Evidence for Plate Tectonics Seafloor mapping using sonar initiated during World War II provided evidence of drifting continents This caused scientists to reexamine continental drift

There are a dozen or so major plates and several smaller ones Move in different directions and at different rates

Sea floor Spreading Seafloor mapping revealed mid-ocean ridges and ocean trenches Harry Hess (1960) proposed the idea of seafloor spreading Ocean crust is created at mid-ocean ridge It is split and carried away then replaced by rising magma

Sea-floor Spreading and Plate Boundaries

Older ocean floor is destroyed at oceanic trench (deepest parts of the ocean) The plate bends downward and slowly plunges back into the earth in a process called subduction This takes place at the subduction zone Most earthquakes occur at edges of plates (plate boundaries)

Sea-floor Spreading and Plate Boundaries

Plate Boundaries Where plates interact 3 types

Break up of Pangaea

Lithosphere plate boundaries Fig. 3-20

1. Divergent Boundaries Places where 2 plates are moving apart Most found on the seafloor, where they form ocean ridges Where new ocean crust is formed (basalt)

High heat flow, volcanism, and earthquakes Causes an ocean basin to grow wider Can occur on a continent, such as East Africa (Rift Valley) May eventually lead to formation of a new ocean

2. Convergent Boundaries Where 2 plates move toward each other 3 types, classified by type of crust: A. Oceanic-Oceanic May result in subduction Creates a volcanic island arc and deep-sea trench

B. Oceanic-Continental Subduction of oceanic produces a trench and a volcanic arc (mountain range on continent) C. Continental-Continental Forms mountain ranges such as the Himalayas

Converging Margins: India-Asia Collision

Sea-floor Spreading and Plate Boundaries

3. Transform Boundaries Crust is not formed or destroyed, but deformed or fractured Usually offset ocean ridges Rarely on continents – San Andreas Fault is the best known exception responsible for most of the earthquakes that strike CA each year

Transform Faults