Overview
This lecture explains the rock cycle, the three major rock types—igneous, sedimentary, and metamorphic—and how to identify each type based on their formation processes and characteristics.
The Rock Cycle
- The rock cycle describes how rocks transform between igneous, sedimentary, and metamorphic types through melting, cooling, eroding, transporting, burying, and changing.
- Earth's rock cycle is driven by plate tectonics, volcanism, erosion, burial, and the movement of rocks through subduction zones.
- The ocean floor is mainly young igneous rocks from mid-ocean ridges, which eventually subduct and recycle.
- Sediments from eroded mountains are transported, buried, and lithified into sedimentary rocks, which can undergo metamorphism if deeply buried.
Comparison to Other Planets
- Earth's dynamic rock cycle contrasts with the Moon's static surface, which lacks volcanism, water, and tectonics; lunar rocks change mainly by meteor impacts.
- Mars shows some evidence of wind- and water-driven sedimentary rocks but is less dynamic, primarily containing igneous rocks.
Identifying Rock Types
Igneous Rocks
- Formed from cooled molten lava or magma.
- Contain intergrown crystals; slow cooling forms large crystals, fast cooling forms tiny ones.
- Typically black or gray, sometimes colorful or sparkly under close inspection.
Sedimentary Rocks
- Composed of grains (clasts) or fragments cemented together.
- Typically dull-colored, clasts can be rounded or angular, cement may be shiny if crystals grew in cavities.
- Includes clastic rocks (from fragments), carbonate rocks (e.g., limestone from shells), and evaporitic rocks (from evaporated salts).
- Only sedimentary rocks preserve fossils and fossil fuels.
Metamorphic Rocks
- Formed by intense heat and pressure that alters rock structure without melting.
- Characterized by foliation: wavy, shiny planar patterns from recrystallized minerals.
- Metamorphic rocks can be sparkly, with marble as an example from metamorphosed limestone.
Key Terms & Definitions
- Rock cycle — the ongoing process of transformation among igneous, metamorphic, and sedimentary rocks.
- Igneous rock — solidified from molten magma or lava.
- Sedimentary rock — formed from cemented fragments or organic matter.
- Metamorphic rock — altered by heat and pressure without melting.
- Clast — a fragment of another rock.
- Lithification — process of turning sediment into rock by compaction and cementation.
- Foliation — wavy or layered textures in metamorphic rocks.
Action Items / Next Steps
- Review the next section: "Mineral Identification of Hand Samples."
- Practice identifying rock types by examining their textures and formation clues.