Divergent Boundaries (aka constructive boundaries) Rising convection currents force 2 plates apart Release of pressure on asthenosphere causes it to become molten and to rise Crust spreads New lithospheric material created

Divergent Boundaries (aka constructive boundaries) - Oceanic Buddies Seafloor spreading As plates made of oceanic crust pull apart, crack in the ocean floor appears Magma oozes up from the mantle  raised ridge (mid-ocean ridge) Magma also spreads outward, forming new ocean floor and new oceanic crust Characteristics of mid-oceanic ridges Shallow earthquakes Minor lava flows The seafloor at the ridges is higher than the surrounding abyssal plain because the rocks are hot and thus less dense and more buoyant. As they move away from the spreading center, they cool and become denser and less buoyant. Example: Mid-Atlantic Ridge (middle of Atlantic Ocean, separating NS America in the west from Eurasian and African plates in the east)

Divergent Boundaries (aka constructive boundaries) - Continental Buddies When two continental plates diverge, a valleylike rift develops As the crust widens and thins, valleys form in and around the area Volcanoes too (may become increasingly active) Early in rift formation, streams and rivers flow into the low valleys and long, narrow lakes can be created. Eventually, the widening crust along the boundary may become thin enough that a piece of the continent breaks off, forming a new tectonic plate At this point, water from the ocean will rush in, forming a new sea or ocean basin in the rift zone Example: Great African Rift Valley

Convergent Boundaries (aka destructive boundaries) - Oceanic meets a foe… Subduction zones Dense oceanic crust dives beneath more buoyant continental crust Characteristics of such zones: A very deep ocean trench next to a high continental mountain range Large numbers of earthquakes that progress from shallow to deep Large numbers of intermediate composition volcanoes. Example: The Andes

Convergent Boundaries (aka destructive boundaries) - Oceanic meets a friend… Subduction zone Both very dense but usually older oceanic crust subducts because colder and slightly denser Strong dragging force creates pronnounced trench Example: Marianas Trench (between Pacific and Philippine plates)

Convergent Boundaries (aka destructive boundaries) - Oceanic meets a friend… Frictional heat at oceanic boundaries high  abundant molten magma  string of volcanoes following trench  series of volcanic islands = island arc Examples: Japanese Ryukyu Islands; Kuril Islands

Pileup of continental material Both pieces of crust are buoyant and are not easily subducted Example: Himalayan mountain range (between Indian and Asian plates) Numerous shallow earthquakes occur But little volcanism Convergent Boundaries (aka destructive boundaries) - Continental meets a friend…

Transform Boundaries Occur in few places To accommodate lateral motion (plates sliding past one another) Examples: San Andreas Fault in California – continental transform boundary Frequent, shallow earthquakes occur Little associated volcanism or topographic relief. Alpine Fault in New Zealand similar to the above Most transform boundaries occur not on land but in short segments along mid-ocean ridges