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Rock types, rock cycle, sedimentary rock properties

Short answer

Rocks are igneous, sedimentary, and metamorphic. The rock cycle transforms them through cooling, weathering, compaction, and metamorphism. Major sedimentary rocks in the Nepal Himalaya include sandstone (hard, granular), shale (fissile, weak), limestone (fizzes in HCl, cavities), and conglomerate; Siwalik rocks are young, weak, and cause landslides.

What are the different types of rocks? Describe the rock cycle with a diagram. Describe the physical properties of major sedimentary rocks in the Nepal Himalaya.

Types of rocks

Rocks are natural aggregates of minerals. The three classes are:

  • Igneous: formed by cooling of magma or lava (granite, basalt).
  • Sedimentary: formed by deposition and lithification of sediments (sandstone, shale, limestone).
  • Metamorphic: formed by change of existing rocks under heat and pressure (slate, schist, gneiss, marble).

Rock cycle

The rock cycle is the continuous transformation of rocks from one type to another.

      MAGMA
     /     ^
 cooling    | melting
    v       |
  IGNEOUS --+--> weathering, transport,
    |            deposition
    |             v
    |         SEDIMENTS
    |             | compaction,
    |             v cementation
    |         SEDIMENTARY
    |  heat &     |
    |  pressure   |
    v             v
      METAMORPHIC --(melting)--> MAGMA

Igneous rocks weather into sediments, which harden into sedimentary rocks. Heat and pressure change these into metamorphic rocks, which melt to magma, forming igneous rocks again.

Major sedimentary rocks of the Nepal Himalaya and properties

RockPhysical propertiesOccurrence
SandstoneGranular, bedded, colour grey, brown, red; hardness 6-7; moderately strongSiwalik (Churia), Lesser Himalaya (Nawakot, Kathmandu complexes)
Siltstone/MudstoneFine-grained, soft, easily weathered; low strengthSiwalik
ShaleFissile, thin layers, soft, slakes in waterLesser and Tethys Himalaya
LimestoneHardness 3, fizzes in HCl; grey; may have solution cavitiesLesser Himalaya (Kathmandu valley, Dhading), Tethys Himalaya
ConglomerateRounded pebbles in cement; hard to weakSiwalik (Upper)
DolomiteLike limestone, harder; weak reaction with HClLesser Himalaya (Nawakot)

Siwalik rocks are young, poorly cemented, and weak, so they cause erosion and landslides.

✓ Answer checked by Claude Opus 5.5 (high effort)

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