3 TYPES OF PLATE
BOUNDARIES
1. Divergent
2. Convergent
3. Transform
Divergent plate boundaries
Almost all the Earth’s new crust forms at
divergent boundaries, but most are not well
known because they lie deep beneath the
oceans.
These are zones where two plates move away
from each other, allowing magma from the
mantle to rise up and solidify as new crust.
One divergent plate
boundary is actively
forming the Red Sea.
Although the Arabian
peninsula and Africa were
once linked to form a
single continent, they are
now being ripped apart.
The white arrows show
the directions the two
plates are moving. Here,
you can see that a new
ocean, the Red Sea, is
being formed as they
separate.
Divergent boundaries build chains of volcanoes and
rift valleys called a mid-ocean ridge.
Mid-ocean ridges are found in the oceans-they are
like mountain ranges on the ocean floor created by
the new lava that is bubbling up!
Little by little, as each batch of molten rock erupts at
the mid-ocean ridge, the newly created oceanic plate
moves away from the ridge where it was created.
MID-OCEAN RIDGES
Atlantic ocean is widening as the Pacific is getting smaller
EXAMPLE OF A
MID-OCEAN RIDGE
Mid-Atlantic Ridge
a mostly underwater
mountain range of the
Atlantic Ocean and
Arctic Ocean that runs
from 87° N (about 333 km
south of the NorthPole)
to sub antarctic Bouvet
Island at 54° S. The
highest peaks of this
mountain range extend
above the water mark, to
form islands.
Pillow lava: new basalt rock formed at
a mid-ocean ridge
3 TYPES OF CONVERGENT
BOUNDARIES
1. OCEAN-CONTINENTAL
2. CONTINENTAL-CONTINENTAL
3. OCEAN-OCEAN
A. Ocean-Continental Collision
In a contest between a dense oceanic
plate and a less dense, buoyant
continental plate, guess which one will
sink?
Ocean plate!
The dense, leading edge of the oceanic
plate actually pulls the rest of the plate into
the flowing asthenosphere and a
subduction zone is born!
At a subduction zone, where the two
plates intersect, a deep trench forms.
These trenches are called deep oceanic or
deep sea trenches.
The next image shows a slice through
the Earth at a convergent plate
boundary. This view illustrates just
one of the ways that plates behave
when they collide. In this case,
subduction is occurring-one plate is
pulled beneath another, forming a
deep ocean trench.
The long, narrow zone where the two
plates meet is called a subduction
zone.
Characteristics of a subduction zone
(ocean-continental collision)
A subduction zone can be located by looking
for curved volcanic mountain ranges on the
land with deep trenches in the water.
(no islands created)
The Cascadia subduction zone is a very long sloping fault that stretches from
northern Vancouver Island to northern California.
An example of volcanic mountains formed by ocean-continental
convergence is the Cascade mountain range, including Mt Hood
and Mt St Helens
• An example of this kind of
convergence produces the
spectacular volcanic
landscape of the
Northwest-the coast of
Oregon, Washington,
Alaska and Canada. This
type of convergent plate
boundary, called a
subduction zone, is known
for producing very strong,
historic earthquakes
• A subduction zone can be
located by looking for
curved volcanic mountain
ranges on the land with
deep trenches in the water.
(no islands created)
2.Continental-Continental
The two plates
override each other,
thickening the
lithosphere and
producing
mountains. The
Himalayas are a good
example of this.
Mountain chains are
formed in this way,
these are not volcanic
mountains. Note the
lack of volcanoes.
Mountain Ranges
Mt. Everest, Nepal
Appalachian Mountains
Formed by collision of two plates.
The Ouachita Mountains in
Arkansas and Oklahoma were
originally part of the
Appalachians as well, but were
disconnected through geologic
history.
Arbuckle Mountains
Located in
Davis, Ok.
Folded
mountains
formed by
plate
collisions.
Turner Falls area of Arbuckle Mountains
3. Ocean-Ocean
Note the trench and volcanic arc. Japan is an
example of such an island arc.
Puerto-Rico Trench
Perspective view of
the sea floor of the
Atlantic Ocean and
the Caribbean Sea.
The Lesser Antilles
are on the lower left
side of the view and
Florida is on the
upper right. The
purple sea floor at
the center of the
view is the Puerto
Rico trench, the
deepest part of the
Atlantic Ocean and
the Caribbean Sea.
Major Ocean Trenches
Trench
Ocean
Depth (meters)
Marianas Trench
Pacific Ocean
10,911
Tonga Trench
Pacific Ocean
10,882
Kuril Trench
Pacific Ocean
10,542
Philippine Trench
Pacific Ocean
10,540
Kermadec Trench
Pacific Ocean
10,047
Japan Trench
Pacific Ocean
9,000
Puerto Rico Trench
Atlantic Ocean
8,605
Peru-Chile Trench or Atacama Trench
Pacific Ocean
8,065
Movement of a transform boundary
Transform Boundaries
The San Andreas Fault is a geological fault
that runs a length of roughly 800 miles (1,300
km) through western California in the United
States. The fault marks a transform boundary
between the Pacific Plate and the North
American Plate.
Movement of the Fault
All land west of the fault on the Pacific Plate is moving
slowly to the northwest while all land east of the fault is
moving to the southwest under the influence of plate
tectonics. The rate of slippage averages approximately 3337mm/yr.
Projected motion indicates that the Gulf of California will
expand northward at the same time that the landmass west
of the fault, including the Baja California peninsula and the
California coast (including Los Angeles) slides past San
Francisco, then continuing northwestward as an island
mass toward the Aleutian Trench, over a period of perhaps
twenty million years.
References
Geology Rocks: PlateTectonics, 2007 Jon Hill &
Katie Davis
http://www.geologyrocks.co.uk/tutorials/plat
e_tectonics_introduction
Wikipedia
The Dynamic Earth, USGS
http://pubs.usgs.gov/gip/dynamic/dynamic.ht
ml