Plate Tectonics
1960’s scientist came to the conclusion that the surface of the Earth is broken up into
sections. These sections they call plates. (Like rafts floating on the surface of the Earth)
Plates - are made of crust (rigid) + upper rigid mantle (rigid) = lithosphere (cold and
hard).
Continental crust = 2.7g/cm (less dense, floats compared to oceanic) granitic
Oceanic crust = 3.0g/cm (more dense, sinks compared to continental) basaltic
Asthenosphere – plates float like rafts on this layer (plastic – hot) crust floats on the
asthenosphere due to its lower density.
Plate Boundary – where two plates meet and interact
IDENTIFY THE PLATES IN THE ESRT pg 5 Color in each plate a different color.
How many do you see?
Now look at the interactions between the plates
Look at their “boundaries”…what are the 3 things they do? Converge (come together),
Diverge (move apart), Transform (slide side by side) (candy bar activity)
Side effect: Earthquakes and volcanoes
Divergent Boundaries – Two plates move apart away from each other
- Seafloor Spreading (Mid-Atlantic ridge: North American plate is
moving away from the Eurasian and African plates)
- Mid-Ocean Ridge – crack in sea floor where the new crust is
being created
- Creation of new oceanic crust / plate
- Great Rift Valley in East Africa – large gap between 2 plates
- ESRT: Arrows moving away from each other away from the
ridge or rift
- Earthquakes are present
- speed = 2 cm / year (fingernail growth)
- youngest crust in formed closest to the rift
-as you move further away from the plate boundary, the seafloor
gets older and colder
Where does this new crust go if the Earth doesn’t expand?
It disappears in other places than its being added. Why? Seafloor crust is denser so it
dives or sinks below continental crust.
Convergent Boundaries - 3 kinds
Ocean Crust to Continental Crust
Subduction zone – The area where oceanic plate sinks
below continental plate into the mantle, gets melted and
recycled back into oceanic plate
- continental volcanoes form above these subduction zones
due to heat and pressure as crust slides below continental
crust
- deep sea trenches can occur here where one crust bends
the other as it sinks or dives below. This creates high
temps (friction) and the molten rock rises to form
volcanoes. (Andes Mountains of S.America formed due to
which two plates? Nazca and S.American plates)
Ocean Crust to Ocean Crust - colder, older crust sinks below creating the same effects
as above
-trenches form (Aleutian Island Trench)
- Volcanic Island Arcs form here
- Mariana Island formed parallel to the Mariana Trench
-deep focus earthquakes
Continental Crust to Continental Crust – no subduction occurs (collision zone)
- both plates are less dense then the asthenosphere
- two plates collide and crumple up forming mountain
ranges (orogenies)
- Shallow Earth Quakes are common here
- No volcanoes because there is no subduction.
- Himalayas in Asia – Indo Australian Plate collided with
Eurasian Plate
Transform Boundaries
- Two plates slide past one another
- They move in opposite direction or same direction at
different rates
- When plates slip past each other suddenly earth quakes
can happen
-Pacific Plate is sliding past the N.American Plate creating
the San Andreas fault (Right lateral strike slip fault)
-shallow earth quakes
Convection currents
Just like soup cooking on the surface of a stove. Hot liquids (less dense) rise until they
are far enough away from the source. Then they cool
(become more dense) and sink back toward the source.
This occurs in the mantle.
Hot spots – Volcanic islands that are created away from plate boundaries. As the plate
moves over the rising magma, it can create an island chain
(Hawaii) over head example