Volcanism and volcanic rocks
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Introduction
Volcanic products
Structure of volcanoes
Lavas
Pyroclastic processes
Pyroclastic materials
Major volcanic landforms
Volcanic rocks
Introduction
Spectacular eruptions from volcanoes have fascinated humans ever since people first
walked on the Earth. In fact, some of the earliest known human-like descendants
(Australopithecus afarensis) used lava fields as protective refuges from wild animals.
Today, over 400 million people on Earth live within reach of volcanic eruptions, with
many of them either dwelling or growing crops on the flanks of dormant and active
volcanoes.
Although volcanoes are commonly seen as being destructive (when they destroy
buildings, infrastructure and farmers' fields), they are also constructive. They add more
land to the surface of the Earth, help us understand about volcanic processes and what
makes up the deep Earth (by transporting fragments to the surface), and, when weathered,
provide us with a nutrient-rich soil for agriculture.
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Volcanic products
The products of volcanic eruptions are lavas, explosive rocks and gases. Lavas vary
widely in composition depending upon their original source magma (which is most
strongly influenced by the plate tectonic setting). Magmas with a low viscosity and
volatile contents (e.g. water vapour, carbon dioxide) are erupted quietly as effusive lava
flows (as seen almost daily at Mauna Loa, Hawaii) whereas lavas with a high viscosity
and/or excess volatile content are erupted explosively and generally result in fragmental
deposits rather than lava flows (e.g. Mount St Helens, USA). Basaltic magma (low-silica
content) is usually erupted as lava flows whereas rhyolitic magmas (high-silica content)
are commonly erupted as pyroclastic deposits or short flows.
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Structure of volcanoes
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Craters and calderas
Formation of craters
Crater Lake, Oregon USA
The Structure of a Volcano
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Craters and calderas
Many volcanoes contain a large depression. Small depressions (i.e. less than 1 km across)
are called craters whereas those over 1 km across are called calderas. Most craters result
from the explosive activity by which the volcanic vent is cleared. However, most calderas
result from subsidence produced by the removal of large volumes of the underlying
magma. Many present-day craters and calderas are filled with water, giving crater lakes.
Formation of craters
The Krakatoa volcano in Indonesia erupted violently in 1883. Prior to the stupendous
eruption, the Krakatoa island group consisted of one large and two smaller volcanic
islands lying around and within a prehistoric caldera. The 1883 eruption resulted in the
removal of two of the three volcanic cones which made up Krakatoa Island and the area
between the three remaining remnant islands was filled by the sea to a depth of 250 m.
The eruptions began in May of 1883 with small explosions and built up in activity. The
large eruption (among the largest ever witnessed by people) occurred between 26 - 27
August and could be heard from a distance of 1000 km. Tsunamis over 40 m in height
killed 36 000 people on the islands of Java and Sumatra. The clouds of ash expelled were
so dense that Jakarta (some 160 km away) was in total darkness by midday 27 August.
The eruption cloud rose to a height of over 80 km and fell over an area of 800 000 km2.
Large (i.e. over 0.5 m across) fragments fell over an area of 100 km2. Because of the
large volume of ash erupted into the atmosphere from this eruption, the entire world
experienced unusually cold weather for the next few years as the ash helped to black out
the sun's rays. Probably over 20 km3 of rock were ejected in this eruption, most of it
being dacitic pumice from the exploding magma as well as rock fragments from the
former old volcanic cones which were blasted in the eruption.
Crater Lake, Oregon, USA
Crater Lake in Oregon, USA, is a large caldera some 10 km across and more than 1200 m
deep which formed about 7000 years ago by the collapse of a pre-existing andesitic cone
known as Mount Mazama. The pyroclastic deposits from this eruption are spread over a
wide area of Oregon and neighbouring states. The deposits of airborne pumice and
basaltic scoria had a volume of about 75 km3, of which some 30 km3 represented former
liquid magma. The volcano collapsed as the vast amounts of magma erupted from the
underlying chamber. Water filled the caldera over time leading to the present crater lake.
The Wizard Islands within this lake are probably younger cones.
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Lavas
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Lava flows
Lava domes
Submarine lavas
Lava flows
The flow of a lava from a vent depends on its viscosity which relates to the composition
and volatile content of the erupting magma. Many lavas are fluid enough to flow away
from a vent under gravity (e.g. basalts and some andesites). More viscous lavas such as
rhyolites and most dacites well up into lava domes (when low in volatiles) or explode in
pyroclastic fall out (when rich in volatiles).
Basaltic lava flows in three distinct forms.
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Pahoehoe flows have a smooth rounded undulating form, often with a ropy
appearance.
Aa flows have a very rough fragmented top and their surface is very sharp and
abrasive.
Block lava flows also have a fragmented top but the individual fragments are
smooth-surfaced many-sided blocks.
Pahoehoe lava (15 cm x 12cm) November 1975 flow, Halemaumau, Kilauea, Hawaii.
Photo: S Humphreys © Australian Museum.
Aa lava (14 cm x 12 cm). Kilauea, Hawaii. Photo: S Humphreys © Australian Museum.
Rhyolitic lava flows are usually short and thick and often travel down a very steep-sided
slope. The Glass Mountain rhyolite flow of Mono Craters, California is 3.6 km long and
75 m thick but was gas-rich.
Highly alkaline basic lavas (low in silica) are extremely mobile. In the 1977 eruption of
the Niyragongo volcano in Zaire, a 20 000 000 km3 lake of nephelinite lava flowed over
an area of 20 km2 in under an hour. The lava was so low in viscosity that in places, it was
only 1 mm thick.
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Lava domes
The most viscous lavas grow into domes, which are forced upwards (simply by pressure
from the underlying magma) through the vent to form a protruding plug or spine. The
weight of the growing plug or spine starts a collapse. The spine loses material from its
sides and top, and crashes down around the base. Some domes expand outwards as well
as upwards by inflation of the hot underlying magma. The Tarawera Complex on the
North Island of New Zealand is an excellent example. Lava domes grow relatively slowly,
about 1 m - 2 m per day.
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Submarine lavas
Many volcanic eruptions occur on the sea-floor. These eruptions often form pillow lavas.
These oval-shaped masses and tubes usually less than 1 m in diameter form outer layers
that are normally glassy (caused by the rapid quenching as cold sea-water hits hot lava)
and they chill from the margins inward. Most pillow lavas are basaltic in composition.
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Pyroclastic processes
Pyroclastic means 'fire broken' and is the term for rocks formed from fragments produced
by volcanic explosions. Magmas with explosive volatile contents can vary in styles of
eruption. The reduction in pressure as the magma approaches the surface drives out
bubbles of dissolved gases in bubbles and expands the volume of the magma. This
degassing magma is driven up into high eruption clouds and also hugs the ground as
pyroclastic flows, clouds of magma droplets, ash clouds and gases. Gases from such
eruptions can be deadly. They include water vapour, carbon dioxide, carbon monoxide,
sulfur dioxide, hydrogen sulfide and hydrofluoric acid.
Case study of an eruption
Ruapehu volcano on the North Island of New Zealand has a vent-related hydrothermal
system and is covered by a 10 000 000 m3 acidic crater lake where volcanic gases
accumulate. The greatest concentration of harmful gases occurs during early watercharged magmatic eruptions. The volcanic ash from these eruptions is six times more
concentrated in fluorine than the original magma. Much of this fluorine forms slowly soluble inorganic compounds, which release fluorine to the soil over a long time period.
However, these fluorine-bearing phases are soluble in the acidic digestion system of
animals. This led to the death of several thousand sheep from fluorosis during the 19951996 eruption. Volcanic gases released during eruptions can create both short-term and
long-term environmental hazards.
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Pyroclastic materials
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Introduction
Scoria cones or cinder cones
Lava tubes
Ash falls
Ash flows
Lahars
Introduction
The pyroclastic material erupted from an explosive volcanic eruption may be ejected as
fragments, resulting in scoria cones or ash fall deposits, or charge outwards in ash flow
deposits. This fragmental material is classified on the basis of grain-size.
Scoria sequence. Mt Shadwell Quarry, Mortlake, Victoria. Photo: I Graham © Australian
Museum.
Scoria cones or cinder cones
Large blocks of lava (bombs) cannot be ejected far from a volcanic vent because of their
weight. Eruptions that eject large amounts of bombs or lapilli (little stones) build up
scoria cones or cinder cones.
Lava tubes
Beginnings of lava tube, eastern shore of Lake Corangamite, Victoria. Photo: I Graham ©
Australian Museum.
These form when the outer surface of a lava flow hardens but the inside remains hot and
lava continues to flow through. Then, when the eruption wanes, most of the lava drains
out forming a hollow lava tube. Some of the largest and most spectacular lava tubes on
Earth are found at Undara in northern Queensland.
Ash falls
The finer materials of explosive volcanic eruptions are ejected into the air, eventually
falling to the ground to form a layer of ash. The extent of the ash fall depends upon the
height that the ash is ejected along with the speed and direction of the prevailing wind at
the time. Most ash clouds are only ejected 1 km into the air and fall back onto the
volcanic cone. Very powerful eruptions may hurl ash over 10 km into the air. These are
relatively rare and occur only a few times every 100 years. When the ash from these
eruptions covers a large area, they form sheet deposits. Because ash falls consist of
relatively unconsolidated material, many deposits rapidly erode and are washed
elsewhere. In the 79 AD eruption of Vesuvius which partly destroyed the Roman town of
Pompeii, the eruption plume rose to a great height and covered the surrounding area with
pumice and hot ash to an extent of 450 km2. As the ash cloud collapsed the surges of
material suffocated most of the victims.
Some ash falls are enormous in extent. The 1783 Laki fissure eruption in Iceland covered
the entire island in ash completely devastating the vegetation and sent a dust cloud over
Europe and North America for several months. The 1883 eruption of Krakatoa threw ash
so high into the atmosphere that the finer dust eventually spread all around the Earth in
just 14 days.
Ash flows
Many of the world's large accumulations of rhyolitic and dacitic debris are of this type.
The volatile-rich material can travel over great distances at fast speed and are also called
nuée ardente (French for fiery cloud). They often occur as avalanches when lava domes
collapse or shake volcanic slopes. In the 186 AD eruption of Taupo in New Zealand, an
ash flow of 30 km3 of material was erupted in under ten minutes and flowed for 80 km in
all directions at a speed of 300 m/s, even flowing straight over mountains in its path.
Lahars
Lahars are mudflows composed of pyroclastic material. They occur where the ground has
a slope and water is present. In tropical areas of high rainfall, thick ash deposits become
water-logged during the wet season, become unstable on the steep slopes and trigger
mudflows. In high areas, mudflows can result from the melting of ice and snow
surrounding volcanic summits. Sometimes water in a crater lake may be discharged when
the volcano becomes active again. Lahars were named from the islands of Indonesia
where they cause loss of many lives.
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Major volcanic landforms
Each type of volcanic massif contains lavas, pyroclastic rocks and intrusions, but these
differ in proportions and compositions. The four main types of large volcanic landforms
are:
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lava plateaus: extensive relatively flat-lying accumulations of lava sheets which
are mainly basaltic in composition (e.g. recent lavas of Iceland).
shield volcanoes: conical-shaped structures consisting mainly of lava flows (e.g.
the volcanoes of the Hawaiian islands).
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strato volcanoes: composite volcanos which contain both lava and pyroclastic
deposits and often change their shape as a result of their explosive volcanic
activity (e.g. Vesuvius).
pyroclastic sheet deposits: extensive relatively flat-lying accumulations of
pyroclastic material (e.g. those of the North Island of New Zealand).
Lava plateaus
These result from numerous eruption of very large volumes of extremely fluid, basaltic
lavas, which accumulate as sheet-like flows covering large areas. These Flood Basalt
Provinces include the Columbia River basalts of Oregon and Washington states, USA,
and the Deccan plateau basalts of India. However, the Siberian basalt province is
probably the most important and may have helped in the extinction of up to 95% of all
life on Earth at the Permo-Triassic (225 million years ago) boundary.
Large plateau lava fields do not necessarily represent exceptional rates of magma supply.
For example, the Columbia River basalts accumulated over 200 000 km3 of basaltic lavas
over 10 million years at an average rate of accumulation of 2 km3 per century. Most
plateau-type eruptions are fissure eruptions, often associated with volcanic centres. These
eruptions tend to be long but narrow and are fed from dykes.
Shield volcanos
Shield volcano, Hawaii. Photo: FL Sutherland © Australian Museum.
A shield volcano builds up around a volcanic centre so that the lavas thin outwards from
the volcano. The Hawaiian Islands are built from shield volcanoes. In a 'Hawaiian-type'
eruption, the basaltic lava flows quietly from a crater or fissure over a large distance.
Repeated eruptions over a long time develop the characteristic shape of a shield - like
cone with gentle outward slopes.
With a slightly more explosive style and a more viscous magma, the shield volcano
passes into steeper cones typical of strato volcanos. An in-between stage between types is
represented by Mount Etna, which rises from sea level to a height of 3.3 km and is some
40 km across at its base. It has grown during the past few million years by frequent but
small eruptions (Mount Etna has erupted 15 times this century).
Strato volcanoes
Strato volcanoes (also known as composite volcanoes) rise up by the accumulation of
alternating lavas and pyroclastic deposits. This gives a steep cone with a summit crater.
The internal structure of these volcanoes can be very complex, with many generations of
dykes, sills, lavas and ash flows and falls. Although andesite is a typical rock in these
volcanoes, rhyolites and basalts are also erupted. A well-known strato volcano is Mount
St Helens in the state of Washington, USA.
Pyroclastic sheet deposits
In some volcanic provinces, large areas are covered by pyroclastic deposits (e.g. central
North Island of New Zealand - the Taupo Volcanic Zone). The volcanic material largely
consists of sheets of rhyolitic ignimbrite which cover an area of over 20 000 km2. The
Taupo Volcanic Zone is still active with large hot springs and three large andesitic strato
volcanoes (Ruapehu, Tongariro and Ngauruhoe) at its southern end. Yellowstone
National Park in the USA contains one of the world's largest rhyolite plateaus. The
volume of the rhyolite lava erupted over the last two million years is over 6000 km3 (in
comparison, Mount Vesuvius contains only 12 km3). As Yellowstone National Park
contains many hot springs, it is still active and eruptions may well occur again.
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Volcanic rocks
Volcanic rocks fall into three main categories:
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Basalts
Volcaniclastic rocks
Pyroclastic rocks
Basalts
Basaltic magmas have erupted throughout most of Earth's history, in oceans and
continents and every tectonic environment. The main areas are:
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Mid-oceanic ridges where they make most of the oceanic crust.
On oceanic islands, where they build up large shields.
At orogenic continental margins and island arcs, related to subduction zones.
On stable continental areas.
Volcaniclastic rocks
Mega volcanic bomb, Anakie Eastern Hill, Anakie, Victoria. Photo: I Graham ©
Australian Museum.
All fragmental volcanic rocks can be described as volcaniclastic.
Blocks are angular fragments of solid rock whereas bombs are fragments flung from
volcanic vents as pasty blobs of magma that were streamlined as they solidified during
their flight.
In addition to size, fragments also vary in composition. They include:
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vitric (glass)
lithic (rock)
crystal (crystal fragments)
Ash-sized clasts are usually vitric or crystal.
Tuff (13 cm x 10 cm) Lake Purrumbete, Victoria. Photo: S Humphreys © Australian
Museum.
Tuffs are volcaniclastic rocks made of ash-sized fragments. Those from ash falls are
often well layered and look like fine-grained sedimentary rocks.
Agglomerate is a term for an accumulation of unsorted deposits of bombs near the
volcanic vent.
When known, the compositional terms and size can be combined in the rock name.
Common examples are lithic lapilli tuff, crystal tuff and dacitic vitric tuff.
Pyroclastic rocks
The term pyroclastic includes fragmental volcanic rocks produced during explosive
volcanic eruptions. Pyroclastic rocks can be classified by their mode of formation into
three main groups:
Ash fall deposits
These form when volcanic material is explosively ejected from the vent up into an
eruption column. There are five main types based on degree of fragmentation (i.e. size of
ash particles): Hawaiian, Strombolian, Subplinian, Plinian and Ultraplinian. Hawaiiantype eruptions have low eruption columns (typically fire fountains) so the deposits have
large particles dispersed over a small area. In contrast, Ultraplinian eruptions have
towering eruption columns (commonly over 25 km high) and deposits consist largely of
small particles dispersed over an extensive area. Ash fall deposits usually drape over the
topography with a uniform thickness. They are generally well-sorted, often being coarsergrained near the source vent and finer-grained away from the vent and can deposit both
on land or in water. Accretionary lapilli are small spherical masses (2mm - 10mm
across) composed of concentrically-layered ash or fine lithic particles, condensed by
moisture in the air. They usually fall within a few kilometres of the vent.
Ash flow deposits
Pyroclastic flow flows under gravity and hug the ground as hot high concentrations of gas
and solid material. Large pumice-rich examples are called ignimbrites. Ash flows are
formed in two main ways:
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Lava dome/flow collapse: This occurs on steep-sided andesitic volcanic cones,
when an unstable actively-growing lava dome or flow collapses under gravity or
through explosion. The flows rush down the flank of the volcano.
Eruption column collapse: When an erupting column of ash becomes too dense to
support itself, gravitational collapse back to the ground occurs, generating a
pyroclastic flow. All ash flow deposits follow the topography and fill valleys and
depressions. The deposits are generally poorly sorted with some lacking any
layering.
Surge deposits
Base surge deposit. Coonal Quarry, Lake Purrumbete, 10 km south-east of Camperdown,
Victoria. Photo: I Graham © Australian Museum.
Pyroclastic surge deposits form by sideways blasts of expanded, turbulent, lowconcentration mixtures of gas and solids. Formation occurs:
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during phreatomagmatic and phreatic eruptions : Here a base surge forms a collarlike, low cloud expanding radially in all directions from the centre of explosion.
They result from the explosive interaction of hot magma or gas with water.
associated with flows: Thin, stratified pumice and ash deposits are often found
associated with pyroclastic flows. They can be found at either the base of the ash
flow unit (ground surges) or at the top (ash cloud surges).
associated with falls : These are similar to ground surges but no pyroclastic flow
is generated. Surge deposits run over the topography and accumulate more thickly
in depressions.