Plate Boundaries

MAJOR TECTONIC PLATES

The Mechanism of Plate Tectonics

• as the material in the asthenospehre is heated, it rises

• as it cools, it is moved horizontally and sinks – pulling along the continental and ocean crust

• movement of material in the asthenosphere is called convection cells

There are three types of boundaries wherecrustal plates meet:• divergent• convergent• transform

Divergent Plate Boundaries• also called rift zone• plates moving apart• upwelling part of convection cell• sea floor is created• forms mid-ocean ridges

Divergent Plate Boundaries

• found at most mid-ocean ridges• ex: Mid-Atlantic Ridge

Divergent Plate Boundaries

Convergent Plate Boundaries• also called subduction zones

• plates move toward one another• downwelling part of convection cell• sea floor destroyed• forms trenches and volcanic island arcs

Convergent Plate Boundaries

Transform Plate Boundaries• plates move past each other• faults are formed

Transform Plate Boundaries

Example: San Andreas Fault

Ring of Fire• around Pacific Ocean• greatest quantity of earthquakes and volcanoes• because edges of Pacific Ocean are boundaries

of Pacific plate

Ring of Fire

• earthquakes around the world