Volcanoes
Mountains
of fire - volcanic eruptions occur wherever magma (molten rock) is able
to reach the Earth's surface. The mountain that forms as the result
of a volcanic eruption is called a volcano.
There are three types of volcanoes each associated with a different
type of volcanic eruption. This topic investigates the characteristics
of volcanoes, volcanic eruptions and the types of volcanoes.
Characteristics of volcanoes
As
hot fluids tend to rise, magma or molten rock deep within the Earth
can force its way up though cracks and fissures in the crust and erupt
through the surface at weak spots in the Earth's crust. Where the magma
passes though, weak rocks that are capped by a stronger rock layer the
magma pools and forms a magma chamber. As the overlying rocks heat up
they crack. The magma is able to travel up through these cracks to reach
the surface. Magma that reaches the surface is called lava.
The process of magma breaking through the surface is known as an eruption
and the hole through which the eruption occurs is called a vent. As
a volcano builds, it forms a bowl shaped blast area or crater. A crater
may have one or more vents and if it is larger than 1.5 km across it
is known as a caldera.
Active, dormant or extinct?
Volcanoes are referred to as active, dormant or extinct depending on
their present and past behaviour. A volcano that has erupted in the
last 100 years is considered to be active. Dormant volcanoes are those
which have not erupted in the last 100 to 1000 years, while those which
have not been active for over 1000 years are said to be extinct.
Volcanic eruptions
Volcanic eruptions can range from quiet streams of lava to violent explosions
of hot gas, rock, steam and ash.
Features of eruptions are:
- Lava flows - molten rock flowing from
a vent pours down the slope of the volcano, moving at speeds of up
to 40 km/hr. As the lava flows away from the vent it cools and hardens,
forming new igneous rock.
- Ash - formed during explosive eruptions,
fine-grained ash can be shot many kilometres into the air and settle
over large surrounding areas.
- Bombs - molten lava lumps hurled skyward
cool and harden in flight, landing as solid rocks. To be considered
a volcanic bomb, the rock must be at least 6cm across, but they have
been recorded as large as cars.
Types of volcanoes
The
three types of volcanoes are named from their shape and structure. Each
forms in a different way from a variety of volcanic materials.
- Shield volcanoes form from runny lava
which flows easily, producing a low shield-like structure around a
vent. Many volcanic islands are the tops of large undersea shield
volcanoes.
- Cinder Cone volcanoes form when viscous
lava, ash and rock build up around a vent. These volcanoes typically
have steep upper slopes with gentler sloping bases.
- Composite volcanoes form when alternating
layers of ash and lava are laid down from a vent. These volcanoes
are similar in appearance to Cinder Cone volcanoes.
Where do volcanoes form?
There
are no active volcanoes in mainland Australia, but there are plenty
of extinct ones. Big Ben on the Australian territory of Heard Island
and others in the Macdonald Group Islands are volcanoes that are considered
active. Volcanoes form in three areas; along the edges of tectonic plates,
over oceanic ridges and on volcanic hot spots. Extinct volcanoes can
be found in all states, but the ones in Queensland and Victoria are
the most typical of volcano shapes.
Where two tectonic plates meet and collide, the older and denser crust
subducts (slides under) the younger, more buoyant crust. This creates
weak spots where magma can come to the surface. The volcanoes forming
the "Ring of Fire" which encircles the Pacific Ocean along
the American and Asian coasts are formed along the boundary of the Pacific
plate.
The second large group of volcanoes has formed along the mid-atlantic
and mid-pacific ridges. Volcanoes on these ridges are created as new
crust is formed where the tectonic plates are moving apart.
The third area of volcanic activity is over hot spots in the Earth's
crust. These locations deep under the crust are usually far from any
tectonic plate boundaries and are parts of the Earth's mantle that are
particularly hot. Periodically, the crust above a hot spot cracks and
a volcano forms. As the crust moves over a hot spot, a chain of volcanoes
can form. This can occur over thousands of years. The Hawaiian islands
have formed over a hot spot and it is thought that many of Australia's
younger volcanoes have formed over hot spots.
Iceland has formed over a hot spot that happens to be in the mid-atlantic
ridge, effectively doubling the lava produced. It is growing larger
as new material is brought to the surface by its volcanoes.
One theory states that Queensland's Glasshouse mountains and a chain
of volcanoes in New South Wales formed over a hot spot and predicts
the next place an eruption could occur is in Tasmania!
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