Overview
This lecture explains the three main types of plate boundaries—convergent, divergent, and transform—and the geological features and events associated with each.
Types of Plate Boundaries
- There are three main types: convergent, divergent, and transform boundaries.
- Dramatic changes in Earth's crust like mountains, volcanoes, and earthquakes occur at plate boundaries.
Convergent Boundaries
- Convergent boundaries form when two plates collide.
- Continental-to-continental collision causes plates to buckle and thicken, forming mountain ranges.
- Oceanic-to-continental collision: denser ocean crust subducts under continental crust, melting and creating volcanoes.
- Oceanic-to-oceanic collision: the older, denser plate subducts under the younger one, melting and forming an island arc.
Divergent Boundaries
- Divergent boundaries form when two plates move away from each other.
- Asthenosphere rises, melts, and forms magma, which erupts as lava to create new crust.
- New lithosphere forms at these sites, often creating mid-ocean ridges or rift valleys.
- Most divergent boundaries are found on the ocean floor, but continental rift valleys also form.
Transform Boundaries
- Transform boundaries occur when two plates slide past each other horizontally.
- Plates scrape, causing sudden slips (earthquakes); movement is not smooth.
- Transform boundaries rarely produce magma or volcanoes.
- The San Andreas Fault is a major transform boundary.
Key Terms & Definitions
- Plate Boundary — the region where two tectonic plates meet.
- Convergent Boundary — plates collide, leading to subduction, mountain building, or island arcs.
- Divergent Boundary — plates move apart, creating new crust.
- Transform Boundary — plates slide past one another, causing earthquakes.
- Subduction — process where one plate moves under another and melts.
- Asthenosphere — semi-fluid layer beneath Earth's crust involved in plate movement.
- Lithosphere — Earth's rigid outer layer, including the crust and upper asthenosphere.
Action Items / Next Steps
- Review examples of each plate boundary type.
- Study the San Andreas Fault and other major world plate boundaries.