VOLCANOES
What is a volcano?
A volcano is a vent or chimney which transfers molten rock known as magma from under
the Earth to its surface. Magma erupting from a volcano is called lava and is the material
which builds up the cone surrounding the vent.
A volcano is active if it is erupting lava, releasing gas or
generates seismic activity. A volcano is dormant if it has not
erupted for a long time but could erupt again in the future.
Once a volcano has been dormant for more than 10 000
years, it is termed extinct.
The explosiveness of a volcanic eruption depends on how easily magma can flow and the
amount of gas trapped within the magma. Large amounts of water and carbon dioxide are
dissolved in magma causing it to behave in a similar way to gas expanding in fizzy drinks,
which forms bubbles and escapes after opening. As magma rises quickly through the
Earth's crust, gas bubbles form and expand up to 1000 times their original size.
Volcanoes can be different in appearance with some featuring perfect cone shapes while
others are deep depressions filled with water. The form of a volcano provides a clue to the
type and size of its eruption which is controlled by the characteristics and composition of
magma. The size, style and frequency of eruptions can differ greatly but all these elements
correlated to the shape of a volcano.
Types of Volcanoes
Shield Volcano
When magma is very hot and runny, gases can escape and eruptions
are gentle with considerable amounts of magma reaching the surface
to form lava flows. Shield volcanoes have a broad, flattened dome-like
shape created by layers of runny lava flowing over its surface and
cooling. Because the lava flows easily, it can move
down gradual slopes over great distances from the volcanic vents. The
lava flows are sufficiently slow for humans to out run or out walk
them. This type of magma has a temperature between 800°C and
1200°C and is called basaltic magma.
Composite Volcano (Strato)
Also known as strato-volcanoes, these volcanoes are characterised by
an explosive eruption style. When magma is slightly cooler it is thick
and sticky, or viscous, which makes it harder for gas bubbles to expand
and escape. The resulting pressure causes the magma to foam and
explode violently, blasting it into tiny pieces known
as volcanic ash. These eruptions create steep sided
cones. They can also create lava flows, hot ash clouds called pyroclastic
flows and dangerous mudflows called lahars. This type of magma has a
temperature between 800°C and 1000°C and is called andesitic magma.
Caldera Volcano
These erupt so explosively that little material builds up near the vent.
Eruptions partly or entirely empty the underlying magma chamber
which leaves the region around the vent unsupported, causing it to
sink or collapse under its own weight. The resulting basin-shaped
depression is roughly circular and is usually several kilometres or more
in diameter. The lava erupted from caldera volcanoes is very viscous
and generally the coolest with temperatures ranging from 650°C to
800°C and is called rhyolitic magma. Although caldera volcanoes are
rare, they are the most dangerous. Volcanic hazards from this type of
eruption include widespread ash fall, large pyroclastic surges and
tsunami from caldera collapse.
Volcanic Hazards
Volcanic hazards include explosions, lava flows, bombs or ballistics, ash or tephra,
pyroclastic flows, pyroclastic surges, mudflows or lahars, landslides, earthquakes, ground
deformation, tsunami, air shocks, lightning, poisonous gas and glacial outburst flooding.
Each hazard has a different consequence, although not all occur in all eruptions or in
association with all volcanoes.
Volcanic ash clouds can damage aircraft engines but ash is not visible by radar, the main
navigation aid for aircraft. There are nine Volcanic Ash Advisory Centres around the world
which use satellites to help track volcanic ash clouds and provide warnings for aircraft.
Measuring Volcanic Eruptions
Volcanic eruptions are measured using a simple descriptive index known as the Volcano
Explosivity Index which ranges from zero to eight. The index combines the volume of
material ejected with the height of an eruption column and the duration of the eruption.
Predicting Eruptions
Volcanic eruptions are very hard to predict as they can erupt without giving any signals
beforehand. A volcano’s eruptive history may provide good clues as to the size and violence
of the next eruption for that volcano. Typically, long periods between eruptions may show
that a volcano’s next eruption will be large and explosive. One problem with using eruptive
history to predict a future event is that only a small number of the world’s volcanoes have a
known history.
Earthquake activity around a volcano indicates that magma is moving below a volcano and
so can precede an eruption. However, eruptions can occur with no change to seismic
activity. Changes in the surface shape may show that magma is rising beneath a volcano.
Keeping track of tiny changes such as bulging can forecast an eruption. Temperature
changes in surface lakes or the ground water near a volcano can be a good early detection
tool of an eruption. However, not all temperature changes are related to volcanic
eruptions.
Where do Volcanoes Occur?
Active volcanoes generally occur close
to the major tectonic plate boundaries.
Many are located along the Earth's plate
boundaries. Active volcanoes are rare in
Australia because there are no plate
boundaries on this continent. However,
there are two active volcanoes located
4,000 kilometres south west of Perth on
the Australian territories, Heard Island
and the nearby McDonald Islands.
The other active volcanoes nearest Australia are in New Zealand, Papua New Guinea,
Indonesia and the Philippines. Gas-rich sticky magmas dominate the Asia Pacific, making
composite volcanoes and calderas the most common varieties in the region. These types of
volcanoes severely threaten lives, property, agricultural lands and lifelines throughout south
east Asia and the Australian region.
VOLCANO GLOSSARY
active volcano
A volcano that is erupting. Also, a volcano that is not
presently erupting, but that has erupted within historical
time and is considered likely to do so in the future.
andesitic magma
A type of magma with intermediate viscosity and silica
content.
basaltic magma
It is primarily made of the rock called basalt which is a rock
found deep within the earth so basaltic magma comes
from deep inside. Basaltic flows are the hottest.
volcanic cone
A volcanic cone is the cone-shaped hill formed as material
from volcanic eruptions piles up around the volcanic vent,
or opening.
dormant volcano
erupt/eruption
extinct volcano
A volcano which is presently inactive but which may erupt
again.
The process by which solid, liquid, and gaseous materials
are ejected into the earth's atmosphere and onto the
earth's surface by volcanic activity.
A volcano that is not presently erupting and is not likely to
do so for a very long time in the future.
lahars
Mud flow of water and volcanic material commonly caused
by the bursting of a crater lake, eruption from a snowcapped volcano or from prolonged torrential rain.
lava
Magma which has reached the surface through a volcanic
eruption. The term is most commonly applied to streams of
liquid rock that flow from a crater or fissure. It also refers
to cooled and solidified rock.
magma
Molten rock beneath the surface of the earth.
magma chamber
The subterranean cavity containing the gas-rich liquid
magma which feeds a volcano.
molten rock
rock liquefied by heat
pyroclastic
Pertaining to fragmented rock material formed by a
volcanic explosion or ejection from a volcanic vent.
pyroclastic flow
Lateral flowage of a turbulent mixture of hot gases and
unsorted pyroclastic material that can move at high speed.
The term also can refer to the deposit so formed.
rhyolitic magma
A type of highly viscous magma with high silica content.
seismic
Relates to vibrations of the earth and its crust.
tectonic plates
Tectonic plates are large plates of rock that make up the
foundation of the Earth's crust and the shape of the
continents.
tephra
Tephra is a general term for fragments of volcanic rock and
lava regardless of size that are blasted into the air by
explosions or carried upward by hot gases in eruption
columns or lava fountains.
volcanic vents
Vents are the locations from which lava flows and
pyroclastic materials are erupted.
Volcano
Explosivity
Index (VEI)
The Volcanic Explosivity Index was devised by Chris
Newhall and Stephen Self in 1982 to provide a relative
measure of the explosiveness of volcanic eruptions
viscous/viscosity The ability of a liquid to flow.