Clastic sedimentary rocks are primarily classified on the basis of
a. grain size.
b. degree of sorting.
c. angularity.
d. mineral composition.
The Correct Answer and Explanation is :
Correct Answer:
a. grain size
Explanation:
Clastic sedimentary rocks are classified primarily based on grain size, which refers to the diameter of individual sediment particles making up the rock. Grain size is the most significant factor because it directly influences the rock’s texture, depositional environment, and formation processes.
1. Grain Size Classification:
Clastic sedimentary rocks are categorized into different types based on grain size:
- Conglomerates and Breccias – Composed of coarse grains larger than 2 mm, with conglomerates having rounded grains and breccias having angular grains.
- Sandstone – Made of medium-sized grains (0.0625 mm to 2 mm), typically composed of quartz, feldspar, or rock fragments.
- Siltstone – Contains fine grains (0.004 mm to 0.0625 mm) that are barely visible to the naked eye.
- Shale – Comprised of the finest grains (less than 0.004 mm), forming thin, layered structures due to compacted clay and silt.
2. Why Grain Size is the Primary Criterion:
- Depositional Energy: Grain size reflects the energy of the environment where the sediment was deposited. Larger grains settle in high-energy environments (rivers, beaches), while finer grains accumulate in low-energy settings (lakes, deep marine basins).
- Transport Distance: Smaller grains travel farther due to lower settling velocity, whereas larger grains deposit closer to the source.
- Compaction and Cementation: Larger grains often result in more porous rocks, while finer grains create dense, well-compacted rocks.
While factors like sorting, angularity, and mineral composition influence rock properties, grain size is the most fundamental characteristic used for classification.
Now, generating an image illustrating different clastic sedimentary rocks classified by grain size.
Here is an educational diagram illustrating the classification of clastic sedimentary rocks based on grain size. It visually represents the different rock types, from conglomerate to shale, along with their depositional environments. Let me know if you need any modifications!