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“Earthquakes”
I. Causes of Earthquakes:
A. Why do earthquakes take place?
1. Rocks act the same way rubber bands work.
2. Applying stress to a rock will cause it to bend or
stretch.
3. Stress causes the rock to deform.
4. When the rock can’t handle the stress it eventually
breaks.
5. The area in which a rock breaks is called a fault.
6. When the break occurs energy is released.
7. The energy causes the rock to vibrate causing an
earthquake.
B. There are three factors that cause breakage in rock.
1. Compression - stress that squeezes rock layers
together.
2. Tension - stress that stretches rock layers apart.
3. Shear - stress that causes the rock layers to slide
by each other.
C. Each one of these forces will cause a different fault.
1. Reverse Faults- Compression stress cause this type
of fault
2. Normal Fault- Tension stress cause this type of
fault.
3. Strike-Slip Faults - Caused by shear stress.
A.) Rocks move sideways past each other.
B.) San Andreas Fault in California is this type.
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II. Earthquake Energy:
A. When rock breaks, energy is released as a seismic
wave.
Energy wave that moves outward,
1. Seismic Wave in every direction, form the center
of the earthquake.
2. Epicenter - is the point on the earth’s surface
directly above the earthquake center.
• III. Types of Seismic Waves:
A. There are three types of seismic waves.
1. Primary Waves (P waves) - Energy waves that
move through the ground in a back and forth
motion.
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Examples of P waves:
A.) Pushing down on your car and then letting
go.
B.) A slinky.
* P waves are the fastest moving earthquake
waves. (6km/s).
2. Secondary Waves (S Waves) - Move through the ground in an
up and down motion.
Examples:
A.) teeter- totter, or seesaw.
* S waves move half as fast as P waves. (3 km/s)
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Seismic Waves Cont.
3. Surface Waves (L Waves) -
Energy waves that
travel along the surface of the earth like waves do
in the ocean. (Rolling motion)
* L waves move the slowest. (1 km/s)
* Cause the most damage during an earthquake.
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Tsunamis:
A. What is a tsunami?
1. Tsunami - An earthquake generated ocean wave.
B. How do they form?
1. An earthquake, landslide, volcanic eruption, or
meteorite impact releases energy.
2. If in the ocean, the energy will push against the
water, generating a powerful wave.
3. These waves can be from 30 to hundreds of feet
high.
4. Most occur in the Pacific Ocean coast of South
America.
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“Volcanoes”
I. What is a Volcano?
A. Volcano - A hole in the earth’s surface that often
forms mountains from eruptions of lava and/ or volcanic
ash.
B. What causes a volcano?
1. Rock has to melt into magma.
2. Magma is forced upward because it is less
dense than the rock layers around it.
3. When magma and other gases reach the
surface it turns to lava or volcanic ash.
4. That forms the volcano.
C. Where do volcanoes occur?
1. A volcano can occur in one of three places.
A.) Rift Zone
B.) Subduction Zone
C.) “Hot Spots”
2. Divergent Plate Boundaries - The area where plates
move apart.
A.) When plates move apart they form large
cracks in the crust called rift zones.
B.) Lava flows out of the rift zone.
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C.) This builds up a volcano.
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Where volcanoes occur cont.
D.) Examples: Volcanoes of Iceland (On the
Mid Atlantic Ridge).
3. Convergent Plate Boundaries - The area where
plates collide into each other.
A.) When plates collide one plate is pushed
downward.
B.) This area is known as a subduction zone.
C.) The heat and pressure of the plate being
downward forms magma.
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Where volcanoes occur cont.
D.) A volcano forms when the magma reaches
the surface.
E.) Examples: Ring of Fire volcanoes in the
Pacific (Mt. St. Helens, Mt. Pinatubo).
4. Hot Spots - Areas of the mantle that are warmer than
other areas.
A.) Rock melts in these areas.
B.) The magma reaches the surface and forms a
volcano.
C.) Example: Hawaiian Islands.
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II. Eruptions & Volcanoes:
A. What causes a violent eruption?
1. Two things determine if a volcano erupts violently or
quietly.
A.) The amount of water and gas trapped in the
magma.
B.) The type of magma.
B. Amount of water and gases trapped:
1. Water and gases in magma act like soda in a can.
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B. Amount of water and gases
trapped cont.
2. If you shake the can and open it the gases explode out of the
can.
3. Gases in magma also try to escape.
4. When the gases escape the volcano explodes.
After
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C. The Type of Magma:
1. Basaltic magma has a low viscosity.
2. Viscosity - The ability of a substance to flow or the
silica content of magma .
3. Gases are released easily.
4. This causes nonviolent eruptions like Mt. Kilauea in
Hawaii.
5. Granitic magma has a high viscosity.
6. Gases build up huge amounts of pressure before
being released.
7. This causes violent eruptions like Mt. St. Helen's in
Washington.
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D. Activity Levels:
1. Volcanoes have three activity levels.
a). Active- A volcano that is erupting or it is
showing signs that it may erupt in the near future.
b.) Dormant- A volcano that is not erupting, and
not showing signs that it will anytime soon. It
could be come active at any time.
c.) Extinct- A volcano that is no longer in an
active volcanic zone. It has very little chance of
ever becoming active again.
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E. Forms of Volcanoes:
1. The form depends on if it was a nonviolent or
violent eruption.
2. There are three basic types:
A.) Shield Volcanoes
B.) Cinder Cone
C.) Composite
3. Shield Volcanoes - a broad volcano with gently
sloping sides, built by nonviolent eruptions of
basaltic magma.
A.) Example: Hawaiian Islands.
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Forms of Volcanoes Cont.
4. Cinder Cone - Volcano that forms from volcanic
material that is violently erupted and piles up into a
steep sided cone mountain.
A.) Tephra - lava and or other volcanic
material that harden into ash, cinders or
bombs.
B.) Example: Paricutin in Mexico (Corn Field
Volcano).
5. Composite Volcano - Volcano that alternates
between violent tephra eruptions and nonviolent lava
eruptions.
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Forms of Volcanoes Cont.
A.) Composite volcanoes are the most violent.
B.) Example: Mt. St. Helens, Mt. Pinatubo.
C.) Pyroclastic Flows are very common.
A dense, destructive mass of very hot ash, lava
fragments, and gases ejected explosively from a
volcano and typically flowing at great speed
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Mt. Kilauea
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Mt. Paricutin
Mt. Etna
Mt. Vesuvius
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Mt. Pinatubo
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Mt. Mauna Loa (Hawaii)
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Magma Chamber (Batholith)- a large underground pool of liquid rock found beneath the
surface of the Earth
Dike- a sheet of rock that cuts across a pre-existing rock body.
Laccolith- Magma that has been injected between two layers of sedimentary rock. The
pressure of the magma is high enough that the overlying rock layers are forced upward,
giving the laccolith a dome or mushroom-like form.
Sill- a sheet of rock that runs parallel to a pre-existing rock body.
Xenolith- is a rock fragment which becomes enveloped in a larger rock during the latter's
development and hardening.
Central Vent- When pressure is strong enough magma shoots up through this primary
vent.
Fissure- Secondary magma vents.
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Structures Inside of a Volcano
Structures of Igneous Rock. Legend: A = magma chamber (batholith); B = dyke/dike; C =
laccolith; E = sill; F = stratovolcano; processes: 1 = newer intrusion cutting through older
one; 2 = xenolith or roof pendant; 3 = contact metamorphism; 4 = uplift due to laccolith
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emplacement.