Warm Up
1. What is an earthquake?
 2. What are the different types of
earthquakes?
 3. What are the different types of
boundaries?
 4. What are the layers of the
atmosphere?
 5. What are the layers of the Earth?

Volcanic Eruptions
and Hazards
Mt. St. Helen
What is a volcano?

vent
cone
conduit
A mountain or hill,
typically conical,
having a crater or
vent through which
molten rock
(magma), rock
fragments, hot
vapor, and gas are
or have been
erupted from the
earth's crust
 The volcano
includes the
surrounding cone
of erupted
magma material.
chamber
What causes the magma to escape
the mantle and come up through
the crust of Earth?

Subduction Zone Volcanoes
–

Divergent Zone Volcanoes
–

Remember that subduction happens at
convergent plates
This results in ridges
Hot Spots
–
These can pop up anywhere 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
The tectonic plate moves over a fixed hotspot
forming a chain of volcanoes.
The volcanoes get younger from one end to the other.
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.
–

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!
–

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).
Types of Volcanoes

An active volcano is a volcano that has had
at least one eruption during the past 10,000
years. An active volcano might be erupting
or dormant.
 An erupting volcano is an active volcano
that is having an eruption...
 A dormant volcano is an active volcano
that is not erupting, but supposed to erupt
again.
 An extinct volcano has not had an eruption
for at least 10,000 years and is not expected
to erupt again in a comparable time scale of
the future.
Why do volcanoes stop
erupting?
all the trapped volatile gasses have
degassed and there is no longer
sufficient pressure to drive the magma
out of the Earth.
OR
 enough heat is lost so that the magma
cools and is no longer buoyant

Pacific Ring of Fire
Volcanism is
mostly
focused at
plate
margins
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

Explosive Eruptions

Explosive volcanic
eruptions can be
catastrophic
 Erupt 10’s-1000’s km3 of
magma
 Send ash clouds > up to
25 km into the
stratosphere
 Have severe
environmental and climatic
effects
 Hazardous!!!
Mt. Redoubt
Above: Large eruption column and
ash cloud from an explosive
eruption at Mt Redoubt, Alaska
Explosive Eruptions

Three products from an
explosive eruption
–
Ash fall
 The
–
Pyroclastic flow

–
fallout of rock, debris and ash
Pyroclastic flows are hot, turbulent,
fast-moving, high particle concentration
clouds of rock, ash and gas.
Pyroclastic surge
 they
don’t have a high concentration
of particles and contain a lot of gas
Pyroclastic flows on
Montserrat, buried
the capital city.
Direct
measurements of
pyroclastic flows
are extremely
dangerous!!!
Pyroclastic Flow - direct impact
Courtesy of www.swisseduc.ch
Pyroclastic Flow - burial
Pyroclastic Flow - burns
Pyroclastic Flow - lahars




Hot volcanic activity can melt
snow and ice
Melt water picks up rock and
debris
Forms fast flowing, high
energy torrents
Destroys all in its path

From above you can see their fine grainsize and the distinctive
slight ridges and grooves that show which way the mass of hot
dusty air was moving.
Pyroclastic Fall
Ash load
Collapses roofs
Brings down power lines
Kills plants
Contaminates water supplies
Respiratory hazard for
humans and animals
Effusive Eruptions

Effusive eruptions are
characterized by outpourings
of lava on to the ground.
Hawaii
Courtesy of www.swisseduc.ch
Lava Flow
It is not just explosive volcanic activity that
can be hazardous. Effusive (lava) activity
is also dangerous.
 Lava flows have temperatures in excess
of 200 degrees Celsius

How can scientists tell if there is
going to be an eruption?
Volcanoes Give Signs

"These signs may include very small
earthquakes beneath the volcano,
slight inflation, or swelling, of the
volcano and increased emission of
heat and gas from vents on the
volcano,"
 said
U.S. Geological Survey (USGS) Volcano
Hazards Program coordinator John
Eichelberger.
Deformation Monitoring

“Tiltmeters” are used to measure the
deformation of the volcano
–
The tiltmeters measure changes in slope as small as one part
per million. A slope change of one part per million is
equivalent to raising the end of a board one kilometer long
only one millimeter!
Deformation Monitoring

Tiltmeters can tell you when new material enters the magma chamber.
A
B
Note the
presence of
earthquakes in
relation to the
deformation.
Often it is a
combination of
events that
fore-warns of
an eruption.
Gas Monitoring

Commonly gas output from a volcano increases or changes
composition before an eruption.
– As magma rises to the surface it releases (exsolves) much
of its gas content.
– This can be measured
Types of volcanoes:

Composite Volcanoes-are a mixture
between shield volcanoes and cone
volcanoes, their eruptions are
explosive.
Types of volcanoes:
Composite Volcanoes
 Shield Volcanoes-are low and flat and
have small, flowing eruptions

Types of volcanoes:
Composite (Strato) Volcanoes
 Shield Volcanoes
 Cinder Cones
- Cones are the tallest
and largest
volcanoes,
and they have
VERY explosive
eruptions.

Types of volcanoes:
Composite Volcanoes
 Shield Volcanoes
 Cinder Cones
 Spatter Cones

Types of volcanoes:
Composite Volcanoes
 Shield Volcanoes
 Cinder Cones
 Spatter Cones
 Super (Complex)
Volcanoes

In Summary..
Volcanoes are extremely hazardous.
 However, the volcano can be
studied, monitored and understood.
 Each volcano is different, and offers
a unique set of dangers
 Plans may be emplaced to help
control potential damage.

Post-lecture Question:

What should geologists do about
volcanic eruptions in the future?
1. Study volcanoes to find out more about how and why
they erupt
2. Monitor the volcanoes
3. Develop hazard mitigation plans
4. Understand the population around volcanoes, i.e. why
do people choose to live near volcanoes?
5. Education
Warm Up
1. What is a divergent boundary? Draw
one.
 2. What is a convergent boundary?
Draw.
 3. What is subduction? Draw.
 4. What is a transform boundary?
Draw.
 5. What is the lithosphere?
 6. What are the layers of the
atmosphere?

Earthquakes

As with volcanoes, earthquakes are not
randomly distributed over the globe
Figure showing
the distribution of
earthquakes
around the globe

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?
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


Friction
causes the
plates to get
stuck, building
pressure and
when the
pressure if
released, an
earthquake
occurs.
 The point
where the
earthquake
starts is called
the focus.
Where do earthquakes form?
Figure showing the tectonic setting of earthquakes
Earthquake key terms:
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.
 Foot
How do they move?
5 ways
 Strike-Slip Quake
(happen at transform
boundaries)
 Normal- Hanging wall
moves down. (divergent
boundaries)
 Reverse/Thrust Quakeshanging wall moves up
(convergent boundaries.)

How do they move?
Horst Quakes
 A horst
represents a
block pushed
upward by the
faulting, and a
graben is a
block that has
dropped due to
the faulting.
 Graben
Quakes

Strike-slip Earthquakes
Normal Earthquake:
Normal Earthquake:
Reverse (Thrust) Earthquake:
Graben:
Horst:
Warm Up
1. What is an earthquake?
 2. Where do earthquakes happen?
 3. What is a strike slip earthquake?
 4. What is a normal earthquake?
 5. What is a reverse earthquake?
 6. What is a horst earthquake?
 7. What is a graben earthquake?

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.
Seismic Activity

Earthquake activity commonly precedes an
eruption
– Result of magma pushing up towards the
surface
– Increase volume of material in the volcano
shatters the rock
– This causes earthquakes
Seismic Activity
Earthquake activity is measured by Seismographs
Seismographs are stationed on the flanks of the
volcano
These record the frequency, duration and intensity of
the earthquakes and report it back to the volcano
observatory.
Earthquakes
Large volumes of magma moving
through the shallow crust can cause
large earthquakes.
 This can lead to building collapse,
slope failure and avalanches

Earthquakes
Destruction after a
volcanic induced
earthquake in Japan
Pyroclastic flow

A pyroclastic flow is a fluidized mixture of
solid to semi-solid fragments and hot,
expanding gases that flows down the flank of
a volcanic edifice. The features are heavierthan-air emulsions that move much like a
snow avalanche, except that they are
fiercely hot, contain toxic gases, and move
at phenomenal, hurricane-force speeds,
often over 100 km/hour. They are the most
deadly of all volcanic phenomena.
Pyroclastic Surge

Pyroclastic Surge: A more energetic
and dilute mixture of searing gas and
rock fragments is called a pyroclastic
surge. Surges move easily up and
over ridges; flows tend to follow
valleys. They are relatively thin; usually
no more than about 10 cm thick,
except in local "drifts" (like snow).