17.3 NOTES: PLATE TECTONICS
-- The outer shell of the Earth is known as the lithosphere. It makes up the solid, relatively rigid outer shell of
the Earth. It is broken up into pieces called plates (tectonic plates).
These plates are made up of 2 different types of material:
-- oceanic crust (makes up the ocean floor) (denser) Made up of the rock type: Basalt
(a denser, darker-colored rock that is high in iron and magnesium)
-- continental crust (what we live on) (less dense) Made up of the rock type: Granite
(a less-dense, lighter-colored rock made up of feldspar & quartz)
Some plates are mostly made up of oceanic crust.
Some plates are mostly made up of continental crust.
Some plates are made up of partly oceanic and partly continental.
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These tectonic plates “float” on top of the next layer of the Earth which is called the asthenosphere.
n It is a layer of rock found in the upper part of the mantle.
n It is unique in that it is a solid but it has the ability to flow like a liquid. This is known as plasticity.
-- What causes this rock to be plastic-like is:
* The intense pressure from the weight of the lithosphere
* The great heat coming up from within the earth
-We think that a lot of this heat is created from radioactive decay
-Evidence of this occurring: Radon Gas à (# 2 leading cause of lung cancer)
There are 3 types of plate boundaries:
* Divergent - plates moving apart
* Convergent - plates coming together
* Transform - plates sliding past one another
Divergent: 2 plates moving away from each other
Ex.: Mid-Ocean Ridge Systems (Mid-Atlantic Ridge: North American Plate & Eurasian Plate)
Red Sea (Arabian Plate & African Plate)
Great Rift Valley in Africa (African Plate & Somalian Sub-Plate)
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Convergent: 2 plates moving toward each other. Because there are 2 different types of crustal material
(oceanic & continental) there can be 3 different types of collisions that can occur.
· Oceanic crust colliding with Continental crust
· Continental crust colliding with Continental crust
· Oceanic crust colliding with Oceanic crust
Oceanic crust colliding with Continental crust
Oceanic crust is denser, it is subducted (pushed down into the Earth).
The region where this occurs is known as a Subduction Zone.
The subducted plate melts due to:
* heat from the asthenosphere
* friction from the plates grinding over one another
Ex.: West coast of S. America: Andes Mtns. (Nazca Plate is subducted under the S. American Plate)
West coast of N. America: Cascade Mtns. (Juan De Fuca Plate is subducted under the N. American Plate)
The most notable volcano found in the Cascades is Mt. Saint Helens
(it is found in the state of Washington and it last erupted in May 1980)
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Continental crust colliding with Continental crust
Both plates have a low density and neither one wants to go into the Earth.
The colliding edges of the plates are crumpled (folded) and uplifted.
Large mtn. ranges (folded mtns.) are produced from this uplifting.
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Oceanic crust colliding with Oceanic crust
One of the Oceanic crusts is subducted (the one that is mostly oceanic is denser).
--This results in the formation of a subduction zone and a very deep trench.
The plate melts due to:
* heat from the asthenosphere
* friction from the plates grinding over one another
Ex.: Aleutian Islands
o Pacific Plate subducting under the N. American Plate
o Also forms the Aleutian Trench
Kuril Islands
o Pacific Plate subducting under the N. American Plate
o Also forms the Kuril Trench
Japanese Islands
o Pacific Plate subducting under the Eurasian Plate
o Also forms the Japan Trench
Mariana Islands
o Pacific Plate subducting under the Philippine Plate
o Also forms the Marianas Trench
* The deepest point on the Earth (6.8 miles below sea level)
Transform Boundary – The plate interaction where 2 plates “slide” past each other.
n The jagged chunks of rock along the edges of the plates will catch, hold energy back and eventually break, releasing the stored energy resulting in:
EARTHQUAKES
Ex.: San Andreas Fault in California
(Pacific Plate sliding past the N. American Plate)
Section 17.4 NOTES: Causes of Plate Motion:
Convection Currents – believed to be the driving force behind plate movements.
- uneven heating within the Earth creates circular flow patterns to develop
within the mantle
· hot material rises (because it is less dense)
· cooler material sinks (because it is more dense)