The sub-classification of sedimentary rocks___:
Try it first: pick an option, then read the explanation.
Correct answer: B. mechanical, chemical, organic
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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.