Which of the following processes is/are part of the Rock Cycle? 1. Lithification 2. Metamorphism 3. Solidification Select the answer using the code given below:

Correct — D, 1, 2 and 3. The rock cycle is the set of processes by which each of the three great rock classes turns into another, and all three named processes belong to it. Lithification is the compaction and cementation of loose sediment into sedimentary rock, and it is how sandstone and shale come into being. Metamorphism is the alteration of an existing rock by heat and pressure without melting, and it turns limestone into marble, shale into slate and sandstone into quartzite. Solidification, the crystallisation of magma or lava, is how igneous rock forms — granite when it cools slowly at depth, basalt when it cools quickly at the surface. Since each of the three is a step in the cycle, no partial code can be right.

• (a)1 only — Keeping lithification alone leaves out both the igneous and the metamorphic routes. A cycle by definition has to include the pathway that makes each class of rock.
• (b)1 and 2 only — This drops solidification, which is the only process on the list that produces igneous rock. Without it there would be no primary rock for weathering to break down in the first place.
• (c)2 and 3 only — This drops lithification, which is the process that turns sediment into sedimentary rock. Weathering and erosion produce sediment, but only lithification converts it into rock.

The rock cycle links three rock classes. Igneous rock forms when magma or lava solidifies. Weathering and erosion break any exposed rock into sediment, which is transported, deposited and then lithified by compaction and cementation into sedimentary rock. Burial under heat and pressure metamorphoses either class into metamorphic rock, and deeper burial melts rock back into magma, closing the cycle. Because the loop can be entered and left at several points, no rock class is permanent, and the same atoms cycle through all three over geological time.

Where a code item lists three processes that all belong to a named system, the full code is usually intended, and the way to confirm it is to check that each process makes a different one of the three rock classes. Here lithification makes sedimentary, metamorphism makes metamorphic and solidification makes igneous, so together they cover the whole cycle and none can be dropped. It is worth adding weathering and erosion to the mental list too, since they supply the sediment that lithification later hardens.

• Lithification is compaction and cementation of sediment into sedimentary rock.
• Metamorphism alters rock by heat and pressure without melting it: limestone to marble, shale to slate.
• Solidification of magma or lava produces igneous rock: granite at depth, basalt at the surface.
• Weathering and erosion supply the sediment that lithification later hardens.
• The cycle closes when deep burial melts rock back to magma, so no rock class is permanent.

• Treating solidification as though it belonged only to volcanoes rather than to the cycle as a whole.
• Confusing lithification, which hardens sediment, with metamorphism, which alters existing rock.
• Assuming a partial code must be right because all three statements look correct.

A three-statement code item where each process corresponds to a different rock class, so the full code follows once each is placed.

• practice — not a real PYQ

  Marble is formed by the metamorphism of which rock?

  • (a)Granite
  • (b)Limestone
  • (c)Basalt
  • (d)Shale

  Answer(b) Limestone — heat and pressure recrystallise its calcite.

• practice — not a real PYQ

  Basalt is an example of which type of rock?

  • (a)Intrusive igneous
  • (b)Extrusive igneous
  • (c)Clastic sedimentary
  • (d)Metamorphic

  Answer(b) Extrusive igneous — it cools quickly at the surface, giving a fine-grained texture.