Transcript Plate Boundaries

Plate Boundaries
M. Billings
NSHS
What is a plate boundary?
Any place where plates “meet”. They are
always places of great stress within the
crust
Plate Movement is driven by
convection currents in the mantle
There are 3 types of
Plate Boundaries!!!
1. Divergent Plate Boundaries
• Two plates are moving in opposite
directions
• Occurs over the Up-welling part of a
convection current in the mantle
• 2 Sub-Types
– Rift Valleys (on continents)
– Mid-Ocean Ridges (in the oceans)
Step 1:
Continental crust is pulled apart causing a valley to form.
Continental crust
Asthenosphere
Step 2:
Fault
earthquakes
Asthenosphere
As the valley gets deeper, faults form and earthquakes occur.
Lakes often form in low areas.
Step 3:
As the crust gets thinner, pressure is released from the mantle and
magma starts to rise. Volcanoes form.
Volcanoes
Step 4
Eventually the crust is pulled apart and magma from the mantle forms
new ocean floor. All ocean floor is produced this way and forms a
mid-ocean ridge which is volcanic.
Mid-ocean ridge
Black & White Smokers
http://rst.gsfc.nasa.gov/Intro/Part2_26x.html
Found at divergent plate
boundaries, black and white
smokers form where cold sea
water seeps into the mid-ocean
ridge and is heated. Once
heated it dissolves minerals.
Many unusual lifeforms live on
black/white smokers and use
these minerals as their food
source instead of light.
http://www.mnh.si.edu/earth/text/4_3_2_2.html
Form on or near midocean ridges
Examples of a Divergent Boundary:
Mid-Atlantic Ridge –
Iceland sits on top of it
East-African Rift
http://www.indiana.edu/~g105lab/1425chap13.htm
2. Convergent Boundaries
• 2 plates are moving toward each other
(sometimes called collision boundaries)
• Occur where convections currents in the
mantle move downward
• Sub-Types
– Ocean-Continent
– Ocean-Ocean
– Continent-Continent
Volcanic Mountain
Chain
Ocean-Continent
trench
Continental
Crust
Ocean
crust
Magma forms as
the ocean plate
melts and rises to
from volcanic
mountains.
Earthquakes
Ocean floor is more dense and is pushed back into the mantle forming a trench .
The part of the ocean crust that is being forced into the mantle produces a
subduction zone. This crust begins to melt and the magma rises to the surface
and forms a chain of volcanic mountains. Earthquakes are numerous!
An example is the Andes Mountains in
South America. Notice the trench to the
west in the ocean.
Ocean-Ocean
Volcanic island arc
trench
The oldest, coldest piece of ocean crust is pushed into the mantle forming a
trench and subduction zone. The magma that forms as this plate melts rises
to form volcanic island arcs (chains). Again, lots of earthquakes
Japan is an example of
an ocean-ocean
boundary. The trench
lies off the east side of
the islands. The islands
always lie on the plate
that is NOT being
subducted.
Continent-Continent
Non-volcanic mountains
Continental Plates collide producing very high non-volcanic mountain chains.
The “boundary” between these plates becomes welded –this area is called the
suture zone. Earthquakes are numerous. Continental crust is too bouyant to
go into the mantle (so there is no subduction).
The Himalayas (where Mt. Everest is found) is an example of a
continent-continent boundary.
3. Transform Plate Boundaries
• Plates sliding past each other
• They connect other types of plate
boundaries
• Not usually convection current connected
Fault
Because plates are
sliding past each other,
these boundaries are
basically faults and
have many
earthquakes.
Transform boundaries also form
between sections of mid-ocean ridges.
A fault is not a simple
straight line. The
earth’s crusts
fractures much like a
glass plate. So a fault
is more like a “zone”
of breakage.
The San Andreas
Fault in California
is a great
example of a
transform
boundary!
Notice most of the world’s volcanoes are found at
plate boundaries!
It takes all 3 types of boundaries to keep the crust
from becoming too large or small for the earth’s
surface.
How will plate tectonics
change the earth in the
future?
Observe the continents on the
next few slides. How are their
locations different?
In the Future…?...
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