Overview
This lecture explains the three main types of rocks—sedimentary, metamorphic, and igneous—and describes how each forms and transforms through the rock cycle.
Sedimentary Rocks
- Sedimentary rocks form from fragments of other rocks or organic material through weathering, erosion, and compaction.
- Three types: clastic (from rock fragments, e.g., sandstone), organic (from biological matter, e.g., coal), and chemical (from mineral precipitation, e.g., limestone).
- Clastic and organic types begin with weathering, then fragments are eroded, transported, and deposited before compacting into rock.
- Chemical sedimentary rocks form when dissolved minerals are left behind as water evaporates, creating compounds like salt or limestone.
Metamorphic Rocks
- Metamorphic rocks are formed when existing rocks change due to high heat or pressure.
- Two categories: foliated (minerals aligned in layers, e.g., gneiss from granite) and nonfoliated (no layered appearance, e.g., marble from limestone).
- Foliation occurs when platy minerals align perpendicularly to pressure; nonfoliated rocks result when minerals do not align or from heat near magma.
Igneous Rocks
- Igneous rocks form when molten material (magma or lava) cools and solidifies.
- Intrusive (plutonic) igneous rocks form inside the Earth, cool slowly, and have large crystals (e.g., granite).
- Extrusive (volcanic) igneous rocks form on Earth's surface, cool quickly, and have fine or glassy texture (e.g., basalt, obsidian).
- Vesicular texture (holey) in some extrusive rocks forms from trapped gas bubbles (e.g., pumice).
Key Terms & Definitions
- Weathering — breaking down of rocks into fragments.
- Erosion — removal and transportation of rock fragments by wind, water, ice, or organisms.
- Compaction — process where sediments are pressed together to form rock.
- Chemical Precipitation — process where dissolved minerals are left behind as water evaporates.
- Foliation — alignment of minerals in metamorphic rocks into layers.
- Intrusive Igneous Rock — rock formed from magma that cools inside the Earth.
- Extrusive Igneous Rock — rock formed from lava that cools on the Earth's surface.
Action Items / Next Steps
- Review the formation processes and examples of each rock type.
- Study the key terms and be prepared to identify rocks by their formation process.