The sub-classification of sedimentary rocks___:
Correct Answer:
B. mechanical, chemical, organic
🪨 Understanding Rock Classification
Rocks are broadly classified into three main families based on how they are formed: Igneous, Sedimentary, and Metamorphic. This question focuses on the next level of classification for Sedimentary Rocks, which is also based on their specific process of formation.
🔬 Detailed Analysis of the Options
B. Mechanical, Chemical, Organic
This is the correct answer. This is the primary genetic sub-classification of sedimentary rocks, describing the three main ways sediments can form a rock.
- Mechanical (or Clastic): Formed from the physical fragments (clasts) of pre-existing rocks that have been weathered, transported, deposited, and cemented together. Examples: Sandstone, Shale, Conglomerate.
- Chemical: Formed when minerals precipitate directly from a solution (usually water). Examples: Rock Salt, Gypsum, some Limestones.
- Organic: Formed from the accumulation of organic debris, such as shells, skeletons, or plant matter. Examples: Coal, some Limestones (Coquina).
A. Volcanic and Plutonic & C. Intrusive and Extrusive
These are sub-classifications of Igneous Rocks, not sedimentary.
- Volcanic (or Extrusive): Igneous rocks that form from lava cooling quickly on the Earth's surface.
- Plutonic (or Intrusive): Igneous rocks that form from magma cooling slowly beneath the Earth's surface.
D. Stratified, un-stratified
This describes the physical structure of a rock, not its genetic origin.
- Stratified means the rock has visible layers (strata). Most sedimentary rocks are stratified.
- Un-stratified means the rock is massive and shows no layering. Most igneous and metamorphic rocks are un-stratified.
📊 Summary: Sedimentary Rock Sub-classification
| Sub-classification | Formation Process | Example |
|---|---|---|
| Mechanical (Clastic) | Cementation of rock fragments. | Sandstone, Shale |
| Chemical | Precipitation of minerals from water. | Rock Salt, Gypsum |
| Organic | Accumulation of organic matter. | Coal, Fossiliferous Limestone |
💡 Study Tips
- Sediments have 3 sources: Think about where sediments come from. They can be physical pieces of rock (Mechanical), dissolved chemicals in water (Chemical), or dead things (Organic).
- Igneous = Location: Remember that igneous rock classification (volcanic/plutonic) is all about the *location* of cooling (outside/inside).
- Stratified = Structure: Stratification is about the final *structure* (layers), not the origin of the material itself.