Geology: Classification of Rocks

Study Notes

Rocks are classified by particle size: conglomerate (largest), sandstone, and shale/mudstone (smallest).
Chemical precipitation contributes to rock formation.

Animal shells composed of calcium carbonate can accumulate on the ocean floor forming organic limestone or chalk.
In swampy regions, vegetation partially decomposes due to low oxygen, resulting in coal formation.

Contact metamorphic: Changes occur due to heat from nearby molten rock, creating narrow bands around igneous rocks.
Regional metamorphic: High pressure from tectonic plate collisions alters rocks, affecting larger areas.

Foliated: Exhibits a smooth appearance with minerals aligned in parallel layers. Examples include slate and gneiss.
Non-Foliated: Has a granular appearance with no alignment of minerals. Common examples include marble and quartzite.

The rock cycle illustrates the continuous transformation of rocks through various geological processes.

Silicate minerals (SiO4 group) are the most abundant on Earth, formed from cooling molten rock.
Cooling conditions influence the type of silicate mineral formed, categorized into:
- Felsic: Light-colored minerals like granite and rhyolite.
- Mafic: Dark-colored minerals like gabbro and basalt.

Hardness: Measured by scratching minerals against known standards using Moh's hardness scale.
Lustre: Describes mineral shine; can be metallic or non-metallic (e.g., glassy, earthy).
Streak: Color of a mineral in powdered form; determined by scraping against an unglazed tile.
Density/Specific Gravity: A measure of mineral density relative to water (1 g/cm³).
Cleavage and Fracture: Cleavage describes how a mineral splits along weak lines; fracture refers to irregular breakage.

Classified by chemical composition: light (felsic) versus dark (mafic) minerals.
Extrusive igneous rocks: Formed from lava, featuring small crystals due to rapid cooling.
Intrusive igneous rocks: Formed from magma, resulting in large crystals from slow cooling.

Formed in layers (strata) adhering to the law of superposition, indicating that layers become younger as you move upward.
Clastic: Made from weathered and eroded sediments that are cemented together.

Soil texture affects aeration, nutrient capacity, infiltration, and water retention:
- Clay: Poor aeration, good nutrient capacity, poor infiltration, good water retention.
- Silt: Medium aeration and nutrient capacity, medium infiltration, medium water retention.
- Sand: Good aeration, poor nutrient capacity, good infiltration, poor water retention.

Soil pH is crucial since specific minerals are accessible to plants at particular pH levels.
NPK (Nitrogen, Phosphorus, Potassium) ratios are essential for effective fertilization.

Law of Superposition: Ages of rocks increase as one moves downward through strata.
Law of Crosscutting: Faults and fractures are younger than the rocks they disrupt.
Law of Inclusions: Rock fragments within a host rock must be older than the host itself.

Index fossils help geologists synchronize the ages of rocks across larger geographical areas.

Radioactive minerals in igneous rocks decay over time, allowing specialists to determine absolute ages of rock samples using technology.

Natural Resources: Materials existing in nature for economic use, categorized as:
- Renewable: Can be replenished sustainably within a short timeframe.
- Nonrenewable: Exist in limited amounts and cannot be replaced quickly.