Evidence for Plate TectonicsSeafloor 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 SpreadingSeafloor 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 interact3 types

Break up of Pangaea

Lithosphere plate boundaries

Fig. 3-20

1. Divergent BoundariesPlaces 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 BoundariesWhere 2 plates move toward each other

3 types, classified by type of crust:

A. Oceanic-OceanicMay result in subductionCreates a volcanic island arc

and deep-sea trench

B. Oceanic-ContinentalSubduction of oceanic produces a

trench and a volcanic arc (mountain range on continent)

C. Continental-ContinentalForms mountain ranges such as

the Himalayas

Converging Margins: India-Asia Collision

Sea-floor Spreading and Plate Boundaries

3. Transform BoundariesCrust is not formed or destroyed, but deformed or fractured

Usually offset ocean ridgesRarely on continents – San Andreas Fault is the best known exception

responsible for most of the earthquakes that strike CA each year

Transform Faults