Chapter 12.2 BC Science 10

• Earth is over 1200 km thick and has four distinct layers:
1.
Crust – outer solid rock layer (granite on land, basalt in oceans)
2.
Mantle – thickest layer, mostly solid except for upper mantle being able to flow like
“thick toothpaste”
3.
4.
Outer core – composed of liquid iron and nickel
Inner core – mostly solid iron, at tremendous temperature and pressure

 We know the
“ Lithosphere ” contains the Earth’s crust and the uppermost part of the upper mantle.
 Just below this is the
“ Asthenosphere ”
 A partly MOLTEN layer of the upper mantle

 The plates move because of convection currents arising in the hot magma of the upper mantle in the
Asthenosphere....




Partly the upper mantle is hot because of Earth’s hot core....
The temp of the upper mantle varies throughout
Large amount of RADIOACTIVE elements (ex:
Uranium) occur in certain spots

When these decay...they release HEAT!

Radioactive URANIUM

Just like in air masses.....
HEAT RISES !
Hot (less dense) material rises…cools….sinks….reheats

 This “mantle convection” is the driving force behind the movement of tectonic plates!

“Spreading Centers” are areas of the Earth’s surface where magma rises up....
(common) (less common)
IN OCEAN....
“ Spreading Ridge ”
On LAND....
“ Rift Valley ”




 When plates “ bump ” into each other, they may subduct....
Consider a “heavy, dense” oceanic plate bumping into a lighter continental plate ....

 The HEAVIER plate slides under the lighter plate.

 When a plate subducts deep into the mantle....it
PULLS the rest of the plate too

 SUBDUCTION ZONES experience more


EARTHQUAKES
!

 A plate boundary an area where two plates are in contact.
 Divergent plate boundaries – areas where plates are spreading apart
 Convergent plate boundaries – areas where plates meet
 Oceanic-continental


Oceanic-oceanic
Continental-continental
 Transform plate boundaries – areas where plates move past each other

1.
Divergent plate boundaries are areas where plates are spreading apart.
– Ocean ridges and continental rifts are examples.
– The Mid-Atlantic Ridge is the longest mountain range on Earth.

2.
Convergent plate boundaries are areas where plates collide.
A.
•
•
•
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
•
•
North America’s west coast 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.

B.
Oceanic-oceanic plate convergence
– The cooler, denser plate will subduct
– under the less dense plate.
Convergence may produce a volcanic island arc, such as those found in Japan, Indonesia, and
Alaska’s Aleutian islands.

C.
Continental-continental plate convergence
– Since both are continental plates, their densities are similar.
– As they collide, their edges fold and crumple, forming mountain ranges.
– The Himalayas are the world’s youngest (and tallest) mountain range, formed as Asia and Africa plates collided 40 million years ago. They are still growing taller today.

3.
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.

Turkey, 1999

Earthquake Precursors
1.
2.
3.
4.
5.
changes in elevation or tilting of land surface fluctuations in groundwater levels magnetic field electrical resistance of the ground
Release of gases

Earthquake Prediction Programs
 include laboratory and field studies of rocks before, during, and after earthquakes

 monitor activity along major faults produce risk assessments

Rocks bend under stress while storing elastic energy. When the strain in the rocks exceeds their strength, breaking will occur along the fault.
Stored elastic energy is released as the earthquake. Rocks“snap back”, or rebound to their original condition.


Body Waves


P-Waves (primary waves)
S-Waves (secondary waves)

Surface Waves
- L – Love Waves
- R – Raleigh Waves (surface, vertical)

 Body waves
 P or primary waves
 fastest waves


 travel through solids, liquids, or gases compressional wave, material movement is in the same direction as wave movement
S or secondary waves


 slower than P waves travel through solids only shear waves - move material perpendicular to wave movement

 Surface Waves
 Travel just below or along the ground’s surface
 Slower than body waves; rolling and side-to-side movement
 Especially damaging to buildings

A direct result of Continental Drift

Seismic wave behavior
 P waves arrive first, then S waves, then L and R


Average speeds for all these waves is known
After an earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter.

Time-distance graph showing the average travel times for P- and Swaves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the
P- and S- waves




Three seismograph stations are needed to locate the epicenter of an earthquake
A circle where the radius equals the distance to the epicenter is drawn
The intersection of the circles locates the epicenter

• Modified Mercalli Intensity Map
– 1994 Northridge, CA earthquake, magnitude 6.7
 Intensity
 subjective of the kind of damage done and people’s reactions to it measure
 isoseismal lines identify areas of equal intensity


Advantages:

No high-tech instruments are required.

Disadvantages:



Damage depends on geologic materials and type of structures in area
Damage varies with distance from epicenter
Subjective - different people may view damage and effects very differently

I. People do not feel any Earth movement.
II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings.
III. Many people indoors feel movement. Hanging objects swing back and forth. People outdoors might not realize that an earthquake is occurring
IV. Most people indoors feel movement. Hanging objects swing. Dishes, windows, and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few people outdoors may feel movement. Parked cars rock.
XI. Most buildings collapse. Some bridges are destroyed. Large cracks appear in the ground. Underground pipelines are destroyed. Railroad tracks are badly bent.
XII. Almost everything is destroyed. Objects are thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.

 Magnitude
 Richter scale measures total amount of energy released by an earthquake; independent of intensity
 Amplitude of the largest wave produced by an event is corrected for distance and assigned a value on an open-ended logarithmic scale

 Increase by 1 whole number means a 10X increase in the Magnitude of the quake
 For every increase on the Richter Scale, the amount of energy released increases 30X
Compare a 5.0 to a 7.0 quake
7.0 has 10 X 10 = 100 times greater magnitude
7.0 has 30 X 30 = 900 times more energy!
Compared to a 1.0 quake, a 7.0 has:
10 X 10 X 10 X 10 X 10 X 10 = 1 000 000 greater strength
And 30 X 30 X 30 X 30 X 30 X 30 = 729 000 000 more energy

Magnitude Number/Year
Less than 2.0
2.0-2.9
3.0-3.9
4.0-4.9
5.0-5.9
6.0-6.9
7.0-7.9
Greater than 8,0
600,000
300,000
49,000
6,200
800
266
18
1

Earthquake intensity
Earthquake magnitude
Richter Scale
Mercalli Scale
Elastic rebound
Fault
Focus
P-wave
Seismic wave
Seismogram
Seismograph
S-wave
Tsunami

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Eruptions will generally be of two types:
 Quiet (Rift) eruptions
 Explosive (Subduction) eruptions
 Each will produce a different type of volcanic cone
(shield or composite)
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 Generally at hot spots, spreading plates
 Mantle comes directly to surface
 Hot lava; low viscosity, flows easily, gases escape easily, mafic lava
 Forms shield volcanoes, flood basalts
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 Found at subduction zones
 Magma low temperature, high viscosity, does not flow easily, gases trapped, hard to predict explosions of felsic lava
 Forms composite (strata) volcanoes, cinder cones, calderas, ash, lapilli, bombs, gas flows, very destructive
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72

 constructed of alternating layers of pyroclastics (ash and rock particles) and rock solidified from lava flows.
 built over long spans of time
 eruption is intermittent, (a few years of intense activity separated by hundreds or thousands of years of inactivity)
 most of the better known volcanoes of the world are composite volcanoes
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Figure 12.32
75

Composite Volcano
Columbia River basalt flow
(Ancient Shield)
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

Pyroclastic flows are superheated gas and particles
(ash & dust or larger cinders, blocks or bombs)
At Mt. St. Helens’s the pyroclastic flow extended up to
8 km with a temp of 700 C
 eruption killed all but 2 of the more than 20,000 people in the town of St. Pierre
78

 Ash from eruptions that falls on surrounding ground
 When ash is carried to high altitudes this can spread great distances
79

MOUNT ST. HELEN’S
Before 1980 Eruption
May 18, 1980
After 1980 Eruption 80

Mt. Pinatubo
Philippines
June 1991
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 brilliant sunsets and sunrises due to fine ash and gases in the stratosphere, (cloud drifts around world)
 Mt. Pinatubo - 22 million tons of sulfur dioxide (SO
2
) formed droplets of sulfuric acid
 blocked some sunlight from reaching Earth, cooling temperatures worldwide by 0.5 C
 An eruption the size of Mount Pinatubo could affect the weather for a few years. o
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ANAK KRAKATAU:
(Child of Krakatoa)
•August 26,1883, Eruption destroyed much of the island of
Krakatau ("Krakatoa") in Indonesia,
•One of the most catastrophic natural disasters in recorded history (heard more than 3000 miles away)
•Fine ashes from the eruption were carried by upper level winds as far away as New York City.
•Volcanic dust in the upper atmosphere affected incoming solar radiation and the earth's weather for several years.
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•
•
• series of large tsunami waves followed main explosion, some nearly 40 meters above sea level more than 36,000 people in the coastal towns and villages killed
Tsunami waves were recorded or observed throughout the Indian Ocean, the Pacific Ocean, the American
West Coast, South America, and even as far away as the English Channel.
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• April 1815 - Tambora Volcano, Indonesia the most powerful eruption in recorded history.
•
•
Tambora's volcanic cloud lowered global temperatures by as much as 3 C o
Even a year after the eruption, most of the northern hemisphere experienced sharply cooler temperatures during the summer months.
• In parts of Europe and in North America, 1816 was known as "the year without a summer."
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