Geomorphology: Plate Tectonics, Rocks and Landforms (NCERT Geography Digest)

Continental drift and plate tectonics

• Continental Drift (Alfred Wegener, 1912): a single supercontinent, Pangaea, surrounded by a single ocean Panthalassa, split (about 200 million years ago) into Laurasia (north) and Gondwanaland (south), which drifted to today's positions. Evidence: the jigsaw fit of the Atlantic coasts, matching rock formations and fossils across oceans (Glossopteris flora, Mesosaurus), placer gold deposits, and tillite (glacial) beds.
• Sea-floor spreading (Harry Hess): new oceanic crust forms at mid-oceanic ridges and spreads outward, being consumed at trenches. Evidence: palaeomagnetism (symmetrical, reversing magnetic stripes either side of ridges) and the youth of oceanic crust (none older than about 200 million years).
• Plate tectonics: the lithosphere is broken into seven major and several minor plates that move over the asthenosphere, driven by mantle convection currents. Three boundary types:
  • Divergent (constructive): plates move apart and new crust forms (the Mid-Atlantic Ridge).
  • Convergent (destructive): plates collide; oceanic plate subducts (trenches, volcanic arcs) or continents collide to raise fold mountains (the Himalayas, from the Indo-Australian and Eurasian collision).
  • Transform (conservative): plates slide past each other (the San Andreas Fault).

Rocks and the rock cycle

• A mineral is a naturally occurring inorganic substance with a definite chemical composition; rocks are aggregates of minerals.
• Igneous rocks form from cooling magma or lava: intrusive (slow cooling underground, coarse-grained, for example granite) and extrusive (rapid cooling at the surface, fine-grained, for example basalt). These are the "primary" rocks.
• Sedimentary rocks form by deposition and compaction of sediments in layers (strata); they may contain fossils (sandstone, limestone, shale).
• Metamorphic rocks form when existing rocks are altered by heat and pressure: limestone to marble, sandstone to quartzite, coal to graphite, shale to slate, granite to gneiss.
• The rock cycle: rocks transform from one type to another over geological time.

Geomorphic processes

• Endogenic (internal) forces build relief: diastrophism (folding, faulting, plate movement) and volcanism.
• Exogenic (external) forces wear down relief (denudation): weathering, mass movement, erosion, transport and deposition by water, wind, ice and waves.
• Weathering (in situ breakdown): mechanical/physical (frost action, exfoliation, thermal expansion), chemical (solution, oxidation, carbonation, hydration) and biological. Weathering prepares material for erosion and is the first step in soil formation.
• Mass movement ranges from slow (creep, solifluction) to rapid (landslides, slumps, mudflows), a key hazard in the Himalayas.

Landforms by agent

How CAPF asks this

• Which two plates raised the Himalayas (Indo-Australian and Eurasian). Type and example of plate boundaries. Evidence for sea-floor spreading.
• Match rock to type (marble is metamorphic, basalt is igneous, limestone is sedimentary).
• Match landform to agent (ox-bow lake to river, cirque to glacier, barchan to wind, stalactite to groundwater).

Authored practice

1. The collision of which two plates raised the Himalayas? (Answer: the Indo-Australian and Eurasian plates.) Authored practice, not a verbatim PYQ.
2. Marble is formed by the metamorphism of which rock? (a) sandstone (b) limestone (c) shale (d) granite. Answer: (b) limestone. Authored practice, not a verbatim PYQ.