Transcript chapter 15A - plate tectonics 1

From Continental Drift Hypothesis to
Plate Tectonic Theory
• Observation: Some of the boundaries of the continents
look as though they had once fit together
• Initial hypothesis, put forth by A. Wegener: In the
geologic past, all the continents formed one
supercontinent, which subsequently broke up and
drifted apart.
Supporting evidence: fossils of the same
type found at these now-separated
continental boundaries
Supporting evidence: evidence of similar
climates at the now-separated areas
Supporting evidence: matching rock types
and orientation of mountain ranges at the
now-separated areas
Continental Drift Hypothesis
Rejected!
How did the continents move through the oceans?
• Tidal influence of the moon? Theoretical
calculations proved this to be false.
• Continental crust plows through or slides over
ocean crust? Observations proved this false.
• The conclusion of the scientific community at the
time was that the continental drift hypothesis, as
presented by Wegener could not be true.
Sea-floor spreading –modifying the hypothesis
The sea-floor spreading hypothesis provided a new
mechanism for the change in the position of the
continents, based largely on post-WW2 observations:
– mapping of underwater mountain ranges (oceanic
ridges) that circle the globe, often parallel to
continental boundaries
– dredging of sea floor sediment and rocks indicated
the age of the oldest ocean crust was much younger
than that of continental crust.
– Recurring patterns of earthquakes and volcanoes in
places such as the Circum-Pacific Belt and along
the oceanic ridges.
Sea Floor Spreading Hypothesis led
to the present Plate Tectonic Theory
The Major Tectonic Plates
Divergent Boundaries
Plates move apart. The mechanism called
Sea-Floor Spreading
Sea-Floor Spreading
• Two plates move apart, creating a rift
valley between them
• Magma (molten rock) from the
asthenosphere pushes into the rift
and creates new seafloor
• Oceanic ridges (underwater mountain
range) develop along well-developed
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. In time,
water fills in and an
ocean basin will
develop
• Arabian peninsula split
from African continent
• Process continues in
East Africa rift valleys
(note lakes filling in
low lying ocean crust).
Some geologists are calling this the divergent
boundary of the Somali Plate
Forming a Divergent Boundary
Convergent Boundaries
• These are boundaries where plates come
together and lithosphere is being deformed
and/or destroyed
• There are 3 types of convergent
boundaries
• In 2 of the 3 types the process that occurs
is called subduction, where oceanic
lithosphere on one tectonic plate is thrust
into the mantle beneath another plate.
Oceanic-continental
convergence
• Denser oceanic slab sinks into
the asthenosphere (subduction)
• Pockets of magma develop and
rise
• Continental volcanic arcs form
(e.g. Andes, Cascades).
Figure 15.14a
Motion at Plate Boundaries
Oceanic-oceanic convergence
• Two oceanic slabs
converge and the
older, denser one
subducts beneath
the younger, more
buoyant one.
• Trench and volcanic
island arc forms as
volcanoes emerge
from the sea.
• Examples include the
Aleutian, Mariana,
and Tonga islands,
and those shown in
the map.
Motion at Plate Boundaries
Continental-continental convergence
• When converging plates contain
continental material at their respective
boundaries, subduction cannot occur, as
the plates are too buoyant to sink.
• The continents collide, causing the crust
to shorten and thicken
• This process produces high-elevation,
non-volcanic mountains over a large area,
such as we see in the Himalayas today.
Figure 15.14c
Convergent Margins: India-Asia Collision
The Major Tectonic Plates
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 transform fault
boundaries include
• San Andreas Fault,
• Alpine Fault
• Anatolian Fault (Turkey)
Figure 15.16
Figure 15.17
Breakup of Pangaea