Download Volcanoes - Geog

Volcanoes And Volcanic Hazards
IB SL
What is a volcano?
A volcano is an opening or vent in the earth’s surface
through which molten material erupts and solidifies as
lava.
Volcanic vent
Definition
• A volcano is an opening through the Earth’s
crust through which hot molten magma (lava),
molten rock and ash are erupted onto the
land.
• Most volcanoes are found at plate boundaries,
although there are some exceptions, such as
the volcanoes of Hawaii.
• Some eruptions let out so much material that
the world’s climate is affected for a number of
years.
Characteristics
• Magma: refers to molten materials inside the Earth’s
interior. When the molten material is ejected at the
Earth’s surface through a volcano or crack at the
surface, it is called lava.
• Chamber: refers to the reservoir of magma located
deep inside the volcano.
• Crater: is the depression at the top of a volcano
following a volcanic eruption. It may contain a lake.
• Vent: is the channel which allows magma within the
volcano to reach the surface in a volcanic eruption.
Where are volcanoes found?
Facts
• The greatest volcanic eruption was Tambora in
Indonesia in 1815. Some 50-80 km3 of
material was blasted into the atmosphere.
• In 1883 the explosion of Krakatoa was heard
as far as 4776 km away.
• The largest active volcano is Mauna Loa in
Hawaii, 120 km long and over 100 km wide.
Cross section of a volcano
Crater
Volcanic
bombs, ash
and gases
Parasitic
cone
Main vent
Magma
chamber
Why do they happen?
A destructive plate boundary is found
where a continental plate meets an
oceanic plate.
Destructive Plate
Boundary
The oceanic plate descends under the
continental plate because it is denser.
As the plate descends it starts to melt
due to the friction caused by the
movement between the plates. This
melted plate is now hot, liquid rock
(magma). The magma rises through
the gaps in the continental plate. If it
reaches the surface, the liquid rock
forms a volcano.
Destructive Plate Boundary
Why do they happen?
At a constructive plate
boundary, two plates move
apart.
Constructive Plate
Boundary
As the two plates move
apart, magma rises up to
fill the gap. This causes
volcanoes at this type of
boundary. However, since
the magma can escape
easily at the surface, the
volcano does not erupt
with much force.
Constructive Plate Boundary
At destructive plate boundaries the lava is viscous
(thick like treacle) and it cannot flow very far from
the volcano’s vent. This makes the volcano steep
sided.
At constructive plate boundaries the lava is
runny and it can flow far away from the
volcano’s vent. This makes the volcano gentle
sided.
Types Of Volcano
• Two contrasting types exist;
• Those dominated by lava, usually found at
diverging boundaries, where basalt can rise
freely to the surface (volcanoes of Iceland)
• Those dominated by Ash, usually found along
subduction zones where large amounts of
pyroclastic material is ejected (Japan - Fuji
Yama)
Cone Shapes
Cone Shape:
Characteristics:
Fissure
Very gentle slope, found at diverging
ocean plates, basaltic lava, can flow over
large distances.
Basic / Shield
Have gentle slopes, steeper than fissure
due to repeated explosions and
subsequent build up of basalt based lava
(Mauna Loa Hawaii).
Cone
Symmetrical in shape, A) acid where thick
viscous lava, rapidly cools, B) Ash / Cinder.
Composite
Very large old volcanoes. Both ash and
lava are deposited (Mt. St. Helens).
Crater / Caldera
Form when a very violent eruption occurs
after a build up of gas beneath the
volcano. Can destroy the magma chamber
leaving a large crater.
Cones
Do all volcanoes erupt?
•Active volcano - liable to erupt e.g. Mt. Etna
•Dormant (sleeping) volcano - a volcano which has
not erupted for many years. For example, Mt
Pinatubo erupted in 1991 after 500 years of
dormancy.
•Extinct volcano - a volcano which has not erupted
for many thousands or millions of years. e.g.
Edinburgh
However, it is often very difficult to tell whether a volcano will erupt
again…El Chichon, Mexico erupted in 1982 after being dormant for
approximately 1200 years!
plate movement
Active volcanoes
Extinct volcanoes
Hot spot
In the diagram above, why are the volcanoes
to the right of the ‘hot spot’ extinct?
Types Of Lava Flow
• This ranges from being extremely thick and
viscous, to highly fluid. The amount of pyroclastic
material also varies. The two types are:
• Aa flow: This is a few metres thick, a mix of
uneven shaped, sharp edged ash and cinder
blocks. It is unstable.
• Pahoehoe flow: Is fluid rather than viscous, but
does not move quickly. Often the surface layer is
static whilst flow continues beneath.
Types Of Eruption
Types Of Eruption
Hawaiian Type
Lava slowly and easily escapes from vent. It is runny, basaltic.
Gases escape easily & gentle sided volcanoes result.
Strombolian type
Gas explosions occur more frequently, small but frequent
eruptions. Cone shaped volcano.
Vulcanian type
Gas emissions involved, more violent but less frequent than
above. Plugs of cooled lava may be ejected in blast.
Vesuvian type
Extremely strong explosions, often after volcano has been
dormant for a while. Gas and ash clouds, which fall over a large
area.
Krakatoan type
Exceptionally violent.
Plinian type
Usually most violent. Massive amounts of lava, gas and pyroclastic
material emitted. Part of the volcano may be removed.
Primary Volcanic Hazards
• Lava flows: A greater threat to property than human life due to
opportunity for evacuation. Lava flows are more dangerous when released
very quickly. They usually cause large but localised economic losses.
• Pyroclastic flows: These are hot rock fragments, lava particles ash and hot
gases. They are linked with subduction zone volcanoes, and the flow moves
very quickly from its source.
• Ash and Tephra fall: Ash is material below two millimetres in diameter
whilst tehpra is anything above this. It is usually formed when magma is
fragmented by explosions, and can stay in the atmosphere causing global
variations in weather patterns. Ash fall do not cause many deaths but can
lead to breathing difficulties.
• Volcanic gases: Gases emitted from volcanoes include, water vapour,
carbon dioxide, sulphur dioxide, hydrogen sulphide, helium and carbon
monoxide. They rarely cause death but can be problematic as many are
heavier than air.
Secondary Volcanic Hazards
• Lahars: Mud flows of volcanic material due to
ash and debris mixing with water. On steep
slopes speeds can reach 22 metres per
second.
• Volcanic landslides: These are slides of rock
and loose volcanic material, which are driven
by gravity.
• Tsunamis: A rare event, but the 1883 eruption
of Krakatoa, did create a 30m high tidal wave.
Activity
Using Essential AS Geography P38-39, answer
the following…
1. What are the differences between “fissure”,
and “vent” volcanoes?
2. How do silica, and volatiles affect volcanic
eruptions?
3. Compare composite and shield volcanoes by
sketching, labelling, and annotating diagrams
of both.