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 spreadsNew lithospheric material created

Divergent Boundaries (aka constructive boundaries)

- Oceanic Buddies Seafloor spreadingAs 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 developsAs the crust widens and thins, valleys form in and

around the areaVolcanoes 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 zonesDense oceanic crust dives beneath more buoyant

continental crustCharacteristics 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 zoneBoth very dense but usually older oceanic crust

subducts because colder and slightly denserStrong dragging force creates pronnounced

trenchExample: 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 arcExamples: 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 BoundariesOccur in few placesTo 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