Plate boundaries refers particularly to places where continental plates touch or have touched in the past.
The major types of plate boundaries are:
- Spreading boundaries. Mid-Atlantic Ridge and Africa's Great Rift Valley are examples. Rift valleys, mountains (orogeny), volcanoes.
- Collision boundaries. These collide, forming a subduction zone (West coast of the Americas) or a continental collision (India with Asia). Mountains (orogeny), valleys, volcanoes.
- Transform or fault boundaries. Here plates grind past each other along transform faults. Earthquakes and volcanoes often occur along the fault.
- Conservative boundaries: little or no relative motion.
All boundaries except the conservative ones are sites of major geological activity.