Overview
This lecture explores the formation, types, and transformation of rocks within Earth's layers, focusing on the rock cycle, plate tectonics, and how surface and deep processes shape rocks over millions of years.
Structure of Earth
- Earth formed 4.5 billion years ago from a collapsing nebula of dust and gas.
- Earth consists of layered shells: atmosphere, crust, mantle, and core.
- The crust is thin, with continental and oceanic types differing in thickness, color, density, and composition.
- The mantle is thick, dense, and changes density with depth; it contains silicates, iron, and magnesium.
- The core has a liquid outer layer and a solid inner layer, both primarily iron and nickel.
Types and Formation of Rocks
- Rocks are collections of minerals; minerals are inorganic compounds with crystalline structure.
- Igneous rocks form from cooled magma or lava; intrusive (e.g., granite) forms below surface, extrusive (e.g., basalt) forms at surface.
- Granite is coarse-grained (large crystals), formed from slowly cooled felsic magma near the surface.
- Basalt is fine-grained (small crystals), formed from quickly cooled mafic lava at the surface.
The Rock Cycle
- Rocks change over time through weathering, erosion, compaction, metamorphism, and melting.
- Weathering and erosion break rocks into sediments that can be transported by water and deposited as layers.
- Sedimentary rocks (e.g., sandstone, limestone, coal) form from compacted and cemented sediments or organic matter.
- Metamorphic rocks (e.g., gneiss) form when rocks undergo intense heat and pressure, causing recrystallization without melting.
- The rock cycle allows rocks to change types in multiple pathways, not always following the same sequence.
Plate Tectonics and Rock Formation
- The Himalayas formed as the Indian and Eurasian plates collided, uplifting rocks and closing the Tethys Sea.
- Subduction recycles crust and sediments, forming new rocks through melting, metamorphism, and uplift.
Key Terms & Definitions
- Crust — Earth's thin, outer solid shell (continental and oceanic types).
- Mantle — Thick layer beneath the crust, rich in silicates, iron, magnesium.
- Core — Innermost layer, with a liquid outer part and solid inner part, made of iron and nickel.
- Mineral — Inorganic, naturally occurring compound with a crystalline structure.
- Igneous Rock — Rock formed from cooled magma or lava.
- Sedimentary Rock — Rock formed from compacted and cemented sediments or organic material.
- Metamorphic Rock — Rock that has changed form due to heat and pressure.
- Weathering — Breakdown of rocks at Earth’s surface by atmosphere, water, and organisms.
- Erosion — Transportation of weathered rock particles by wind, water, or ice.
- Rock Cycle — Ongoing process of rock transformation among igneous, sedimentary, and metamorphic forms.
Action Items / Next Steps
- Review the rock cycle diagram and examples of each rock type.
- Read about local geology and Indigenous histories in your region.