Chapters 4 & 5: Volcanoes
What you will learn
1. How plate tectonics leads to volcanic activity
2. The parts of a volcano and the products of a volcanic
eruption
3. External or extrusive landforms (volcanic)
4. Internal or intrusive landforms (plutonic)
5. Effects of volcanoes – positive and negative using case
studies
6. Predicting volcanoes using a case study.
7. Know how this topic appears on the exam paper
Volcano Intro Clip
• http://www.youtube.com/watch?v=uZp1dN
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Common Exam Questions
1. Explain how the study of plate tectonics has
helped us to understand the occurrence of
volcanoes
2. Discuss the positive (and negative) impacts of
volcanoes
3. Examine the processes that have led to the
formation of any two volcanic landforms
Mind Map Volcanoes
• In groups think of everything you remember
about volcanoes since Junior cert
• How they occur
• Effects
• Examples etc
Activity: Take down the mind map from the board
Activity: label the diagram of a
volcano
Parts of a volcano Keywords
•
•
•
•
•
Vent
Fissure
Cone shaped mountain
Magma chamber
Lava
The Distribution of Volcanic
Activity
Volcanoes occur in three types of locations
1. Mid Ocean Ridges (at constructive plate
boundaries)
2. Subduction zones (at destructive plate
boundaries)
3. Hotspots in the middle of the plates
Volcanoes at Divergent Margins
• As plate separate, magma rises through
the crust and out through the vent or
fissure in the volcano; the lava cools and
solidifies to form new crust (or a new layer
to the volcano)
• Mid-ocean ridges are created in this way
• Lava plateau form where the lava spreads
out rather than forming a volcano
Volcanoes at Convergent
Margins
• Subduction occurs where the heavier plate is
pulled down under the lighter plate due to gravity
and is melted deep in the mantle
• This produces an explosive viscous (thick) lava;
eruptions are violent due to intense build up of
pressure
• Dome volcanoes are steeply sloping cones with
convex sides; formed because the lava is so
thick and barely able to flow; it cools and
solidifies forming a dome; layers build up but
could also be destroyed by very violent
eruptions
Volcanoes at Hotspots
• Hot spots form when a rising column of
very hot magma called a thermal plume
forces its way through a fault in the
oceanic crust; they do not drift with the
plates; they have a fixed position
• Shield volcanoes form where the lava runs
freely & quickly creating gentle slopes
Remember we have done this
already in chapter 1….
• Can you remember the connection
between plates and volcanic activity?
• Activity: Read the sample answer on
volcanoes and plate boundaries. In groups
summarise the answer in bullet points into
your copy books
Active, Dormant & Extinct
Volcanoes
• Active – almost constantly erupting
• Dormant – no eruption for more than 100
years
• Extinct – no eruptions in historic times
Products of Volcanic Eruptions
Keywords
1.
2.
3.
4.
5.
Lava
Pyroclasts and pyroclastic flows
Poisonous gases
Ash, pumice and dust
Water Vapour
1. Lava
• Two types of Lava – Acidic and Basic
• Type of lava changes depending on the
plate boundary
• Contains silica – The amount of silica
determines how sticky the lava is.
Lava
• Acid lava is high in silica which prevents
gases escaping; is very explosive; found
at convergent (destructive) plate margins
• Basic lava is low in silica; lava flows
freely; found at divergent (constructive)
plate margins. Gentle lava eruptions
Activity: In pairs fill in the following
tables
Properties
Silica Content
Trapped Gases
Type of eruption
Typical location
Examples
Acid Lava
Basic Lava
2. Pyroclasts
• Hot ash, lava and rock fragments that are
thrown out of the volcano. Sometimes
called volcanic bombs.
• Pyroclastic Flows are boiling clouds of ash
and rock which travel at great speeds
(600km/hr) down the sides of the volcano.
Also called Nuée ardentes.
3. Poisonous Gases
• Carbon dioxide
• Chlorine
• Sulpher dioxide
4. Ash, Pumice and Dust
• Power of the eruption turns rock into a fine ash
inside the vent.
• The ash is thrown many kilometres into the sky
and may be carried by winds across the world.
Can disrupt air travel.
• When lava is thrown from the volcano it becomes
full of air bubbles. If it cools quickly the bubbles
become trapped in the rock and it is now called
pumice. Some pieces contain so much air that
they float in water.
5. Water Vapour
• Volcanoes along subduction zones
release huge amounts of water vapour
• (From the sea water that’s carried into the
mantle by the sinking oceanic plates)
• As the water vapour rises into the air it
cools quickly creating torrential rain.
• This can trigger lahars
Lahars
• Word that describes a mixture of water
and rock fragments flowing down the
slopes of a volcano.
• Looks like a large amount of wet concrete
• Lahars vary in speed and size
• Volcanic eruptions can trigger one or more
lahar by quickly melting snow and ice on a
volcano.
Activity
• Work in pairs to discuss the positive and
negative effects of volcanoes.
Good Effects of Volcanoes
• Energy – geothermal energy (hot water) is piped
to many homes and businesses in Iceland
• Food production is abundant in volcanic regions;
lava cools to form basalt which weathers to a
mineral-rich soil
• Tourism – volcanoes & geysers attract tourists in
the millions e.g. Mt Vesuvius, Italy & Old
Faithful, Yellowstone National Park, USA
• New land is created Iceland and Hawaii are two
examples
• Minerals
Bad Effects of Volcanoes
• Toxic gases (carbon dioxide, hydrogen, carbon
monoxide & sulphur) are released
• Pyroclasts (fiery volcanic bombs) rain down on
surrounding areas
• Nuée Ardente are clouds of hot gases & pyroclasts
running down the side of a volcano at high speed
• Lahar is a fast moving volcanic mudflow
• Jokulhlaups are glaciers which are melted by an eruption
beneath them
• Climate change may occur if eruptions are particularly
devastating
• Can disrupt air travel
Volcanic Landforms
• Intrusive (plutonic): occur beneath the surface
but are visible because of weathering and
erosion.
• Extrusive (volcanic): Features found on the
earth’s surface.
Intrusive (plutonic) Landforms
1.
2.
3.
4.
5.
Batholiths
Sills
Dykes
Laccoliths
Lopoliths
Extrusive (volcanic) Landforms
1. Volcanic cones - shield volcanoes and
volcanic domes.
2. Lava Plateaux
Intrusive Landforms
1.
2.
3.
4.
5.
Batholith
Sill
Dyke
Laccoliths
Lopoliths
Intrusive Landforms
1. Batholith – a large mass of igneous rock
(granite) that formed as magma from the
mantle pushed into the crust and slowly
cooled down and became solid.
Activity:
Read the notes in your book about
Batholiths. Summarise the main points and
draw the diagram in your copy.
• Sills: Magma forces its way between layers of rock and
becomes solid as large flat areas of igneous rock.
• Dykes: Magma slices across rock layers and becomes
solid and forms a wall of basalt or granite. It runs
perpendicular to the rock layers.
• Laccoliths: Magma seeps between rock layers pushing
them up to form dome like structures.
• Lopoliths: Form the same way but the weight of the
magma causes the rock to sag downwards.
In pairs name the following intrusive
features
Extrusive (volcanic) Landforms
1.
2.
3.
4.
Volcanic cones
Shield volcanoes
Volcanic domes
Lava plateau
Extrusive Landforms
• Volcanic Cones: magma rises from the crust.
Gas bubbles expand and help the magma to
force its way through the cracks. Once the
magma reaches the surface an eruption
occurs which gradually forms a volcanic cone.
There are two types
• 1) shield volcanoes 2) volcanic domes
Shield Volcanoes
• Broad, gentle, concave slopes e.g. Mauna Loa,
Hawaii.
• Associated with hotspots – highly fluid basic
lava.
• Basic lava flows easily over the ground for
many kms before it becomes solid.
• Mount Loa is 4,160m above sea level but from
its base it is 10,000m high. 2km taller than
Mount Everest.
Volcanic Domes
• Mounds that form when acidic lava is erupted
slowly and piles up covering the vent rather
than flowing away.
• Common at subduction zones.
• Steep sided convex slopes that are very
unstable and may callopse causing massive
landslides.
• Mount St. Helen USA.
Lava Plateau
•
Steep sided flat topped uplands of basalt that
cover large areas
• Formed by large but less explosive eruptions of
highly fluid basic lava that pours from long narrow
openings or fissures in the crust.
• E.g. Antrim Plateau in County Antrim.
• Have the same characteristics
1. Cover large areas of land
2. Have a layered structure
3. Each lava eruption makes the plateau higher.
Name the features
Exam Question
• Discuss the formation of any two volcanic
landforms.
Investigative Learning
• In groups investigate how volcanic eruptions
can be predicted.
• Use your book to help. Find at least 4
methods.
• Discuss the results with the class.
Homework
• Write into your copies how volcanoes can be
predicted.
• Finished chapter today so traffic light the
keywords and the chapter.
End of chapter
• Look back at the chapter objectives you wrote
at the start of the chapter.
• Have you achieved these or are you still
working on them??
• Traffic light your progress