1. Year 9 Science – Seaview High School
Mountains of Fire
Year 9 Science – Seaview High School

2. Explosive evidence
Most changes to the Earth’s surface are very slow to occur.
Volcanic eruptions can be very explosive and are the most obvious evidence of these changes.
For example, in 1883, the volcano Krakatoa exploded. The sound was heard over 4000 km away and the resultant tsunami drowned about people.

3. How are Volcanoes Formed?
Volcanoes form when magma
(molten rock) forces its way
to the surface through a
weakness in the Earth’s crust.
Once the magma reaches
the surface we call it lava.
Lava is a mixture of magma
and gases, including steam,
carbon monoxide and
hydrogen sulfide (‘rotten egg’
gas).

4. What Comes Out?
As the pressure builds up the volcano begins to show signs of an imminent eruption.
Smoke and ash billow from the vents.
During the eruption lava flows from the vents and red-hot fragments of rock, dust and ash, steam and other gases shoot out of the crater.
Exploding gases can destroy parts of the volcano.
At times, larger fragments may be thrown from the volcano. These are called volcanic bombs, or lava bombs.

5. Dancing With The Lava

6. A Volcano Broken Down Lightning can sometimes be seen in the
ash clouds. Colliding particles of ash and
lava create static electricity
An ash cloud explodes
from the crater and up
to 30 kilometres into the
atmosphere.
Main volcanic vent
Ash settles on
the volcano surface
The volcanic crater is the
point where the magma
reaches the surface and
becomes known as lava.
Blocks of hot rock known
as lava bombs are ejected
Pressure from gas
and magma mixing
underground forces
magma up the main vent
and branch pipes.
Avalanches and landslides
are common during eruptions
Branch pipe
The initial heat blast from
the eruption ‘boils’ trees by
turning the water in plants
and animals to steam.
Melted snow and ice turn
into destructive mudflows
that travel down valleys.
Magma gathers in
the magma chamber
before it is forced
to the surface.

7. Types Of Volcanoes
Lava that erupts from a volcano can be very runny, like honey, or very thick, like toothpaste.
Runny volcano will move away from the volcano site creating large plains of basalt.
Such plains exist around Melbourne.
Thick lava will stick close to the volcanic site resulting in large, cone shaped volcanoes.

8. Types of Volcanoes
Mauna Loa in Hawaii is an example of a shield volcano
Popocepetl is an active composite
volcano in Mexico
Sunset Crater in Arizona, USA,
is an example of a cinder cone volcano

9. Shield Volcanoes Some of the largest volcanoes on Earth.
Forms from layers of very runny lava which can form puddles, rivers and streams (950˚C).
Lava can flow for kilometres creating gently sloping sides.
Usually don’t explode and are usually easy to avoid by car or even by foot.
Occur at hotspots or mid-ocean ridges
Skjaldbreidur Herbst, Iceland is famous for its shield volcanoes

10. Composite Volcanoes Also known as strato-volcanoes
Formed by alternating layers of rock and lava
This is why they are called composite.
Often high altitude. Think of images of snow capped volcanoes such as Mt. Fuji
Can appear to be extinct, but erupt explosively
Occur along subduction zones
Left: Mt Fuji, Fuji, Japan
Right: Vesuvius, Napoli
Italy
Both are examples of
composite volcanoes

11. Cinder Cone Volcanoes A steep conical hill that forms above a vent.
Most common volcanic land forms on Earth.
Usually form in groups around the edges of shield or composite volcanoes.
Built from lava fragments called cinders which are ejected from the vent and fall back near the vent.
Relatively small, 250 m tall and 500 m diameter
Paricutín is a
cinder cone
volcano located
in Mexico.

12. Types of Volcanoes
ations/VolcanoTypes.html
Volcanic lightning as
Mount Shinmoe-dake in
Kyushu, Japan erupts.

13. Birth of a Volcano
On a winter’s day, in Mexico, 1943, a small crack opened up in a corn field.
As cinders shot out of the crack, a farmer attempted to fill it with dirt.
By the next day, the crack had opened to over 2 metres in diameter.
By a week later, a cone 150 metres tall had formed!
Explosions from the crater continued, destroying the local village of Paricutin.
Within a year, the cinder cone had reached 300 m in altitude.
Eruptions stopped in 1952 with the cone reaching a height of 410 metres.

14. Subduction Zones
Most of the world’s volcanoes occur near subduction zones (for example, the ‘Ring of Fire’).
As the dense oceanic plate subducts beneath the less dense continental plate, the rock is pushed down into the mantle and is turned to molten rock.
As this occurs, water is forced down with the subducting plate.
This water turns to gas (steam) and escapes back towards the surface through any path it can find.
The molten rock follows these paths and forces its way to the surface causing an eruption.
eruption.swf

15. Hot Spots
Volcanoes that occur away from the edges of the tectonic plates can be explained by hot spots.
Hot spots occur at areas of crust directly above regions of particularly hot molten rock in the mantle.
Volcanoes in western Victoria and Queensland have been formed this way.
The Hawaiian island chain has also been formed this way.
It is believed a large hot spot sits below Yellowstone National Park in central United States of America.