Sediments, the fragmented bits of rock, minerals, and organic matter, undergo significant changes as they are transported from their source to their final resting place. Here's a breakdown of the key physical alterations:
1. Size Reduction:
* Abrasion: As sediments collide with each other and with the channel bed during transport, they get worn down. This process, known as abrasion, reduces the size of particles.
* Attrition: Similar to abrasion, attrition is the breaking down of particles through collisions with each other. However, attrition specifically involves the impact of smaller particles on larger ones.
* Dissolution: Some minerals are soluble and can dissolve in water, particularly acidic water. This process reduces the size of particles and changes their composition.
2. Shape Modification:
* Rounding: Angular fragments from the source gradually become more rounded as they are transported. This is primarily due to abrasion and attrition, which smooth out sharp edges and corners.
* Sphericity: The degree to which a particle approaches a perfect sphere is called sphericity. As sediments travel, they tend to become more spherical, though this is influenced by mineral properties and transport conditions.
3. Sorting:
* Sorting: Sediments are rarely perfectly sorted. As they are transported, they are separated based on their size, density, and shape. For example, currents can selectively carry finer particles further than coarser particles.
* Well-sorted: Sediments that have undergone extensive transport will generally be well-sorted, meaning they consist of particles of similar sizes.
* Poorly-sorted: Sediments that have undergone minimal transport will be poorly sorted, with a wide range of particle sizes.
4. Mineral Composition:
* Weathering: During transport, sediments are exposed to weathering processes that can alter their mineral composition. For example, chemical weathering can dissolve certain minerals, leaving behind more resistant ones.
* Recrystallization: Some minerals can recrystallize during transport, forming new minerals with different compositions. This is particularly common in sedimentary environments with high temperatures or pressures.
5. Deposition:
* Depositional Environment: The environment where sediments are deposited has a significant influence on their characteristics. For example, sediments deposited in high-energy environments like rivers will be coarser and less rounded than those deposited in low-energy environments like lakes.
* Sedimentary Structures: The way sediments are deposited creates sedimentary structures like bedding, cross-bedding, and ripple marks. These structures provide clues about the environment in which the sediments were deposited.
Impact on Sediment Characteristics:
The changes in sediment characteristics during transport provide valuable insights into:
* Source rock: The original rock from which the sediments were derived.
* Transport history: The distance and conditions under which the sediments were transported.
* Depositional environment: The environment in which the sediments were ultimately deposited.
By studying the physical characteristics of sediments, geologists can reconstruct the past history of an area, understand the processes that have shaped the landscape, and even predict future geological events.
