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Volcanoes
The cause of it all…

What causes volcanoes to erupt???

The shift in the Earth’s plates are what causes
volcanoes to form.

As the plates join or separate some of the molten
rock is exposed.

viscosity, resistance of a fluid to flow.
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.

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).
Explosive
Rapidly escaping gas bubbles rip apart magma,
fragmenting it.





Explosive volcanic
eruptions can be
catastrophic
Erupt 10’s-1000’s
km3 of magma
Send ash clouds >25
km into the
stratosphere
Have severe
environmental and
climatic effects
Hazardous!!!
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

Pyroclastic flow

Pyroclastic surge
Pyroclastic flows on
Montserrat, buried
the capital city.
Effusive
magma leaks out onto the surface passively as lava
flows.

Effusive eruptions are
characterized by
outpourings of lava on
to the ground.
Courtesy of www.swisseduc.ch
Hawai
i
Where do volcanoes erupt???
•
Convergent plate boundaries – where two of Earth’s plates join
together RING OF FIRE
•
Divergent plate boundaries – where two of Earth’s plates move
apart MID OCEAN RIDGES
•
Hot spots – active areas below the earths crust
Structure of a
Volcano
ash cloud
lava
central
vent
side vent
old layers
of lava
magma
Earth’s crust
Structure of a Volcano

Magma – the molten, or liquid-like, rock
within the Earth

Lava – magma that reaches Earth’s surface

Vent – an opening through which molten (liquid-like)
rock flows onto Earth’s surface

Fissures: side vents, usually accompanying the main
vent.
Crater/caldera: large, depression created by the
emptying of the magma at great speed. The volcano
collapses creating a large crater.
Ash cloud: smoke and debris usually consisting dust,
hot rock fragments (tephra), gases, water vapor, and
other particles.


Types of Volcanoes
There are three major types of volcanoes:
based on composition of
magma

Shield volcanoes
 Composite

volcanoes
Cinder cone volcanoes
Shield Volcanoes
• The magma inside a shield volcano is rich in iron
and magnesium and is very fluid.
• Since the magma is very fluid, the lava coming
out of the volcano tends to flow great distances.
• When shield volcanoes erupt, the flowing lava
gives the volcano the shape of a gently sloping
mountain.
Shield Volcanoes

Eruptions of shield volcanoes are mild and can occur several
times.

Happens in HOT SPOTS (place where earth’s crust is weak and
lava may flow continuously)

Mauna Loa in Hawaii is an example of a shield volcano.
Shield Volcanoes
Composite Volcanoes
• The magma inside a composite volcano is rich in
silica and much thicker than magma from a
shield volcano.
• Gases get trapped inside this thicker magma.
• Eruptions from composite volcanoes can be
flowing lava or explosions. The explosive
eruptions come from the trapped gases and
produce cinders and ash.
Composite Volcanoes
• These different types of eruptions are what give
composite volcanoes their alternating layers of
lava and cinders.
• Composite volcanoes have much steeper slopes
than shield volcanoes.
• Happens at subduction zones
• Mount Fuji in Japan and Mount St. Helens in the
USA are examples of composite volcanoes.
Composite Volcanoes
Cinder Cone Volcanoes
• The magma inside a cinder cone volcano has
large amounts of gas trapped in it.
• Eruptions from cinder cone volcanoes are
violent and explosive because of all the gas
trapped in the magma.
• The large amounts of hot ash and lava thrown
out of the vent fall to the ground forming the
cone shape that these volcanoes have.
Cinder Cone Volcanoes
• Cinder cone volcanoes are usually only
active for a short time and then become
dormant (inactive).
• Paricutin in Mexico is an example of a
cinder cone volcano.
Cinder Cone Volcanoes
Volcanic Products

Pyroclastic
flows / falls

Lahars

Lava

Volcanic ash
Pyroclastic flows
Pyroclastic flows are
mixtures of hot gas, ash
and other volcanic
rocks travelling very
quickly down the
slopes of
volcanoes. They are
one of the most
dangerous hazards
posed by volcanoes.

Pyroclastic flows are
so hot and choking
that if one is caught in
one the person will
certainly be
killed. Because these
flows are very fast they
cannot be out-runned!

Pyroclastic Fall
Ash load
 Collapses roofs
 Brings down
power lines
 Kills plants
 Contaminates
water supplies
 Respiratory
hazard for
humans and
animals
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
•term
that describes a hot or
cold mixture of water and
rock fragments flowing down
the slopes of a volcano and
(or) river valleys.
Volcanic ash

Volcanic ash is a
volcanic rock which is
exploded from a vent
in fragments less than
2mm in size.

Volcanic ash-particles
are like small sharp
glass-particles that
damage anything they
come across.

During heavy ash-rains
houses and buildings
may collapse, people
and animals may die
by lack of oxygen.
Pahoehoe Lava
Pahoehoe lavas
flow smoothly and
are often formed by
small volumes of
hot, fluid
basalt. The higher
the volume of lava
emitted the faster
the current.
 . When the
pahoehoe lava flow
cools, it often
solidifies to a
smooth surface.

Aa Lava

Aa flows are emitted
from the vent at high
rates ranging to 50km
an hour, often with
much lava
fountaining.

Aa flows are animated
with sporadic bursts of
energy. They may push
down houses, walls and
forests.

However, the hallmark
of aa lava flows is the
very rough surface it
produces when it cools
and solidifies.
Lava flow differences
Volcano Monitoring
Volcano
Observatories are
set up on all active
volcanoes that
threaten the human
population. These
are designed to
monitor and
potentially to
predict the eruptive
behavior of the
volcano in question.
• Seismicity
• Deformation
• Gas Output
These three are the most important in
monitoring active volcanoes.
Volcanic Gas

Volcanic gas is
contained within
magma. As the
magma rises to
the Earth's
surface the
gases are
released. Becau
se some gases
are toxic they
can suffocate
people.
Seismometers


Seismometers are
instruments that
measure and record
motions of the
ground, including
those of seismic
waves generated by
earthquakes.
When a volcano is
ready to erupt, the
magma chamber
starts filling up,
creating cracks in the
surrounding rocks, i.e.
earthquakes
Global Impact

When a volcano
erupts it throws out a
lot of ash. At short
notice this ash can be
very harmful to the
environment, but on
the long term the ash
layer, which contains
many useful minerals,
will be converted to a
very fertile soil.

Volcanoes provide
resources for energy
extraction, also called
geothermal
resources. Heat from
the earth's crust is
being converted to
energy. The big
advantages to this
type of energy are
that it is very clean
and the resources are
nearly inexhaustible.
Global Impact

Sea Floor Spreading- new crust is formed at the
bottom of the ocean floor.
Final Exam Focus Question
Most of the gas erupted from a volcano is
steam, the remainder is…
 A) hydrogen sulfide B) Carbon Dioxide

Although volcanic ash is talked about a lot,
the most significant global effects are
produced by:
 A) heat from lava flows
 B) melting of glaciers during eruptions
 C) Destruction of vegetation

Final Exam Focus

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Why do earthquakes cause such large
death tolls in 3rd world countries?
A) more tsunamis
B) poor hospitals
C) stronger earthquakes
D) weak but heavy construction
materials
What is a tsunami?
A) measure of energy released by an
earthquake
B) A seismic sea wave
Final Exam Focus

Why do volcanoes occur at subduction zones?

A) compression heats the rocks

B) The descending slab begins to melt

C) tension opens cracks so material from the core
can rise.