Overview
This lecture explains the three main types of rocks—igneous, sedimentary, and metamorphic—including their formation, classification, and key characteristics.
Introduction to Rocks
- Rocks are naturally formed solids made up of associations of minerals.
- The composition and mineral associations in rocks determine their classification and origin.
- Geologists classify rocks based on their mineral content.
Igneous (Magmatic) Rocks
- Igneous rocks form from the cooling and solidification of magma.
- Plutonic (intrusive) rocks cool slowly deep underground, resulting in large, visible crystals (e.g., granite, gabbro).
- Volcanic (extrusive) rocks cool quickly on the surface, forming fine-grained textures (e.g., rhyolite, basalt, pumice, obsidian).
- Granite contains quartz, feldspars, and micas; gabbro and basalt are darker, rich in ferro-magnesian minerals.
Sedimentary Rocks
- Sedimentary rocks form from the accumulation and consolidation of sediments from pre-existing rocks.
- Detrital sedimentary rocks are classified by grain size: shales (fine), sandstones (medium), conglomerates/breccias (coarse).
- Chemical sedimentary rocks form from precipitation of dissolved minerals, including limestone (calcium carbonate), rock salt (halite), and gypsum.
- Sedimentary rocks often form distinct layers called strata.
Metamorphic Rocks
- Metamorphic rocks arise when existing rocks are subjected to high temperatures and pressures, causing mineral changes.
- Laminar (foliated) metamorphic rocks have layered structures due to mica minerals (e.g., slate, schist, gneiss).
- Slate splits into thin sheets; schist displays shiny surfaces; gneiss features banded mineral layers.
- Crystalline (non-foliated) metamorphic rocks break irregularly and are often uniform (e.g., marble from limestone, quartzite from sandstone).
Key Terms & Definitions
- Mineral — a naturally occurring, inorganic solid with an ordered atomic structure.
- Magma — molten rock beneath the Earth's surface.
- Plutonic (Intrusive) Rock — igneous rock formed deep underground with large crystals.
- Volcanic (Extrusive) Rock — igneous rock formed at the surface with small or invisible crystals.
- Detrital Sedimentary Rock — rock formed from fragments of other rocks.
- Chemical Sedimentary Rock — rock formed from mineral precipitation.
- Metamorphism — process where rocks change due to heat and pressure.
- Foliated (Laminar) Rock — metamorphic rock with a layered appearance.
- Non-foliated (Crystalline) Rock — metamorphic rock without a layered structure.
Action Items / Next Steps
- Review the classification of rock-forming minerals if needed.
- Watch supplementary videos for deeper understanding of igneous, sedimentary, or metamorphic rocks.
- Prepare any questions on rock types for the next lecture.