Transcript Volcanic Eruptions and Hazards

Volcanic
Eruptions
Mt. St. Helen
What is a volcano?

vent
cone

conduit
magma
chamber
A volcano is a
vent or 'chimney'
that connects
molten rock
(magma) from
within the Earth’s
crust to the
Earth's surface.
The volcano
includes the
surrounding cone
of erupted
material.
What causes the magma to escape
the mantle and come up through
the crust of Earth?
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Subduction Zone Volcanoes
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Divergent Zone Volcanoes
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Remember that subduction happens at
convergent plates
This results in ridges
Hot Spots
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These can pop up anywhere where the crust is
weak and thin, even in the middle of plates
What are Hotspot Volcanoes?

A hotspot is a location on the Earth's
surface that has experienced active
volcanism for a long period of time
The Hawaiian island chain are
examples of hotspot volcanoes.
Photo: Tom Pfeiffer / www.volcanodiscovery.com
What is the difference in magma and
lava?

Magma

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Molten (melted) mantle beneath the surface of the
Earth
Lava
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Magma that reaches the surface
How and why do volcanoes erupt?

Hot, molten rock (magma) is
buoyant (has a lower density than
the surrounding rocks) and will rise
up through the crust to erupt on
the surface.
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Same principle as hot air rising, e.g.
how a hot air balloon works
When magma reaches the surface
it depends on how easily it flows
(viscosity) and the amount of gas
(H2O, CO2, S) it has in it as to how
it erupts.
How and why do volcanoes erupt?

Large amounts of gas and a high
viscosity (sticky) magma will form
an explosive eruption!
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Think about shaking a carbonated
drink and then releasing the cap.
Small amounts of gas and (or) low
viscosity (runny) magma will form
an effusive eruption

Where the magma just trickles out of
the volcano (lava flow).
Ring of Fire
The Ring of Fire has 452 volcanoes and is
home to over 75% of the world's active
and dormant volcanoes
 The Ring of Fire is a direct result of plate
tectonics and the movement and collisions
of crustal plates

Earthquakes

As with volcanoes, earthquakes are not randomly
distributed over the globe
Figure showing
the distribution of
earthquakes
around the globe
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At the boundaries between plates, friction causes
them to stick together. When built up energy
causes them to break, earthquakes occur.
Where do they happen?

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Most often they happen where the plates meet
(fault lines)
Sometimes in the middle of the plate where the
crust becomes very heavy and drops like a sink
hole. EX Mississippi River delta
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The point
where the
earthquake
starts is called
the focus.
Where do earthquakes form?
Figure showing the tectonic setting of earthquakes
Earthquake key terms:
Foot wall: The plate that doesn’t move
during an Earthquake.
 Hanging wall: The plate that moves
during an Earthquake.
 Fault plane: The plane along which the
break between two plates occurs.
 Fault line: The line in the surface of the
Earth caused by the fault plane.

How do they move?
5 ways
Strike-Slip Quake (happen at
transform boundaries)
Normal- Footwall and the
hanging one is the one that
moves.
Reverse/Thrust Quakeshanging wall moves up
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How do they move?
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Horst Quakes
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The graben are the
downdropped
blocks and the
horst are the
upthrown blocks
that lie next to the
graben
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Graben Quakes
Strike-slip Earthquakes
Normal Earthquake:
Reverse (Thrust) Earthquake:
Graben:
Horst:
How do you tell how severe an
earthquake is?
Earthquakes generate seismic
waves which can be detected
with a sensitive instrument called
a seismograph .
 The Richter Scale is based in
energy released as measured by
maximum wave amplitude on a
seismograph

Richter Scale
Richter scale no.
No. of earthquakes per
year
Typical effects of this magnitude
< 3.4
800 000
3.5 - 4.2
30 000
4.3 - 4.8
4 800
Most people notice them, windows rattle.
4.9 - 5.4
1400
Everyone notices them, dishes may break, open doors
swing.
5.5 - 6.1
500
Slight damage to buildings, plaster cracks, bricks fall.
6.2 6.9
100
Much damage to buildings: chimneys fall, houses move
on foundations.
7.0 - 7.3
15
Serious damage: bridges twist, walls fracture, buildings
may collapse.
7.4 - 7.9
4
> 8.0
One every 5 to 10 years
Detected only by seismometers
Just about noticeable indoors
Great damage, most buildings collapse.
Total damage, surface waves seen, objects thrown in the
air.