Plate Boundaries  According to the Plate tectonic theory, three boundaries exist at the edges of each tectonic plate. 1) Divergent Boundary (Ridge) 2) Convergent Boundary (Trench) 3) Transform Boundary Reference: pages Figure 19.17, pg. 528

1) Divergent Boundary  Plates move apart, resulting in upwelling of molten material from the mantle to create new ocean floor.  Features on the ocean floor called Ridges, show this form of plate movement.  Tensional forces cause the plates to move apart. OceanOcean Crust MohoMagmaContinental Crust  Divergent plate boundaries are referred to as constructive margins since new oceanic crust is being formed there.  Examples include; Mid-Atlantic ridge and Mid-Indian ridge.

1) Divergent Boundary A If tensional forces, due to the motion of convection currents in the upper mantle, begin beneath continents, it can cause continents to split into two or more smaller segments. B A spreading center is formed and a rift valley is formed as a result of the associated faulting (normal) and thinning of the crust. Eventually the thinning becomes significant enough for ocean water to move into the rift valley A B C D Evolution of Rift Valley into a Divergent Boundary: (Page )

1) Divergent Boundary C Ridge (Rift) volcanism and seafloor spreading begin and a divergent plate boundary is produced. D As both continue, the segments (i.e., landmasses) on both sides of the ridge continue to move apart from one another. A global example of a current rift valley is the East African rift valley. A B C D Evolution of Rift Valley into a Divergent Boundary: (Page ) Examples include; East African Rift Valley

A) Ocean – Ocean Convergent Boundary  Features called ocean trenches are formed at these boundaries. Lithosphere is destroyed as one oceanic slab descends beneath another.  Example include the Japan island arc and the Japan trench.  Compressional forces cause plates to move together, causing one slab of lithosphere to be consumed into the mantle initiating volcanic activity which creates volcanoes to form on the ocean floor.  Fluid, basaltic magmas feed the volcanic islands and form shield volcanoes. 2) Convergent Boundaries: (3 types)

B) Ocean – Continent Convergent Boundary  Deep ocean trenches form adjacent to the zone of subduction. These trenches can range up to thousands of kilometers long and 8 – 10 km deep. Lithosphere is destroyed as one oceanic slab descends beneath another.  At depths of about 100 km the oceanic plate and parts of the mantle partially melt producing viscous magmas. This molten rock rises slowly where it cools and solidifies at depths producing plutons. However, some magma may reach the surface and erupt through composite volcanoes as violent volcanic eruptions.  Compressional forces cause an ocean plate and a continent plate to move together, causing the more dense ocean plate to sink into the asthenosphere. This region where the ocean plate sinks is called a subduction zone. 2) Convergent Boundaries: (3 types)

B) Ocean – Continent Convergent Boundary  If the subduction occurs beneath continental crust, a continental volcanic arc is produced (such as the Rocky mountains in North America and the Andes mountains in South America). 2) Convergent Boundaries: (3 types)

C) Continent – Continent Convergent Boundary  Such a collision occurred when India collided with Asia forming the Himalayan mountains and also during the formation of the Appalachian mountains  Compressional forces cause two continental plates to move together. Because of the low density of continental crust neither plate will subduct and the two plates ram into one another causing the continental margins to compress, deform, and uplift forming folded mountains. 2) Convergent Boundaries: (3 types)

 Where lithospheric plates slide past one another in a horizontal manner, a transform fault is created. Earthquakes along such transform faults are shallow focus earthquakes. Lithosphere is not created or destroyed at these boundaries.  One of the largest such transform boundaries occurs along the boundary of the North American and Pacific plates and is known as the San Andreas Fault. Here the transform fault cuts through continental lithosphere.  Most transform faults occur where oceanic ridges are offset on the sea floor. Transform Fault Fracture Zone Oceanic Ridge (spreading center) 3) Transform Boundary  The convection currents exert shear forces on both plates forcing them to slide past one another.

Sample Problem Using a labelled diagram, briefly describe what happens at a mid-ocean ridge. OceanOcean Crust MohoMagmaContinental Crust Answer: Tensional forces cause the plates to move apart resulting in upwelling of molten material from the mantle to create new ocean floor.