
  • Continental plate is crumpled and pushed upwards to form mountain ranges
  • Orogenesis involves

folding, faulting, volcanic activity, igneous intrusion and metamorphic to form mountains

  • Forms of mountains
  1. Mountains ridge
  • chain of mountains or hills that form a continuous elevated crest for some distance
  1. Mountain range/barrier/belt/chin/system
  • geographic area containing numerous geologically related mountains
  • g. Himalayas, Alps, Andes etc.


  1. Fold Mountains
  • continental crust floating above the mantle is forced upwards to form hills, plateaus or mountains
  • Fold mountains are formed when
  • two plates collide under stress
  • there is an imbalance in weight of overlying rocks
  • there are variations in mantle flow
  • there is an intrusion or magma
  • Fold mountains are formed under compressive forces
  • Crust is folded into
  1. Syncline
  • Trough or Down fold of the crust
  1. Anticline
  • Crest or Up fold of the crust
  • Many active volcanoes are found in fold mountains
  • Major fold mountains
  • Himalayas in Asia
  • Alps in Europe
  • Andes in South America
  • Rockies in North America
  • Urals in Russia

2- Block or Fault Mountains

  • Block mountains are formed when the crust rifts or faults
  • Block mountains are formed under both tension and compressive forces
  • One side of the fault rises relative to the other forming a mountain
  • Formations
  • The uplifted blocks form block mountains or horsts
  • The dropped blocks form graben and rift valleys
  • Major Block mountains
  • Rift valley in Africa
  • Narmada river Valley in India between the Vindya and Satpura horsts
  • Rhine valley in Europe
  • Vosges mountains in France
  • Black forest in Germany


3- Volcanic Mountains

  • Formed when molten rock or lava erupts, and piles upon the surface
  • Types
  • Shield Volcano
  • Strato volcano
  • Dome mountains
  • Major Volcano mountains
  • Mount Fuji in Japan
  • Mount Pinatubo in the Philippines
  • Navajo Mountain in the United States
  • Mount Catapaxi in Ecuador

4- Plateau or Erosion mountains

  • Formed by denudation or wreathing forces
  • Large mountains are subjected to the agents of erosion which results in plateaus
  • Major plateaus of erosion – Deccan Plateau