Overview
This lecture introduces the structure, layers, and composition of the Earth, focusing on how we study them, their properties, and the importance of the Earth's magnetic field.
Earth's Layers and Analogies
- The Earth is made of distinct layers: crust, mantle, liquid outer core, and solid inner core.
- Analogies used: jawbreaker (distinct layers), crispy M&M (outer shell, filling, center).
- Only the crust has been directly explored; deeper layers are studied indirectly.
Surface and Composition
- Earth's surface is divided into land (lithosphere) and water (hydrosphere).
- Water covers more surface area than land.
- Four main elements in Earth as a whole: iron, oxygen, silicon, and magnesium.
- Minerals are inorganic, solid, and crystalline, forming rocks.
- Glasses are non-crystalline solids, such as obsidian.
- Three main rock types: igneous (cooled from liquid), metamorphic (formed with heat/pressure), and sedimentary (from weathered material).
- Organic materials (wood, coal, oil) are rare due to decomposition by oxygen.
- Metals are solid and made of metallic elements.
Earth's Layers in Detail
- Crust: Outermost layer, thin (2-40 miles), with variable thickness under oceans and mountains.
- Mantle: Thick, solid but flows, comprises 82% of Earth’s volume, made of ultramafic material (iron and magnesium silicates).
- Core: Outer core is liquid (creates magnetic field); inner core is solid; both primarily iron and nickel.
Crust Types and Lithosphere
- Continental crust: thicker, less dense (2.7 g/cm³), granite composition.
- Oceanic crust: thinner, denser (3.0 g/cm³), basalt composition.
- New oceanic crust is formed from mantle material; continental crust is not being created.
- Lithosphere: rigid outer layer, includes crust and upper mantle, forms tectonic plates.
- Asthenosphere: softer, flowing solid below lithosphere; allows plates to move.
Investigating Earth's Interior
- Direct sampling only possible for crust; deeper layers studied via seismic waves produced by earthquakes.
- P-waves (compressional) travel through solids and liquids; S-waves (shear) only through solids.
- Shadow zones from seismic waves reveal liquid outer core and solid inner core.
- Moho (Mohorovičić discontinuity) marks the boundary between crust and mantle.
Earth's Magnetic Field
- Generated by movement of molten metals in the outer core.
- Magnetic field (magnetosphere) protects Earth from solar wind and radiation.
- Magnetic storms and auroras are caused by solar particles interacting with the magnetosphere.
- Mars lacks a strong magnetic field, contributing to atmospheric and water loss.
Key Terms & Definitions
- Crust — The Earth's outermost solid layer, composed of continental and oceanic types.
- Mantle — The thick layer beneath the crust, made of flowing solid rock.
- Core — The central part of Earth; outer core is liquid, inner core is solid.
- Lithosphere — Rigid outer shell, comprising the crust and upper mantle.
- Asthenosphere — Softer, plastic upper mantle below the lithosphere.
- Moho — Boundary between crust and mantle.
- Igneous rock — Formed from cooled liquid magma/lava.
- Metamorphic rock — Formed by heat and pressure changing existing rocks.
- Sedimentary rock — Formed from weathered and re-cemented materials.
- P-wave — Primary earthquake wave, compressional, travels through solids and liquids.
- S-wave — Secondary earthquake wave, shear, travels only through solids.
- Magnetosphere — Earth's region dominated by its magnetic field.
- Mineral — Inorganic, solid, crystalline substance making up rocks.
Action Items / Next Steps
- Watch the supplemental videos on seismic waves, p-wave and s-wave shadow zones, and Earth's magnetic field.
- Prepare for the next lecture topic: minerals.