Australian
Volcanoes Slide Set
Slide 1. Australia's nearest plate
boundaries
The
outer layer of the Earth is broken into about a dozen
semi-rigid plates made of the crust and the uppermost
mantle. The plates are slowly being moved around the
planet by underlying convection currents in the hotter,
flowing mantle. The majority of the Earth's volcanic
activity occurs at the boundaries of these plates and
the way the plates interact at these boundaries governs
the style of volcanism.
The Australian continent lies on the
Indo-Australian Plate. This plate is moving northwards
away from Antarctica at approximately 7 cm a year.
As the Plate moves it collides with the Eurasia Plate
and the Pacific Plate which lie to the north and east.
This collision causes melting of the mantle and part of
the subducted crust. The molten rock can reach the
surface and form volcanoes, shown in this slide as red
triangles. The chain of volcanoes on this plate boundary
continue around the Pacific Ocean rim where the Pacific
Plate and other plates collide. This chain of volcanoes
is known as the 'Ring of Fire'. Currently there are no
active volcanoes on the Australian continent the
only continent without an active volcano.
Slide 2. Australia's ancient volcanoes
The
Australian continent has not always been volcanically
quiet. Evidence for volcanism throughout geological time
can be found in every state and territory. Volcanism
over the last 60 million years has occurred down the
east coast of Australia. Volcanologists have divided
this volcanic activity into lava fields areas where
large amounts of lava flowed from diffuse dykes and
pipes over a wide area (shown in red); and central
volcanoes areas where volcanism was produced from
either a single central vent or a cluster of vents
(shown in orange). It is now thought that the central
volcanoes were produced as the Australian continent
moved over a hot spot in the underlying mantle which
'melted' through the plate to form the volcano. As the
continent moved northward, the stationary hot spot
formed volcanoes further to the south on the continent.
Therefore the rocks of central volcanoes down the east
coast become younger as you move southward.
Slide 3. Shield volcano, Hawaii
Shield
volcanoes have very gentle slopes of only a few degrees
and can be many tens of kilometers in diameter. They are
common above hot spots but also form above subduction
zones. Mauna Loa is a shield with a summit caldera. The
volcano rises more than 9000 m above the ocean
floor and is the largest volcano on Earth. Mauna Loa is
also one of the most active volcanoes, with 38 eruptions
in the last 160 years, the most recent was in 1984. The
caldera, called Mokuaweoweo, is 4.3 km long and
2.5 km wide. The walls of the caldera are
180 m tall. Pit craters can be seen in the
foreground of this image. Mauna Kea, an extinct shield
volcano, is in the background. Mauna Kea has moved to
the edge of the Hawaiian hot spot and erupts less
frequently. Mauna Kea last erupted 3500 years ago but it
will erupt again. (Slide : USGS)
Slide 4. Stratovolcano, USA
Stratovolcanoes,
often called composite volcanoes, can form where a
tectonic plate is being forced under another tectonic
plate (subduction zone). This slide is of Mount St.
Helens in Washington State, USA, just prior to its
eruption in March 1980. Stratovolcanoes are made up of
layers of pyroclastic material (ash, bombs etc) and lava
and can have slopes close to 30°. The lava is normally
very viscous and will form a lava dome over the vent
rather than a lava flow. This dome can block the feeder
pipe causing pressure to build up due to escaping gas.
These pressures can cause the dome to explode. (Slide :
USGS) Other examples of stratovolcanoes are Mt Fuji
(Japan), Mt Ruapehu (New Zealand), Mt Vesuvius (Italy)
Slide 5. Big Ben Australian
stratovolcano
While the Australian continent has no active volcanoes,
a number do exist on Australian Territory. Big Ben,
which is a stratovolcano, dominates Heard Island in the
great Southern Ocean . The volcano has erupted at least
eight times since 1910 from a vent called Mawson Peak,
named after Australia's famous Antarctic explorer. Many
of the eruptions were subglacial and most were moderate
in size and explosive. Only three historic eruptions
produced lava flows. The most recent eruption of Big Ben
was in 1993. (Slide: Kevin McCue, AGSO)
Slide 6. MT Canololas, NSW Shield
volcano
In some locations on Earth magma (molten rock) from a
hot spot in the mantle can rise through the overlying
plate. When this occurs, basaltic lava can reach the
surface and form a Shield volcano. Basaltic lava is not
very viscous and can flow many kilometers from the vent.
Over time the accumulated flows can form a very large
volcano, often many tens of kilometers across.
This slide is a distant photograph of
Mt Canobolas, an eastern Australian central volcano,
near Orange, NSW. Mt Canobolas is an extinct shield
volcano which last erupted around 1113 million years
ago. The eruptions took place from around 30 vents
within 30 km of the current summit. The main vent
erupted over the entire life of the volcano while most
of the other vents were only active for short periods.
While the upper part of the volcanic structure has now
been removed by erosion, the flanks of the shield can
still be seen. (Slide : Gary Lewis, AGSO)
Slide 7. PUU OO Fire Fountain,
Hawaii
Some small explosive eruptions produce a lava fountain,
a jet of incandescent lava shot into the air. Fountains
commonly reach heights of 10100 m. This photo is
of a lava fountain at the rare height of 460 m at
the Puu OO vent of Kilauea volcano in Hawaii. There
were 44 eruption episodes with lava fountains at Puu OO
between 1983 and 1986. As the small droplets of lava
fall back to the surface they cool, forming cinders. The
cinders pile up to form a cinder cone, a common type of
volcano. The Puu OO cone grew to 255 m and
covered an area of 42 km2.
Cinder cones are monogenetic, meaning
they erupt for a few months or years and then never
erupt again. Cinder cones are common at hot spots,
subduction zones, and in rift zones. They can form on
shield volcanoes or stratovolcanoes or in volcanic
fields made only of cinder cones. (Slide : USGS)
Slide 8. Mount Fox, QLD Cinder
Cone
Mount Fox, located 50 km west of the Queensland
town of Ingham, is a pyroclastic cone which formed
around 560 000 years ago. The cone is 120 m
high and has a shallow crater. A lava flow extends away
from the southern base of the cone.
Similar cinder cones can be found in
western Victoria. Mt Elephant is the highest in the
region and, in the past, the pyroclastic material has
been quarried for a number of uses such as providing the
'volcanic rock' found in many gas barbeques. (Slide : P
Whitehead)
Slide 9. Mount Gambier, SA
Mount Gambier formed about 50004300 years ago making
it one of the youngest volcanic features on the
continent of Australia. Mount Gambier is a complex of
maar volcanoes. A maar volcano is a low-relief, broad
volcanic crater. Blue Lake now fills one of the craters.
Maars form by numerous shallow explosive eruptions. The
explosions are caused by rising magma that encounters
groundwater. (Slide : M Duggan, AGSO)
Slide 10. Glasshouse Mountains
Mount Tibrogargan, QLD
The Glasshouse Mountains, located 50 km north of
Brisbane, Qld, are the remnants of plugs and domes of a
central volcano which erupted between 25 and 34 million
years ago. The mountains were sighted and named by Capt.
James Cook when he sailed up the east coast of Australia
in 1770. He thought that they resembled the shape of
fancy glasshouses being constructed by the wealthy in
Britain during the late 1700s.
This slide is of Mt Tibrogaran, one of
the larger plugs, and it is rhyolite in composition.
(Slide : M Duggan, AGSO)
Slide 11. Elevation image of Tweed
volcano, NSW/QLD
The Tweed volcano is a 100 km-wide shield volcano
which straddles the coastal NSWQueensland border and
which last erupted 20 million years ago. The
central plug of the volcano is Mt Warning : the first
place to receive sunlight each morning on the Australian
continent.
This volcano was once around
2000 m higher than its present eroded surface and
during its volcanic history, covered nearly
7000 km2 with lava. Today over 4000 km2
remained covered by lava after erosion.
This slide is of a digital elevation
image of the Tweed Volcano. This image uses colors to
represent heights red being the highest points and
blue being the lowest points. It is easy to recognize
the flanks of the volcano as well as the eroded central
area and Mt Warning. (Slide : AGSO & AUSLIG)
Slide 12. The Breadknife
Warrumbungles, NSW
The Breadknife is a spectacular example of an igneous
dyke that has been exposed by the erosion of the
surrounding rocks. This dyke is part of the eroded
Warrumbungle Volcano in central NSW a central
volcano which first erupted 17 million years ago and
stopped around 13 million years ago.
The magma which formed the Breadknife
was injected into loose accumulations of ash and cinders
on the flanks of the large volcano. The molten material
cooled to form this knife-shaped band of hard igneous
rock. Over time the softer ash and cinder accumulations
have eroded away to expose the feature. (Slide : M
Duggan, AGSO)
Slide 13. Cape Hillsborough, QLD
The remnants of this volcano, which is found in
Queensland just north of Mackay, are cliffs containing
the layers of lava which flowed from the vent between 31
and 34 million years ago. The lava is basaltic in
composition and originated from vents now eroded away by
the sea. The central area of this volcano was once to
the north-east of the Cape. (Slide : M Duggan, AGSO)
Slide 14. Mount Schank, SA
Mt Shank, one of the youngest volcanoes in Australia, is
part of the Newer Volcanics Province, a complex of
volcanic centres across south-eastern Australia. The
Newer Volcanics Province covers an area of
15 000 km2 and contains over 400 vents
including Mt Gambier. Mt Shank is a scoria cone formed
when magma came in contact with groundwater. Charcoal
found beneath the volcanic deposits has been dated at
18 000 years old suggesting that the volcano was
active from then till 540 years before present. (Slide :
Mines and Energy, SA)
Slide 15. Australian volcanologists
at work
While
there are no active volcanoes on the Australian
mainland, our near neighbors (Indonesia, New Guinea and
New Zealand) all have to cope with the hazards
associated with active volcanism. Australian
vulcanologists are often called upon to assist when
major volcanic events disrupt human activity, such as
the recent eruptions at Rabaul. This slide show
vulcanologists measuring the amount of volcanic ash that
accumulated throughout the Rabaul township. This
information provides them information which they can use
to minimize future risk to human life and property
because of the regions active volcanoes.
Related Links:
Australian Geological Survey
Organization
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