Transcript Plate Boundaries…

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
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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
Convergent Boundaries (aka destructive boundaries)
- Continental meets a friend…
 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
Transform Boundaries
 Occur in few places
 To accommodate lateral motion
(plates sliding past one another)
 Examples:
 San Andreas Fault in California –
continental transform boundary
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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