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 involved There 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.