Plate Boundaries

2. Divergent boundaries occur
along spreading centers
where plates are moving
apart and new crust is
created by magma pushing
up from the mantle. Picture
two giant conveyor belts,
facing each other but slowly
moving in opposite
directions as they transport
newly formed crust away
from the ridge crest.

3. Mid-Atlantic Ridge
The rate of spreading along the Mid-
Atlantic Ridge averages about
2.5 centimeters per year, or 25 km
in a million years.
Seafloor spreading
over the past 100-200 million years
has caused the Atlantic Ocean to
grow from a tiny inlet of water
between the continents of Europe,
Africa, and the Americas into the
vast ocean that exists today.

4. They also cause volcanoes to occur. An example of this is in
Iceland The island of Iceland is actually being pulled apart,
and the result are lava fountains, like Krafla Volcano.
Krafla

5. This process, called rifting, is still in progress comes in the many active and
semi-active volcanoes located along the Rift (designated by red triangles on
the map). Further evidence of volcanic activity along the rift is provided by
the presence of numerous boiling hot springs.
African Rift Valley
A 6,000-mile crack in
the earth's crust, stretching
from Lebanon to
Mozambique is known as the African Rift
Valley. The force of the plates pulling apart
caused huge chunks of the crust to sink and
force up molten rock in volcanic eruptions.
Rifting

6. The Red Sea
(just off the
east coast of
Africa) is
spreading.
The land to
either side is
actually
pulling away!
The Red Sea
continues to
get bigger each
year.

7. Convergent boundaries exist where one plate is
subducted underneath another plate.
Subduction - one plate sinks underneath another plate
Where convergent plates exist, the earth’s crust is being
destroyed as it slowly moves underneath another section
of crust and melts into the mantle.

8. Ocean-Continent Collision:
Underneath the ocean
water there are a number
of long, narrow, curving
trenches thousands of
kilometers long and 8 to 10
km deep cutting into the
ocean floor. Trenches are
the deepest parts of the
ocean floor and are created
by subduction.

9. Ocean-Continent Collision:
Off the coast of South America, the oceanic Nazca Plate is pushing into
and being subducted under the continental part of the South American
Plate. At the same time, the South American Plate is being lifted up,
creating the Andes Mountains.
Strong, destructive earthquakes and the rapid uplift of mountain ranges
are common in this region. Such earthquakes have been known to jolt the
land up several meters.
Andes Mountains

10. Ocean-Ocean Collision:
When two oceanic plate collide, it
can result in the formation of
volcanoes, too. One oceanic plate
sink beneath the other, and over
millions of years, the erupted lava
and volcanic debris pile up on the
ocean floor. Finally, a volcano rises
above sea level to form an island
volcano. Such volcanoes are
typically strung out in chains called
island arcs.
This is how the
Aleutian Islands
have formed and
why they
experience
numerous strong
earthquakes.

11. Continent-Continent Collision:
The Himalayan Mountains
were created when two
continental plates met head-
on,and neither was subducted.
Continental rocks are
relatively light and, like two
colliding icebergs,
resist downward
motion. Instead,
the crust buckled
and was pushed
upward and
sideways. Viola!
Mountains!

12. India collided into Asia 50 million
years ago, causing the Eurasian
Plate to crumple up and override
the Indian Plate.
After the collision, the slow
continuous convergence of the two
plates over millions of years
pushed up the Himalayas and the
Tibetan Plateau to their present
heights.
The Himalayas, towering as high
as 8,854 m above sea level, are the
highest continental mountains in
the world.
Continent-Continent Collision:

13. Most transform
boundaries are found on
the ocean floor. A few
occur on land, an example
is the San Andreas fault in
California. Transform
boundaries are the result
of two plates sliding past
each other.

14. • Stretches about 1,300 km long and in some
places tens of kilometers wide.
• Slices through two thirds of the length of
California.
• The Pacific Plate has been grinding
horizontally past the North American Plate
for 10 million years, at an average rate of
about 5 cm/yr.
• Land on the west side of the
fault (on the Pacific Plate) is
moving in a northwesterly
direction relative to the land
on the east side of the fault
zone (on the North American
Plate).
The San Andreas Fault

15. Why???

17. So what will the
earth look like in
the future?

We can only
guess…

26. All of this movement on the
earth’s surface…

What does it cause?
Let’s take a closer look…
Cross Section of a Milky Way…
What does each ingredient represent?

27. •Carefully unwrap your Milky Way
candy bars.
•With someone at your table, attach the
two pieces of candy so that one side
touches.
•Slowly slide the two “plates” (candy)
past each other and watch what
happens.
•What kind of plate boundary are you
demonstrating?
•Draw it next to the term on your web!

28. •Carefully unwrap your two Milky Way
candy bars.
•Attach the two pieces of candy so that
one side touches.
•Slowly slide the two “plates” (candy)
past each other and watch what
happens.
•What kind of plate boundary are you
demonstrating?
•Draw it next to the term on your web!

29. 1.Now split the next two plate boundaries so
that you only demonstrate one more…
Your candy can only withstand one more
demonstration before this gets too messy!
2.Watch as two people at your table…
- Start with the candy pieces touching
- Slowly apply compression force (push
them together)
•What forms?
•What kind of plate boundary is this?
•Draw it next to the term on your web!

30. 1.Now split the next two plate boundaries so
that you only demonstrate one more…
Your candy can only withstand one more
demonstration before this gets too messy!
2.Watch as one person at your table…
- Starts with the candy pieces touching
- Slowly apply compression force (push
them together)
•What forms?
•What kind of plate boundary is this?
•Draw it next to the term on your web!

31. •So you probably know what is next…
•Have another two people carefully bite off
one sliver from a side of the chocolate on
each piece of candy.
•Attach the two “exposed” sides.
•Carefully pull them apart.
•What is exposed from the candy?
•What kind of plate boundary is this?
•Draw it next to the term on your web!

32. •So you probably know what is next…
•Have another person carefully bite off one
sliver from a side of the chocolate on each
piece of candy (don’t do anything for the
Starburst).
•Attach the two “exposed” sides.
•Carefully pull them apart.
•What is exposed from the candy?
•What kind of plate boundary is this?
•Draw it!

33. Think…
• What kind of boundary is at the uplift zone?
• What kind of boundary is at the spreading
center?
• What kind of boundary is at the subduction
zone?
QuickTimeª and a
TIFF (Uncompressed) decompressor
are needed to see this picture.

34. Think…
• What kind of boundary is at the uplift zone?
CONVERGENT
• What kind of boundary is at the spreading center?
DIVERGENT
• What kind of boundary is at the subduction zone?
CONVERGENT
QuickTimeª and a
TIFF (Uncompressed) decompressor
are needed to see this picture.