Overview
This lecture explains the structure of Earth's lithosphere, types of tectonic plates and boundaries, and how plate movement leads to earthquakes and geological features.
Earth's Structure and Lithosphere
- The lithosphere is Earth's cool, rigid outer layer, broken into large moving plates.
- Plates can be fully oceanic, fully continental, or a mix of both types.
- The mantle, beneath the crust, is a 2900 km-thick layer of high-magnesium silicate rock.
- The lithospheric mantle is cool and rigid, while the asthenosphere below is hotter and ductile.
- Lithospheric plates are made of crust plus the rigid uppermost mantle and move as a single unit.
- Earthquakes occur in brittle lithospheric rock; the ductile asthenosphere does not produce earthquakes.
Types of Lithosphere
- Continental lithosphere ranges from 150 to 200 km thick and has buoyant, silica-rich rocks, forming continents above sea level.
- Oceanic lithosphere is 50–140 km thick, with denser, iron- and magnesium-rich rocks forming the ocean floor below sea level.
Plate Boundaries
- Plate motion is grouped as transform (plates slide past), divergent (plates move apart), and convergent (plates collide).
- Transform boundaries (e.g., San Andreas Fault) have horizontal motion and cause earthquakes up to magnitude 8.
- Divergent boundaries (e.g., mid-ocean ridges) create new oceanic crust and mostly shallow, small earthquakes.
- Convergent boundaries occur when plates press together, often leading to subduction (one plate moves beneath another).
Earthquakes and Geological Features
- Over 75% of earthquakes occur near convergent boundaries, where largest magnitude events and tsunamis happen.
- Subduction zones create deep earthquakes (up to 700 km) and features like trenches and volcanoes.
- Continental-continental collisions create mountain ranges and frequent shallow earthquakes.
- Diffuse boundary zones exist where deformation is spread over large areas, and intraplate earthquakes can occur away from boundaries.
- Most tectonic activity and features such as volcanoes, mountains, and trenches occur at or near plate boundaries.
Key Terms & Definitions
- Lithosphere — Rigid outer shell of Earth, broken into tectonic plates.
- Asthenosphere — Hot, ductile layer beneath the lithosphere allowing plate movement.
- Plate Tectonics — Theory describing movement of Earth's lithospheric plates.
- Subduction — Process where one plate moves beneath another at a convergent boundary.
- Transform Boundary — Plate boundary where plates slide past each other horizontally.
- Divergent Boundary — Plate boundary where plates move apart and new crust forms.
- Convergent Boundary — Plate boundary where plates move toward each other and collide.
Action Items / Next Steps
- Review diagrams of Earth's layers and major plate boundaries.
- Read textbook section on plate tectonics and earthquake generation.