1. 12.2 Features of Plate Tectonics
Earth is over 1200 km thick, and has four distinct layers.
These layers are the crust, mantle (upper and lower), outer core, and inner core.
Crust - outer solid rock layer (granite on land, basalt in oceans)
Mantle - thickest layer, mostly solid except for upper mantle being able to flow like “thick toothpaste”
Outer core - composed of liquid iron and nickel
Inner core - mostly solid iron, at tremendous temperature and pressure
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2. 12.2 Features of Plate Tectonics
Tectonic plates make up the lithosphere,
which floats on the asthenosphere.
The lithosphere is the crust and upper portion of the upper mantle.
The asthenosphere is the molten layer of the upper mantle.
Heat to keep asthenosphere molten comes from radioactive elements.
A convection current forms as hot, low density rock rises
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3. Plate Motion (continued)
Continents, attached to the tectonic plates, float in the magma of the asthenosphere.
As magma is heated in the asthenosphere, convection currents form.
Rising magma can reach the surface at ridges (in the oceans) or rifts (on land).
The magma cools when it reaches the surface, solidifies, and is pushed aside as new magma pushes from below. This is called ridge push.
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4. Plate Motion (continued)
Tectonic plates are all moving at the same time.
There are 12 large tectonic plates, and many smaller ones.
Where continental and oceanic plates meet, subduction occurs.
More dense oceanic plate subducts under the lighter continental plate.
By “slab pull”, the rest of the plate follows.
Large earthquakes and volcanoes are found in subduction zones.
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5. Much like what happens when heating a pot of soup.
Mantle Plume - Hot, less dense mantle material is forced upward, once it reaches the surface it cools and sinks.
Much like what happens when heating a pot of soup.
When it reaches the surface it forces the plates apart at spreading centers, called rifts or rift valleys
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6. Quick Check
1. Name the four layers of Earth, in order from the inside out.
(a) _____________________________________ (b) ___________________________________
(c) _____________________________________ (d) ___________________________________
2. What is the difference between the densities of oceanic crust and continental crust? __________
________________________________________________________________________________
3. What is the source of energy for convection currents and hot-spot activity in Earth’s mantle?
4. In terms of the crust and the mantle, describe:
(a) the lithosphere _______________________________________________________________
(b) the asthenosphere ____________________________________________________________
5. What is a mantle plume? __________________________________________________________
6. How does a rift valley form? _______________________________________________________
7. What happens in subduction? ______________________________________________________
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7. A plate boundary is where two plates are in contact.
Plate Interactions
A plate boundary is where two plates are in contact.
The way the plates interact is based on the type of plate, and the direction the plates are moving relative to each other.
Divergent plate boundaries - where plates are spreading apart
Convergent Plate boundaries - where plates meet
Transform plate boundaries - where plates move past each other
Tectonic plate boundaries, and their relative movement to each other.
See page 523
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8. Plate Interactions (continued)
Divergent plate boundaries are where plates are spreading apart.
Ocean ridges and continental rifts are examples
The Mid Atlantic Ridge is the longest mountain range on Earth.
Convergent plate boundaries are where plates collide.
Oceanic-Continental plate convergence
The oceanic plate subducts under the continental plate, forming a trench.
Cone-shaped volcanoes can form from magma seeping to the surface.
This is how the volcanic belt of the
Pacific Northwest has formed.
Mountain ranges like the Coast Mountain
range also form from the collision.
Earthquakes can occur when subduction,
ridge push and slab pull stall.
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The convergence of an oceanic and a continental plate.
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9. Ocean-Continental Convergent Boundary
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10. Plate Interactions (continued)
Oceanic-Oceanic plate convergence
The cooler, more dense plate will subduct under the other.
Convergence may produce a volcanic island arc, such as those found in Japan, Indonesia and Alaska’s Aleutian islands.
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11. Plate Interactions (continued)
Continental-Continental plate convergence
Since both are continental plates, densities are equal.
As they collide, edges fold and crumple, forming mountain ranges.
The Himalayas are the world’s youngest (and tallest) mountain range.
They formed as Asia and Africa plates collided 40 million years ago.
They are still growing taller today.
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12. Plate Interactions (continued)
Transform plate boundaries are where plates move past each other.
Usually are found near ocean ridges.
Since rock slides past rock, no mountains
or volcanoes form.
Earthquakes and faults are very common, though.
Earthquakes often form from the friction between
moving tectonic plates.
This accounts for 95% of all earthquakes.
The Juan de Fuca convergent plate boundary west of
Vancouver Island has many earthquakes.
Large earthquakes hit this region every
years.
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13. Tectonic plate movement
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14. Tectonic plate movement
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15. Hawaii is moving toward Japan at a rate of 8.3 cm/ yr
Using satellite imagery and laser technology plate movement can be tracked.
Hawaii is moving toward Japan at a rate of 8.3 cm/ yr
Maryland is moving away from England at 1.7 cm / yr
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16. Quick Check
1. List three kinds of plate interactions. _________________________
_________________________ ______________________
2. Identify the geographical features that are typical of
(a) two oceanic plates converging _____________________________
___________________________________________________________
(b) an oceanic plate and a continental plate converging ________________
____________________________________________________________________
(c) two continental plates converging _______________________________________
(d) two continental plates diverging ________________________________________
(e) two oceanic plates diverging ___________________________________________
3. What is the relative motion that occurs between two plates that meet at a transform boundary?__________________________________________________
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17. Describing Earthquakes
Earthquakes are very difficult to predict.
Scientists understand why they happen, but it is very difficult to predict their timing, exact location and strength.
Their build-up happens underground, over very long periods of time.
What we do understand has helped prepare structures to survive them.
Alaska, one of the most violent quakes of recent times.
It lasted 7 minutes, It was felt in Texas, The ground moved up and down 6m
A giant ocean wave was set off 200km away, which rushed to shore and moved a tonne ship on to land.
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18. Earthquakes
Earthquakes occur along faults. The rock can move vertically or horizontally. The force of the moving plates cause rocks to bend stretch and break.
An earthquake is the vibrations produced by breaking rock at a fault.
An earthquake occurs when the friction of the rock temporarily halts their movement at a fault.
The forces are still applied to the rock building up stress at the points where the rock is stuck
The rock is bent and stretched. The earth has been storing energy as it bends this is called potential energy.
Eventually it is bent beyond its elastic limit and it breaks. As soon as the earth breaks, it releases energy as it moves, this is called Kinetic Energy.
The energy is released in the form of seismic waves.
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19. Describing Earthquakes
The focus of the earthquake is where the pressure is finally released.
The epicenter is the point on the surface directly above the focus.
Earthquakes occur at various depths, depending on the plates involved.
Earthquakes at the surface tend to cause more damage.
See page 528
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20. Describing Earthquakes (continued)
Seismic waves form when the energy of an earthquake is released.
Seismology is the study of these waves.
These waves reveal the source and strength of an earthquake.
They also help us learn about the composition and distances of the Earth’s interior.
Types of earthquake waves:
P – waves
S – waves
L – waves
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24. Quick Check
1. What is the difference between the focus of an earthquake and the epicentre of an earthquake?
________________________________________________________________________________
2. What are three kinds of earthquake waves and how do they differ?
(a) ____________________________________________________________________________
____________________________________________________________________________
(b) ____________________________________________________________________________
(c) ____________________________________________________________________________
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25. Describing Earthquakes (continued)
Seismic waves behave differently in different Earth layers.
Knowing this, scientists can learn much about both earthquakes and the interior of Earth.
Seismometers are used to measure seismic wave energy.
Early seismometers just measured if the ground shook or not.
Some seismometers measure horizontal movement, others vertical movement.
A seismogram is produced, showing when an earthquake started, how long
it lasted, and the magnitude.
1 in magnitude = 10X stronger
A magnitude 6 earthquake is 100X more powerful than a 4.
Since seismic waves travel at different speeds, a distance-time graph, revealing the focus.
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26. Determining the epicenter of an earthquake
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27. The movement of tectonic plates causes volcano formation.
Volcanoes
The movement of tectonic plates causes volcano formation.
Composite volcanoes - this is the stereotypical volcano, erupting and belching smoke and ask everywhere, found along SUBDUCTING plate boundaries
Layers of ash and thick lava (magma outside Earth) form a tall cone.
As magma reaches the surface, it cools, hardens and traps gases below.
Pressure builds, eventually there is an eruption.
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28. Volcanoes
Shield volcanoes - these are not found at plate boundaries, but instead form over hot spots
(a weak spot in the normal lithosphere).
Thin magma/lava flows out from a hot spot and forms a low, wide cone.
All of the Hawaiian and Galapagos Islands were formed by shield volcanoes
Ex; the Hawaiian Islands were formed at hot spots. The Pacific Plate is moving north west and the lava that is coming to the surface is forming new islands. ***Therefore the northern most island is the OLDEST and the southern most island is the YOUNGEST***
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29. Rift eruptions - these occur along long cracks in the lithosphere
Volcanoes
Rift eruptions - these occur along long cracks in the lithosphere
These are not explosive, but release
massive amounts of lava.
Occur at spreading ridges in the ocean or on land
lava flows out of fissures (or cracks) in the crust
not violent, but can release a lot of lava.
The Columbian River Plateau, from Canada through Washington and Oregon into California is a rift volcano. 30 million years old.
Take the Section 12.2 Quiz
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30. Impact of Volcanoes
The ash, gas and debris that explode out of the top or rush down the sides of a volcano are called pyroclastic flow.
Can move at speeds of up to 300 km/hr – they actually ride on a cushion of air.
is superheated to 1000oC
Responsible for most of the loss of life and damage to property caused by volcanoes in the 20th century
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32. Ash can go 10 km high, and be carried around the world.
Lahar - if a volcano erupts that is covered in snow and ice, the snow and ice will melt and wash down the sides of the volcano with all of the mud and debris.
In 1985, in Columbia, a volcano erupted and the lahar buried a town 100km away in 4 hr with 5 meters deep of mud and debris. 23 people died, the mud was like concrete and hardened quickly.
Ash can go 10 km high, and be carried around the world.
In 1815, a volcano in India, released so much ash that it block the sun for months. It caused the earths temperature to drop dramatically. Major famines occurred because crops could not grow due to the frost in JULY.
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35. Some areas are covered by as much as 600 feet.
The largest landslide in recorded history swept down the mountain at speeds of 150 miles per hour and buried the Toutle River under an average of 150 feet of debris.
MSH was 9,677 feet before the eruption and 8,363 feet after.   1,314 feet was removed by the May 18th eruption
Some areas are covered by as much as 600 feet. 
The lateral blast swept out of the north side of MSH at 300 miles per hour creating a 230 square mile fan shaped area of devastation reaching a distance of 17 miles from the crater. 
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36. Temperatures as high as 660 OF and the power of 24 megatons of thermal energy, it snapped 100 year old trees like toothpicks and stripped them of their bark.
Pyroclastic flows rolled out of the crater for 9 hours after the eruption.  Covering 6 square miles they sterilized the remaining soil with temperatures nearing 1,300 OF.
The massive ash cloud grew to 80,000 feet (18 kilometers) in 15 minutes and reached the east coast in 3 days.  Although most of the ash fell within 300 miles of the mountain finer ash circled the earth in 15 days and may continue to stay in the atmosphere for many years.
The lava dome rises 876 feet above the crater floor and is about 3,500 feet in diameter. 
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