Overview
This lecture covers convergent plate boundaries, their types, geological features, and related hazards, emphasizing why these areas are the most dangerous on Earth.
Introduction to Convergent Boundaries
- Convergent plate boundaries are where two tectonic plates move towards and collide with each other.
- They are the site of Earth’s most hazardous geologic events, including violent earthquakes and explosive volcanoes.
Types of Convergent Boundaries
- There are three main types: oceanic-continental, oceanic-oceanic, and continental-continental plate collisions.
- Each type creates different geological features and hazards.
Oceanic-Continental Convergence
- Oceanic crust (thinner, denser) always subducts beneath continental crust (thicker, less dense).
- Subduction forms deep ocean trenches and folded volcanic mountain ranges on the continent.
- Water and sediments subducted with oceanic crust cause partial melting, forming magma that rises to create explosive volcanoes.
- Earthquakes occur when plates get stuck and then slip; underwater movement can generate tsunamis.
- Example: Cascadia Subduction Zone creates the Cascade Mountains, including Mount Rainier and Mount St. Helens.
Oceanic-Oceanic Convergence
- Both plates are oceanic; the older, colder, and denser plate subducts under the younger one.
- Subduction forms volcanic islands called island arcs.
- Explosive volcanism, large earthquakes, and tsunamis occur.
- Example: Japan is formed by this process; Mount Fuji is a prominent volcanic result.
Continental-Continental Convergence
- Continental crust doesn’t subduct due to low density; instead, crust crumples and pushes upward forming large mountain ranges.
- No volcanism or tsunamis, but strong earthquakes, landslides, and avalanches are common.
- Example: The Himalayas formed from India colliding with Asia.
Summary of Hazards
- Oceanic-Continental: volcanic chains, large earthquakes, tsunamis, landslides.
- Oceanic-Oceanic: island arcs, large earthquakes, tsunamis, landslides.
- Continental-Continental: high mountains, large earthquakes, landslides, avalanches; no volcanoes or tsunamis.
Key Terms & Definitions
- Convergent Boundary — A plate boundary where two plates collide.
- Subduction Zone — Area where one plate sinks beneath another due to density differences.
- Trench — Deep depression in the ocean floor formed at a subduction zone.
- Island Arc — A chain of volcanic islands formed above a subducting oceanic plate.
- Mega-thrust Earthquake — A large, powerful earthquake occurring at a subduction zone.
- Avalanche — Rapid movement of snow, ice, and rock down a mountain slope.
Action Items / Next Steps
- Review these notes to prepare for upcoming modules on earthquakes, tsunamis, and volcanoes.
- Keep notes handy for future reference as hazards are discussed in more detail.