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 Plate Characteristics
•Spreading boundary
•Rift Valleys
•volcanoes, earthquakes
Divergent Plates examples:
Rift Valley of eastern Africa
Mid-Atlantic ridge
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
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.