Rock Cycle

What is the rock cycle?

  • Nature's recycling program involving three main rock types: igneous, sedimentary, and metamorphic.

  • A continuous process of rock formation, breakdown, and transformation by Earth's internal heat, pressure, and surface conditions.

  • Any rock type can transform into any other via:

    • Weathering/erosion

    • Heating/pressure (metamorphism)

    • Melting/solidification (igneous processes).

  • Simplified flow:
    1) Igneous rocks form from cooling magma/lava.
    2) Weathering breaks igneous rocks into sediments.
    3) Sediments compact and cement into sedimentary rocks (lithification).
    4) Heat/pressure transforms them into metamorphic rocks.
    5) Metamorphic rocks can melt into magma, restarting the cycle.

Igneous rocks (fire rocks)

  • Formation: Cooling and solidification of molten rock (magma/lava).

  • Texture depends on cooling rate:

    • Slow cooling (intrusive/plutonic): large crystals, coarse-grained (e.g., granite, diorite, gabbro).

    • Rapid cooling (extrusive/volcanic): small crystals or glassy (e.g., basalt, obsidian, pumice).

  • Timescale: Intrusive cooling can take from 103extto106extyears10^3 ext{ to } 10^6 ext{ years}.

Sedimentary rocks (layered rocks)

  • Formation: Accumulation, compaction, and cementation of sediments (lithification).

  • Sediments: Particles from weathered rocks, minerals, or organic material.

  • Lithification:

    • Compaction: squeezes out water/air.

    • Cementation: minerals bind sediments (e.g., silica, calcite, iron oxides).

  • Timescale: Millions of years for formation.

  • Types:

    • Clastic (detrital): from rock fragments (e.g., sandstone, shale, conglomerate).

    • Chemical: from minerals crystallizing out of water (e.g., limestone from dissolved CaCO3\mathrm{CaCO}_3, rock salt).

    • Organic: from plant/animal remains (e.g., coal, chalk).

Metamorphic rocks (transformed rocks)

  • Formation: Existing rocks change due to high heat, pressure, or chemical processes (metamorphism).

  • Effects:

    • Heat: recrystallization of minerals.

    • Pressure: denser, more compact rocks; may cause foliation (layered/banded).

  • Classification:

    • Foliated: layered texture (e.g., slate from shale, gneiss from granite).

    • Non-foliated: uniform crystalline texture (e.g., marble from limestone).

The rock cycle in action (summary of processes)

  • Rocks weather into sediments.

  • Sediments become sedimentary rocks via lithification.

  • Sedimentary rocks become metamorphic rocks under heat and pressure.

  • Metamorphic rocks can melt into magma.

  • Magma cools into igneous rocks.

  • Any rock type can transform into another at various stages.