9-3 Theory of Plate Tectonics The Earth’s crust and upper mantle ( lithosphere) are broken into huge pieces called plates Earth’s lithospheric plates move slowly relatively to each other, driven by convection currents in the mantle.

Causes of plate motion THE CONVECTION CURRENTS Wilson said convection currents of Earth drive plate motion. Hot material (magma) of deep mantle moves upward at the same time cooler , denser slabs of oceanic lithosphere sink into the mantle. Creates convection currents beneath the plates that cause the plates to move.

Effects of plate motion The plate move on average of 5 cm / yr. The result of this motion could be earthquakes, volcanoes and mountain building.

Divergent Boundaries Boundary ( margin) between two plates that are moving apart or rifting This rifting causes spreading of the seafloor.

Divergent Boundaries Their spreading centers are located along the crests of mid-ocean ridges. Others are spreading on the mountain. These are constructive plate margins because produce new oceanic lithosphere.

Formation of a rift valley When the forces of plate motion begin to stretch apart the lithosphere, hot rock plumes rise up from the mantle, bending the crust upward , weakening and fracturing it. The fractures let magma to reach the surface. Ex ; Rhine Valley in Europe and the Great Valley in East Africa.

Convergent Boundaries Boundaries between two plates that are colliding and producing features including ; Trenches Volcanoes Mountain ranges

They are three types 1)Oceanic-continental Ocean plate colliding with a less dense continental plate Continental volcanic arc; Volcanic mountains produced in part by the subduction of oceanic lithosphere.

Andes Mountains, South America

2) Oceanic-Oceanic ocean plate is colliding with another ocean plate The less dense plate slides under the more dense plate creating a subduction zone called a Trench.

Aleutian Islands, Alaska

3) Continental-continental A continental plate colliding with another continental plate resulting a continental volcanic arc develops along the continent margin. Continental lithosphere is less dense of oceanic lithosphere, no subduction happens but a collision between two continents and forming complex mountains.

3)Transform Fault Boundaries Boundary between two plates that are sliding past each other without destroying the lithosphere. Earthquakes creating along this fault Ex: San Andreas fault in California where Pacific plate is sliding over North American plate.

San Andreas Fault, CA

Questions... What are the three types of boundaries? Give an example of each. Which boundary has a subduction zone? What occurs at a subduction zone? What causes plates to move? How is a convection current formed?