Theory of Plate Tectonics

The Modern Synthesis

The modern synthesis:

The Earth’s surface is broken into about a dozen major plates

Plates “float” or “slide” on

a highly viscous layer

-- the asthenosphere

Plates are made up of both continental and oceanic

crust,

Main action in plate tectonics occurs at the

edges of the plates

Over the course of geologic time these

boundaries are the site of the splitting,

shifting and crumpling of the continents

In a human time scale these boundaries are

marked by the presence of volcanoes and the occurrence of

earthquakes

Volcanoes of the World (Smithsonian Institution Global Volcanism Program)

Types of Plate Motion

Divergent Plates

Divergent Plate Characteristics

•Spreading boundary

•Rift Valleys

•volcanoes, earthquakes

Divergent Plates examples:

Rift Valley of eastern Africa

Mid-Atlantic ridge

Heezen and Tharp

Convergent Plates

At convergent plate boundaries the response to

plate collision will vary with the type of crust

involvedThere are three possible

combinations:

Oceanic crust converging with

oceanic crust:

characteristics•subduction of one plate

•formation of an island arc

•Volcanoes, earthquakes

Oceanic crust converging with oceanic crust:

example Japanese Islands

Oceanic crust converging with continental crust:

Characteristics

•Formation of mountain ranges

•Volcanoes, earthquakes•Trenches

Oceanic crust converging with

continental crust: example

Andes Mountains

Continental crust converging with continental crust:

Characteristics

•Overriding of one plate•formation of mountain ranges

•earthquakes

Continental crust converging with

continental crust: example Himalayas

Transform Boundaries

Characteristics

•“side-slip boundary”

•create fault zones

•earthquakes

Transform Boundaries

example San Andreas

Recap Picture of the Three Types of Tectonic Plate Boundaries

Cross section by José F. Vigil from This Dynamic Planet

250 million years

150 million years

50 million years

Causes of Plate Motions

The Driving Force of Plate Movements is Mantle Convection

The rising part of a convection current causes up and out forces driving

plates away from one another by a process called

RIDGE PUSH

The downward part of a convection current

causes downward forces pulling plates down at convergent boundaries.

The weight of the subducting plate helps

pull the remaining lithosphere down. This

process is called SLAB PULL

Most scientists agree that convection currents in the mantle

cause plate movement but it is debated; what causes these

convection currents and whether or not these currents change

positions.