Transcript Chapter 4

Ch – 15 Plate Tectonics
Fig. 6.10, p.139
Plate tectonics map showing Somali Plate
What happens at a divergent
plate boundary
Sea Floor Spreading
• Two plates move apart
• Mantle material upwells to create new
seafloor
• Mid-Oceanic ridges (underwater
mountain range) develop along welldeveloped divergent boundaries
• Mid-Atlantic Ridge
• East Pacific Rise
Figure 15.10
Sea Floor Spreading on Land
• Sea floor spreading
adds thin, lowelevation ocean crust
to landmass.
Eventually water fills
in
• Arabian peninsula
split from African
continent
• Process continues in
East Africa rift valleys
(note lakes filling in
low lying ocean crust)
• Somali Plate?
What happens at a
convergent plate boundary
• Subduction
• Oceanic-continental convergence
• Denser oceanic slab sinks into the
asthenosphere
• Pockets of magma develop and rise
• Continental volcanic arcs form (e.g.
Andes, Cascades)
Figure 15.14a
What happens at a
convergent plate boundary
• Subduction (Cont’d)
• Oceanic-oceanic convergence
• Two oceanic slabs converge and the
older, denser one descends beneath
the younger, more buoyant one.
• Forms volcanoes on the ocean floor
• Volcanic Island Arcs forms as
volcanoes emerge from the sea
• Examples include the Aleutian,
Mariana, and Tonga islands
Figure 15.14b
Fig. 6.10, p.139
Ring of Fire
What happens at a
convergent boundary
Continental Collision (no
subduction)
• Continental-continental convergence
• When subducting plates contain
continental material, two continents
collide
• Can produce non-volcanic mountain
ranges such as the Himalayas
Figure 15.14c
What happens at Transform Fault
Boundaries
Conservative boundary (no loss or gain
of lithosphere)
Plates slide past one another
• Most transform faults join two segments
of sea-floor spreading
• Significant non-oceanic tranform fault
boundaries include
• San Andreas Fault,
• Alpine Fault
• Anatolian Fault (Turkey)
Figure 15.16
Figure 15.17
Modern discoveries supporting Plate
Tectonic Theory
• Mid-ocean ridges – underwater mountain chains
that circle the globe and often mimic the shape
of the coastline
• Symmetry of magnetic polarity across mid-ocean
ridges
• Distribution and depths of earthquakes and
volcanoes
• Relatively young age of the oceanic crust (less
than 180 million years)
• Lack of deep-ocean sediment
Paleomagnetism: study of ancient magnetic fields
•The inner and outer core of the
Earth cause the earth to act like
a magnet, with north and south
poles
•Iron minerals orient themselves
towards the north pole as lava
solidifies on the earth’s surface
and become fixed in that
direction.
•Ancient lavas tell us the
strength and direction of the
earth’s magnetic field during
geologic history.
Magnetic polarity reversals
•At irregular time intervals, the “magnet turns around”.
•Lava that solidified during these reversals allows us to
determine the date of these reversals.
•For example, volcanic rocks dated to 760,000 years ago
in several locations, including California, show evidence of
reversed magnetic polarity.
Magnetic polarity reversals on the ocean floor
•Development of oceangoing magnetometers
allowed remote mapping
of the magnetic field of
the ocean crust.
•Symmetrical pattern at
mid-ocean ridges could
best be explained by the
sea-floor spreading
hypothesis, which was
still being debated at the
time.
Paleomagnetic reversals recorded by basalt
flows at mid-ocean ridges
Age of the Sea Floor
Evidence from ocean drilling
• Age of deepest sediments indicates ocean
crust much younger than continental crust,
which supports both subduction and seafloor spreading hypotheses.
• Lack of sediments at at mid ocean ridges
supports seafloor spreading.
• Age distribution of ocean crust indicates
location of divergent and convergent
boundaries
Hot spots and mantle plumes
• Caused by rising plumes of mantle
material
• Volcanoes can form over them
(Hawaiian Island chain)
• Originate at great depth, perhaps at the
mantle-core boundary
Figure 15.18
Earthquake depths and distributions
• Shallow
earthquake (red)
occur along the
oceanic ridge
systems.
• Deep
earthquakes
(green-blue)
occur on the
“Ring of Fire” .
• Earthquake
depth increases
in direction of
subducting
plate.
What drives plate motion?
• Convective flow in the mantle is the underlying driving
force for plate movement.
• Mantle convection and plate tectonics are part of the
same system. Warm, buoyant rock rises and “cold”
dense rock, e.g. subducting plates, sink.
• Plate tectonic movements are ultimately due to
unequal heat distribution in the earth’s interior.
Slab-pull, ridge-push, slab suction
• Descending “cold”oceanic crust pulls the plate in
direction of subduction. This is probably the main
mechanism of plate motion
• Elevated ridge system pushes the plate away from
ridge
• Descending plate causes suction which pulls the two
sides closer.
Plate-mantle convection: Descending
plates, along with rising mantle plumes
cause convection within the mantle