In regard to erosion, the surface of the Earth is constantly changing. The time over which this erosion occurs is dependant upon your reference to time; days, months, years, decades or more. For example, if we are talking about erosion of sedimentary rocks, the time period is relatively short compared to the erosion of igneous rock.
That being said, generally, erosion occurs from the forces of wind, rain, heat, and grinding - usually associated with the erosion of continental plates. This will not be an environmental answer as much as a physical science answer.
The breaking down of rock may follow the following cycle (a process that eventually returns to the starting point);
- Erosion- the breaking down of a substance to small particles, increasing the surface area of a rock
- Deposition- The depositing of the broken down material from erosion settling. This deposited material may then pick up other materials other than what may have been broken down by the erosion, i.e., shells, bones, organic fragments
- Compaction- As the deposited material settles, over time (see the reference to time, above), the layers a squeezed together. Often the squeezed layers are visible if the deposited material eventually becomes exposed
- Cementation- With the addition of water, a glue, so to speak, is formed that seepes between all the weak points in the compacted material, bonding the layers together.
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