Volcanoes
- see also:
- volcano warning and monitoring sites:
Vulcanicity:
- the venting of either:
- molten rock - "magma" from pockets called
"magma chambers" with it being extruded as lava
or crystallising underground to form either bodies of plutonic
rock or dykes.
- volcanic gases from vents called fumaroles
- volcanoes perform a crucial role in maintaining the earth's environmental
balance by:
- pouring out new raw materials such as gases and lava which hardens to
form new islands, ocean floor and continental crust
- being outlets for pressures that have built up beneath earth's surface
- volcanoes cause death by:
- burns from pyroclastic flows rushing down the mountain which may
exceed the speed of sound and effect 20-30km away and in the case of a
super-volcano, some 100km away.
- volcanic ash:
- will cause jet engines to fail & damage airplane windscreens
preventing visibility
- if it falls down at substantial amounts may cause suffocation and
if the load is heavy enough, will cause buildings to collapse
- when mixed with the floodwaters from thunderstorms, produces a
deadly landslide effect
- ash from a super-volcano in Yellowstone Park would directly affect
3/4 of USA and reach Europe.
- aerosols of sulphuric acid reaching the stratosphere where they can
remain for several years blocking the sun over most of the world, such
that with a super-volcano, one could expect mean temperatures to drop by
12degC at high latitudes causing permanent snow for several years and by
15degC in the tropics disrupting the monsoons causing drought &
famine.
- eruption of other toxins such as flourine resulting in potentially
crippling or lethal flourosis in the region as occurred in Iceland in
1753 from the 7 month long emissions.
- super-volcanoes occur rarely, the last, some 73,000yrs ago, but can
cause a global cataclysm, wiping out much life on earth by lowering global
temperatures as above. There are some 40 super-volcano sites around the
world, most are dormant, but the Yellowstone Park super-volcano erupted some
2.5million, 1.3million & 740,000yrs ago, and thus although remotely
likely to do so in the next 100yrs, it may erupt at any time. Certainly the
amount of magma lying underneath is sufficient to do so.
- types of volcanoes:
- central volcanoes:
- the majority of volcanoes are of the central type in which the
activity is localised at one main vent situated above a pipe-like
conduit or neck. A conical volcanic mountain with a steep-walled
crater at the summit is built up, which has come to be regarded as
the typical volcano, exemplified by cones famous for their beauty
and grandeur such as Fuji Yama, Mt Egmont.
- the crater wall may become breached resulting in a lava flow on
that side
- eg. Mt Franklin at Daylesford; Mt Warrenheip near Ballarat;
- lava often breaks through from vents on the flanks of large
volcanoes forming parasitic or adventive cones
- eg. Mt Napier near Hamilton; Mt Etna;
- embryonic volcanoes that do not grow beyond the stage at which
after a pipe is formed in the crust by explosive gases, a low ring
of tuff & ejected blocks is built up around a crater is called a
maar which may be filled in with water to form a lake
- eg. Lake Keilambete near Terang; Lake Purrumbete & twin
lakes Gnotuk & Bullenmerri near Camperdown
- eg. Lake Burrumbeet, Lake Learmonth near Ballarat
- small cones of approx. 100m height with steep sides of
~45deg consist almost entirely or very erosion-resistant scoria and
persist as scoria cones or cinder cones
- eg. these are a feature of volcanic areas of western Victoria
(eg. flanks of Mt Napier, Hamilton)
- fissure volcanoes:
- the eruptions occur from a more or less straight fissure in the
crust several kilometres in length, from which vast floods of lava
may be extruded
- examples include:
- the 30km long fissure near Laki, Iceland which in 1783
extruded lava up to 250m thick over 500 sq. km.
- Mt Gambier district, SA
- near Mt Eccles, Vic.
- volcanic cauldrons:
- a roughly circular or elliptical fissure surrounding a central
block of crust which, lacking support, subsides into an underlying
magma chamber
- lava & tuffs are extruded from the surrounding fissure, which
remains as a ring dyke after the vulcanicity ceases
- the subsiding central block becomes flooded with lavas & is
known as a volcanic cauldron
- because of the resistance of the volcanic rocks to erosion, these
old volcanic complexes constitute prominent features for millions of
years
- examples include:
- deeply fissured cauldrons near Healesville, Warburton, &
Marysville
- mountain masses such as the Dandenongs, the Macedon Range,
parts of the Strathbogies near Violet Town
- shield volcanoes:
- basalts build very broad & relatively low volcanic piles
called shield volcanoes
- examples of these are Hawaii; Mt Cotteril north of Werribee
- lava domes:
- composed entirely of lavas, but having much steeper sides &
being very small in comparison with shield volcanoes, are lava
domes, also called tholoids.
- they may form from viscous lava within the crater of a large
volcanic cone, or may be extruded quite separately as an isolated mamelon
resulting from internal accretion and swelling, as with tooth paste
squeezed up through a hole in a cardboard sheet.
- examples of mamelons:
- The Camel's Hump, Hanging Rock & Brock's Monument near
Macedon
- calderas:
- volcanic pits that unlike volcanic craters which are constructional,
calderas are the result of destruction
- types of calderas:
- explosion caldera:
- results from the top of a conical volcano being exploded off
leaving a caldera with a central smaller cone
- eg. Mt Vesuvius, Monte Somma
- collapse caldera:
- results from the volcanic cone base collapsing into the
underlying magma leaving a shallow lake
- volcanic sink caldera:
- steep sided sinking of the central part of the volcano
- eg. Mauna Loa, Hawaii
- lakes occupying craters or calderas are called crater lakes
- eg. Mt Gambier SA; Mt Eccles near Macarthur; Lakes Gnotuk &
Bullenmerri near Camperdown; Tower Hill;
- products of volcanoes:
- the lavas and gases that are extruded reach 1200degC in the throat of
the volcano and such temperatures result in oxidation of elements
including hydrogen and ferrous iron when the magma & gases make
contact with air at or near the surface
- much of the basaltic lava and underlying magma is 600-900degC
- lavas derived from different volcanoes, or in some cases from the same
volcano at different times, may vary widely in chemical and mineral
composition, so that the resultant rocks are also very different
- basaltic lavas:
- very fluid, may travel at 50km an hour but usually 10kph is more
usual, and reach tens of kilometres
- examples:
- Tyrendarra flow from Mt Eccles, Vic which is 48km long
- flows that entered Yarra Valley & dammed the river near
Fairfield
- as lava cools, it contracts, which causes it to crack, and with
slow cooling, the cracks are regularly arranged so that the rock is
divided into columns with are usually 6 sided in
cross-section (maybe 5 or 7 sided) and form perpendicular to the
flow with a well developed set in its base extending upwards and a
less well developed set extending downwards from its surface.
- eg. Organ Pipes, Keilor
- eg. much of central & eastern Tasmania has vertical
columns of dolerite formed in evenly, slowly cooling intrusions
of magma (called sills) which are approximately horizontal and
some tens or hundreds of metres thick
- when basalt lavas flow into the sea, they do not form pumice as do
acid lavas but instead develop into a piled up mass of rounded pillows,
each of the order of a metre or two across
- eg. lower part of a flow in Toolern Creek near Exford Bridge
in the Werribee Plains
- silica-rich lavas:
- more viscous, may advance a few metres per day
- when these acid lavas flow into the sea, they may form pumice -
contains innumerable small gas cavities like froth so that it is
light enough to float in water - which are blown up by steam to the
surface
- volcanic gases:
- water vapour, carbon dioxide, and generally, smaller amounts of
nitrogen, hydrogen, halides & sulphurous compounds as well as
volatile solids such as sulphur & some halogen compounds which
will deposit as incrustations on cool surfaces.
- fragmental products:
- large blebs, small droplets of liquid lava, pieces of solidified
lava, other rock pieces
- sub-crustal rocks such as the green mineral olivine, usually
coated with basalt to form "olivine bombs"
such as are found among the scoriae of many young volcanic cones
from Mt Gambier to the Anakies near Geelong.
- consolidated volcanic ejectamenta of all kinds are called pyroclastic
rocks
- unconsolidated or partially consolidated fragmentary deposits are
generally known as tephra
- finer particles are called ashes which if thick
deposits build up exhibiting stratification & become
consolidated are called tuff. Ashes laid down from nuees
ardentes (hot cloud) are flow-tephra, and if
the fragments are sufficiently hot they adhere & compact to form
welded tuffs - if these are silica-rich (acid) rocks,
then they are called ignimbrite.
- lapilli is applied to all kinds of solid fragments from a few mm
to a few cm, although rough pieces of cellular rock full of gas
cavities (vesicles), are generally called scoriae or cinders
- although the inexorable advance of a lava flow as it engulfs farms and
villages is awe-inspiring, the most disastrous eruptions are actually
those in which huge volumes of hot, ash-laden gases are projected
down the flanks of the volcano & spread out over the surrounding
country as with Pompeii in 79AD by eruption of Vesuvius and St Pierre in
Martinique in 1902.
Volcanoes
in Australia:
- vulcanicity and earthquakes tend to occur primarily at the junctions of
the crustal plates which are mainly:
- the edges of the Pacific Ocean (including Japan, Phillipines, Papua
New Guinea, NZ, western coast of South America and North America - the
San Andreas Fault)
- north-eastern coast of Mediterranean through Turkey, Iran
- southern aspect of Indonesia
- the Australian belt of
volcanism extending along the east coast is one
of the world's most extensive volcanic zones.
- fortunately, the Australian continent is now quite distant from the edges of
the Australian Plate and thus vulcanicity is extinct and major earthquakes
rare.
- all volcanoes in Australia are extinct (the last eruption was Mt Schank
near Mt Gambier ~5,400yrs ago), the most recent being those in
south-western Victoria which are Cenozoic
( Pliocene - from 6-7 million years
ago to the last such as most of the large scoria cones which were built
around 20,000-25,000yrs ago) in contrast to the much older ones along the east
coast which are Devonian
(345-405 million yrs ago)
- Victoria:
- Bacchus Marsh Volcano:
- Lerderderg State Park is 75 km NW of
Melbourne
- is a NW trending graben (20 km EW, 5
km NS)
- It contains 160 m thick lava flows fed by dykes.
- eruptions 55-64 million years BP
- see also Organ Pipes National Park in Keilor
for old lava streams
- NE Victorian alpine region:
- eruptions 25-44 million years BP
- western Victoria:
- Sth Australia:
- Mt Gambier:
- Last eruption 2900 BC. Central volcano
eruption, explosive, pyroclastic flows, lava flows.
Tremors still occur under these volcanoes.
Mantle plume may currently lie offshore.
Contains 4 water-filled craters. Blue Lake,
Valley Lake, Leg of Mutton Lake, Browne's Lake.
- Mt Schank:
- Mt Schank volcano consists of 2 dry craters.
Explosive eruptions, pyroclastic flows and
lava flows.
- erupted ~5000 yrs BP ie ~3000BC
- NSW:
- Burning Mountain (Wingen):
- Burning mountain is a burning underground
seam of coal. It has been burning for 5 000 years.
The cracks are located on a lightly forrested
hillside in rural NSW.
The vents emit hot gases and sulphur. Until
1866 Burning Mountain was proclaimed as Australia's only mainland
volcano.
- Southern Highlands:
- erupted 31-55 million years BP
- Sydney:
- Qld:
- Atherton (Cairns): erupted 100,000yrs BP
