The understanding of mountain building, or orogenesis, has evolved significantly, with recent views largely dominated by the theory of plate tectonics. This paradigm provides a comprehensive framework for explaining the formation of most major mountain ranges globally.

Recent Views on Mountain Building (Plate Tectonics): Modern geology attributes mountain building primarily to the interactions of Earth's lithospheric plates. The key processes include:

1. Convergent Plate Boundaries: This is the most significant mechanism for mountain building. When two plates move towards each other, the immense compressional forces lead to deformation of the crust.

   • Oceanic-Continental Convergence: An oceanic plate subducts beneath a continental plate. The continental crust is uplifted, folded, and faulted, often accompanied by volcanic activity (e.g., Andes Mountains, Cascade Range). Sediments scraped off the subducting plate also contribute to accretionary wedges.
   • Oceanic-Oceanic Convergence: One oceanic plate subducts beneath another, leading to the formation of volcanic island arcs (e.g., Mariana Islands, Japanese Archipelago). Over time, these arcs can collide with continents or other arcs, contributing to larger mountain belts.
   • Continental-Continental Convergence: When two continental plates collide, neither subducts significantly due to their buoyancy. Instead, the crust is intensely folded, faulted, and thickened, leading to the formation of massive mountain ranges (e.g., Himalayas from the collision of the Indian and Eurasian plates, Alps from the collision of the African and Eurasian plates).

2. Hotspot Volcanism: While not directly related to plate boundaries, hotspots can create volcanic mountains as plates move over a stationary mantle plume (e.g., Hawaiian Islands).

3. Extensional Tectonics (Block Faulting): In some regions, tensional forces can cause the crust to stretch and break into large blocks. Some blocks are uplifted (horsts) and others subside (grabens), forming block mountains (e.g., Basin and Range Province in the western USA).

Division of World Mountains on the Basis of their Genesis: Based on their formation processes, mountains can be broadly classified into several types:

1. Fold Mountains: These are the most common type, formed by the intense folding and faulting of sedimentary and metamorphic rocks due to compressional forces at convergent plate boundaries. They are characterized by anticlines and synclines. Examples include:

   • Young Fold Mountains: Himalayas, Alps, Rockies, Andes (formed in the Cenozoic Era, still tectonically active).
   • Old Fold Mountains: Appalachians (North America), Urals (Russia), Aravallis (India) (formed in earlier geological eras, heavily eroded).

2. Block Mountains (Fault-Block Mountains): These mountains are formed when large blocks of the Earth's crust are uplifted or tilted along fault lines due to tensional or compressional forces. They often have steep, straight sides. Examples include:

   • Sierra Nevada (USA)
   • Vosges Mountains (France) and Black Forest (Germany)
   • Salt Range (Pakistan)

3. Volcanic Mountains (Accumulation Mountains): These are formed by the accumulation of volcanic material (lava, ash, cinders) erupted from the Earth's interior. They can occur at convergent plate boundaries (subduction zones) or over hotspots. Examples include:

   • Mount Fuji (Japan)
   • Mount Kilimanjaro (Tanzania)
   • Andes (many peaks are volcanic, e.g., Cotopaxi)
   • Hawaiian Islands (hotspot volcanism)

4. Dome Mountains: These are formed when molten magma pushes up the Earth's crust from below, creating a dome-shaped bulge. The overlying sedimentary layers are uplifted and often eroded away, exposing the igneous core. Examples include:

   • Black Hills (South Dakota, USA)
   • Adirondack Mountains (New York, USA)

5. Erosional or Residual Mountains: These are not formed by tectonic activity but are remnants of elevated plateaus or landmasses that have been deeply dissected and eroded by agents like rivers, wind, and glaciers over long periods. The harder, more resistant rocks remain as mountains. Examples include:

   • Aravalli Range (India, though also an old fold mountain, its current form is largely residual)
   • Mesa and Butte formations (e.g., in the American Southwest)
   • Some parts of the Scandinavian Mountains.