1. ACTIONS AT PLATE BOUNDARIES

2. Divergent Boundaries
• Oceanic ridges - continuous elevated zones on floor of all major ocean basins. Rifts at the crest of ridges represent divergent plate boundaries.
• Seafloor spreading produces new oceanic lithosphere.

3. Divergent Boundaries
• Rift valleys - deep faulted structures found along the axes of divergent plate boundaries. Can develop on seafloor or on land.

4. Average spreading rates around entire world are about 5 cm per year
Divergent Boundaries
Average spreading rates around entire world are about 5 cm per year

5. Examples: the Andes, Cascades, and the Sierra Nevadas.
CONVERGENT BOUNDARIES
Oceanic-Continental
• Denser oceanic slab sinks into the asthenosphere.
• Pockets of magma develop and rise.
• Continental volcanic arcs form in part by volcanic activity caused by the subduction of oceanic lithosphere beneath a continent.
Examples: the Andes, Cascades, and the Sierra Nevadas.

6. Examples: the Aleutian, Mariana, and Tonga islands.
CONVERGENT BOUNDARIES
Oceanic-Oceanic
• Two oceanic slabs converge and one descends beneath the other.
• Often forms volcanoes on the ocean floor.
• Volcanic island arcs form as volcanoes emerge from the sea.
Examples: the Aleutian, Mariana, and Tonga islands.

7. CONVERGENT BOUNDARIES
Subduction zone - one oceanic plate is forced down into the mantle beneath a second plate.
If Mount Everest, the highest mountain on Earth at 8,848 meters (29,030 ft), was set in the deepest part of the Mariana Trench, there would be 2,076 meters (6,810 ft) of water left above it

8. CONVERGENT BOUNDARIES
Continental-Continental
• two continental plates collide.
• Can produce new mountain ranges, such as the Himalayas.

9. CONVERGENT BOUNDARIES
Mount Everest

10. Transform Fault Boundaries
Transform fault boundary - plates grind past each other without destroying the lithosphere.
Most join two segments of a mid-ocean ridge.

11. ACTIONS AT PLATE BOUNDARIES

12. Section 9.3 Review
A zone where oceanic crust dives beneath continental crust is called a zone of
reduction.
divergence.
subduction.
rifting.