Geological agents; geological cycle

Geological agents are natural forces that wear down, move and rebuild the earth's surface. They are:

1. Running water (rivers), the most important agent.
2. Groundwater, working below the surface.
3. Glaciers (ice).
4. Wind.
5. Sea waves, tides and currents.
6. Gravity (mass movement), volcanism and organisms (including humans).

Each agent does three kinds of work: erosion (wearing away), transportation (carrying) and deposition (laying down), which together shape landforms.

Geological cycle

The geological cycle is the continuous, slow, cyclic process by which the rocks of the earth's crust are formed, destroyed and re-formed into other kinds of rock. It has no start or end. It links the rock cycle, the hydrological cycle and plate tectonics.

                 cooling
    Magma --------------------> Igneous rock
      ^                          |        |
      |              weathering, |        | heat,
      |                 erosion  |        | pressure
      | melting                  v        |
      |                     Sediments     |
      |          compaction,     |        |
      |          cementation     v        v
      |                   Sedimentary --> Metamorphic
      |                      rock   heat,    rock
      |                           pressure    |
      +---------------------------------------+

• Magma cools to igneous rock.
• Weathering, erosion, transport and deposition give sediments, which become sedimentary rock by lithification (compaction and cementation).
• Heat and pressure change any rock (igneous or sedimentary, or older metamorphic rock) into metamorphic rock.
• Deep burial and melting return the rock to magma.

Uplift brings deep rocks back to the surface, where weathering starts the cycle again. Engineers use this idea to understand why different rock types occur at a site.