New layers are formed primarily through the deposition of particles from water or air.
This process, which leads to the creation of layered sedimentary rocks, relies on several key principles:
Sediment Deposition
- Particles in Suspension: Materials like sand, silt, clay, and organic matter are carried by water (rivers, oceans) or air (wind).
- Settling: When the transporting medium (water or air) slows down, the particles begin to settle out due to gravity. Heavier particles settle first, followed by lighter ones.
- Accumulation: Over time, these settled particles accumulate, forming layers.
Steno's Law of Original Horizontality
This fundamental geological principle, proposed by Nicolas Steno, states that sedimentary layers are initially deposited horizontally. This means that any tilted or folded layers we see today have been deformed after their initial formation.
Lithification
- Compaction: As more layers accumulate, the weight of the overlying sediments compresses the lower layers.
- Cementation: Minerals dissolved in water precipitate between the sediment grains, binding them together. Common cementing agents include silica, calcite, and iron oxides.
Formation Environments
The specific environment where deposition occurs influences the type of layers formed. Examples include:
- Rivers: Form layers of sand and gravel.
- Lakes: Form layers of fine-grained silt and clay.
- Oceans: Form layers of various sediments, including sand, silt, clay, and the remains of marine organisms.
- Deserts: Form layers of sand through wind deposition.
In summary, new layers are formed by the settling and accumulation of particles from water or air, followed by compaction and cementation to form solid rock.
