Download PowerPoint

17.4
THEORY OF PLATE TECTONICS
• The theory of plate tectonics states that the
lithosphere is divided into 12 large sections (plates)
and about 20 smaller ones.
• These plates ‘float’ on the more dense, fluid-like
asthenosphere
VOCABULARY
theory of plate
tectonics
divergent boundary
rift valley
convergent boundary
subduction zone
oceanic–oceanic
convergent boundary
oceanic–continental
convergent boundary
continental–continental
convergent boundary
transform boundary
strike–slip fault
17.4
THEORY OF PLATE TECTONICS
• Three types of boundaries exist where tectonic
plates meet. These create Earth’s geological
features and events.
• Ridges, rifts, volcanoes and
earthquakes are created at divergent
boundaries, where plates are moving
away from each other.
• At convergent boundaries (where
plates move toward each other), we
find mountains, trenches, subduction
zones, volcanoes, and earthquakes,
depending on the types of plates
involved.
• Earthquakes and strike-slip faults are
created along transform boundaries,
where plates move past each other in
opposite directions.
VOCABULARY
theory of plate
tectonics
divergent boundary
rift valley
convergent boundary
subduction zone
oceanic–oceanic
convergent boundary
oceanic–continental
convergent boundary
continental–continental
convergent boundary
transform boundary
strike–slip fault
17.4
Theory of Plate Tectonics
THREE TYPES OF
PLATE BOUNDARIES
17.4
Divergent Boundaries
 Continental Rifts
• When spreading centers develop within a continent, the
landmass may split into two or more smaller segments,
forming a rift.
East African
Rift Valley
17.4
review
Convergent Boundaries
 A subduction zone occurs when one oceanic
plate is forced down into the mantle beneath a
second plate.
 Oceanic-Continental
• Denser oceanic slab sinks into the asthenosphere.
• Pockets of magma develop and rise.
• Continental volcanic arcs form in part by volcanic activity
caused by the subduction of oceanic lithosphere beneath a
continent.
• Examples include the Andes, Cascades (Juan de Fuca), and
the Sierra Nevadas.


Cascadia Subduction Zone
West of Vancouver Island, and extending from the north tip of the
Island to northern California, the oceanic Juan de Fuca plate is
moving towards North America at about 2-5 cm/year. This region
is called the Cascadia subduction zone. Here, the much smaller
Juan de Fuca plate is sliding (subducting) beneath the continent
(it is about 45 km beneath Victoria, and about 70 km beneath
Vancouver). The ocean plate is not always moving though. There
is good evidence that the Juan de Fuca and North America
plates are currently locked together, causing strain to build up in
the earth's crust. It is this squeezing of the crust that causes the 300
or so small earthquakes that are located in southwestern British
Columbia each year, and the less-frequent (once per decade,
on average, damaging crustal earthquakes (e.g., a magnitude
7.3 earthquake on central Vancouver Island in 1946). At some
time in the future, these plates will snap loose, generating a huge
offshore "subduction" earthquake - one similar to the 1964 M=9.2
Alaska earthquake, or the 1960 M=9.5 Chile earthquake. Current
crustal deformation measurements in this area provide evidence
for this model. Geological evidence also indicates that huge
subduction earthquakes have struck this coast every 300-800
years.
Convergent Boundaries
OCEANIC-CONTINENTAL
CONVERGENT BOUNDARY
17.4
17.4
Convergent Boundaries
 Oceanic-Oceanic
• Two oceanic slabs converge and one descends
beneath the other.
• This kind of boundary often forms volcanoes on the
ocean floor.
• Volcanic island arcs form as volcanoes emerge
from the sea.
• Examples include the Aleutian, Mariana, and
Tonga islands.
17.4
Convergent Boundaries
OCEANIC-OCEANIC
CONVERGENT BOUNDARY
17.4
Convergent Boundaries
 Continental-Continental
• When subducting plates contain continental
material, two continents collide.
• This kind of boundary can produce new
mountain ranges, such as the Himalayas.
17.4
CONVERGENT BOUNDARIES
Continental-Continental
Convergent Boundary
17.4
CONVERGENT BOUNDARIES
‘Collision of India and Asia’
17.4 Transform Boundaries
 At a transform fault boundary, plates grind past
each other without ‘destroying’ the lithosphere.
 Transform faults • When the faults ‘overcome’ the
friction holding them in place, the
‘slip’ causes earthquakes
17.4 Transform Boundaries
TRANSFORM
FAULT
BOUNDARIES
18.1
CAUSES AND EFFECTS OF PLATE
MOVEMENT
• Scientists now believe that three forces work together
to move the tectonic plates:
1. mantle convection
2. ridge push
3. slab pull
VOCABULARY
mantle convection
ridge push
slab pull
•Forces at plate boundaries produce landscape features such as mountains,
trenches, and island arches.
18.1
CAUSES AND EFFECTS OF PLATE
MOVEMENT
• Uplifted mountains, volcanic belts, and island arcs
are found parallel to convergent boundaries.
• Ocean trenches lie along convergent boundaries.
• Mid-ocean ridges and rift valleys mark divergent
boundaries.
VOCABULARY
mantle convection
ridge push
slab pull
18.1
CAUSES AND EFFECTS OF PLATE
MOVEMENT
 Scientists believe tectonic plates are moved by three forces:
o Mantle convection is the idea that there is a treadmill-like
current in the asthenosphere that moves the crust that sits
above it. (hot magma rises in one area and cool magma sinks
in another)
• Convective
flow is the
motion of matter
resulting from
changes in
temperature.
18.1
CAUSES AND EFFECTS OF PLATE
MOVEMENT
Convection causes Ridge Push and Slab Pull...
 Scientists generally agree that convection
occurring in the mantle is the basic driving
force for plate movement.
Ridge
Push
Slab
Pull
Ridge
Push
Slab
Pull
Ridge
Push
18.1
CAUSES AND EFFECTS OF PLATE
MOVEMENT
 Scientists believe tectonic plates are moved by three forces:
o Ridge Push is the idea that the hot magma pushing up the
mid-ocean ridges is also pushing the ocean plates apart.
18.1
CAUSES AND EFFECTS OF PLATE
MOVEMENT
 Scientists believe tectonic plates are moved by three forces:
o Slab Pull is when one plate slides underneath the other. The
subduction zone causes the bottom plate to be pushed back
into the mantle and this pulls the plate away from the midocean ridge.
18.1
CAUSES AND EFFECTS OF PLATE
MOVEMENT
 Mountains form at convergent boundaries.
o Ocean – continent boundary causes mountains to form along
the coast. These often push land that was once underwater up
high causing ocean fossils to be found in the mountains.
18.1
CAUSES AND EFFECTS OF PLATE
MOVEMENT
 Mountains form at convergent boundaries.
o Continent – continent boundary causes mountains to form
from the uplifting of the two continent edges. This can also
cause former coast lines and marine fossils to be found in the
mountains.
18.1
CAUSES AND EFFECTS OF PLATE
MOVEMENT
• Rifts form at
divergent
boundaries.
• If this happens on land
a rift valley forms.
• Often volcanoes also
form along these rifts
since the crust is
thinner.
CAUSES AND EFFECTS OF PLATE
18.1
MOVEMENT
 Trenches can form at a convergent boundary.
o They occur where one plate is subducted under the other. If
there are mountain chains along a coast then you know that a
few hundred kilometers out into the ocean that there must be a
trench.
18.1
CAUSES AND EFFECTS OF PLATE
MOVEMENT
 Trenches can form at a convergent boundary.
o When trenches form at oceanic-oceanic convergent
boundaries, a deep ocean trench is also formed
o The Mariana trench is Earth’s deepest trench, still sinking, and
is ~ 11 km below the surface of the ocean!
QUESTIONS
PG. 508 # 1 – 9
PG. 521 # 2