ACTIONS AT PLATE BOUNDARIES

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.

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

Average spreading rates are about 5 cm per year Divergent Boundaries Average spreading rates are about 5 cm per year

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

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.

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.

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

CONVERGENT BOUNDARIES Mount Everest

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

ACTIONS AT PLATE BOUNDARIES

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