Download Theory of Plate Tectonics II

Plate Tectonics
Plate Tectonics
Types of
Plate
Boundaries
Divergent
Convergent
Transform
 Plates move apart, resulting
in upwelling of material from
the Mantle to create new sea
floor.
Divergent
 Plates move together,
causing one of the slabs of
lithosphere to be consumed
into the Mantle as it decends
beneath the overriding plate.
Convergent
 Plates slide past each other,
without creating or destroying
lithosphere.
Transform Fault
W. W. Norton. Modified from Cox and Hardt, 1986.
Each plate is bounded by a combination of these
types of boundaries.
Transform
Divergent
Convergent
We can’t observe these
different types of plate
boundaries evolving over
time, because the time
scale is too great, but:
We can see these
different types of plate
boundaries in different
stages of development by
going to different
locations on the Earth.
Divergent Plate Boundaries
Most divergent plate boundaries are situated
at the crests of mid-ocean ridges.
Sea Floor Spreading
As plates move away from the ridge axis, the gaps
are filled with molten rock that oozes up from the
hot asthenosphere.
Atlantic Ocean Floor
The material cools slowly to produce new sea floor, and
injections of magma add new crust between the
diverging plates.
Atlantic Ocean Floor
This mechanism has produced the floor of the Atlantic
Ocean during the past 165 million years.
Some spreading
centers are much
younger than the
mid-Atlantic ridge.
The Red Sea is the
site of a recently
formed divergent
plate boundary.
Here the Arabian
peninsula separated
from Africa (about 20
million years ago) and
began to move toward
the northeast.
The Red Sea might
resemble the way the
Atlantic ocean looked
in its infancy.
Development of a Spreading Center
When a spreading
center develops
within a continent,
the landmass
splits into smaller
segments as
Wegener
proposed for the
breakup of
Pangaea.
Development of a Spreading Center
Fragmentation of a
continent is initiated
by an upward
movement of hot
rock from below.
This causes
upwarping of the
crust above the hot
rising material.
Crustal stretching
associated with this
doming generates
tensional cracks.
Large downfaulted
valleys are generated
by this process.
The East African
Rift Valleys are an
example of this type
of feature.
Rift
Valley
East African
Rift Valleys
If this spreading
process continues, the
rift valley will become
wider and deeper, and
eventually be
transformed into an
ocean.
Fig. 11.34
U.S. Geological Survey
Fig. 11.33
W. W. Norton
Development
of a
Spreading
Center
Convergent
Plate
Boundaries
Convergent
Plate
Boundaries
Oceanic - Continental
Oceanic - Oceanic
Continental - Continental
The Earth’s Crust
Oceanic - Continental Convergent Plate Boundary
Types of
Plate
Boundaries
Divergent
Convergent
Transform