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Volcanoes and Volcanic Deposits
IN THIS LECTURE
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Monogenetic and Polygenetic volcanoes
Shield Volcanoes
Flood Basalts
Scoria Cones
Maars and Tuff Cones and Rings
Monogenetic versus polygenetic
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Volcanoes can be subdivided into two types
– Monogenetic
• volcano built up by the products of one eruption or eruptive
phase
• Simple magma conduit system used during only one eruption or
one prolonged eruptive phase
• Example – Surtsey and Heimey
– Polygenetic
• volcano resulting from many eruptions, separated by relatively
long periods and often involving different magmas.
• Complex “plumbing systems” with intricate complicated conduit
systems used many times during different eruptive phases.
• Example – Hawaii
Volcanoes and Plate Tectonics
Characteristics of Volcanic rocks
Products of Volcanic Eruptions
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Tephra - 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. Tephra includes large dense blocks and
bombs, and small light rock debris such as scoria, pumice, reticulite, and ash. As
tephra falls to the ground with increasing distance from a volcano, the average
size of the individual rock particles becomes smaller and thickness of the
resulting deposit becomes thinner. Small tephra stays aloft in the eruption cloud
for longer periods of time, which allows wind to blow tiny particles farther from
an erupting volcano.
Pumice is a light, porous volcanic rock that forms during explosive eruptions. It
resembles a sponge because it consists of a network of gas bubbles frozen
amidst fragile volcanic glass and minerals. All types of magma (basalt, andesite,
dacite, and rhyolite) will form pumice. Pumice is similar to the liquid foam
generated when a bottle of pressurized soda is opened--the opening
depressurizes the soda and enables dissolved carbon dioxide gas to escape or
erupt through the opening. During an explosive eruption, volcanic gases dissolved
in the liquid portion of magma also expand rapidly to create a foam or froth; in
the case of pumice, the liquid part of the froth quickly solidifies to glass around
the glass bubbles.
Products of Volcanic Eruptions
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Scoria is a vesicular (bubbly) glassy lava rock of basaltic to andesitic
composition ejected from a vent during explosive eruption. The bubbly
nature of scoria is due to the escape of volcanic gases during eruption.
Scoria is typically dark gray to black in color, mostly due to its high iron
content. The surface of some scoria may have a blue iridescent color;
oxidation may lead to a deep reddish-brown color.
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Tuff is the general name for consolidated ash. The material forming a
tuff may be composed of (a) crystals ejected from the volcano; (b)
small fragments (<4mm) of lava or other rock types; (c) lapilli and (d)
fragments of a glassy nature. Tuffs often show sedimentary features
such as bedding and grading
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Ignimbrites are welded tuffs that form when the layers of tuff
material were so hot when they were deposited that that the edges of
the fragments weld together. Often ignimbrites display well-developed
banding resulting from flattening of glass shards and other fragments
and can often be mistaken for rhyolitic lavas.
Products of Volcanic Eruptions
Volcanic ash - consists of rock, mineral, and volcanic
glass fragments smaller than 2 mm (0.1 inch) in diameter,
which is slightly larger than the size of a pinhead.
Volcanic ash is not the same as the soft fluffy ash that
results from burning wood, leaves, or paper. It is hard,
does not dissolve in water, and can be extremely small-ash particles less than 0.025 mm (1/1,000th of an inch) in
diameter are common. Ash is extremely abrasive, similar
to finely crushed window glass, mildly corrosive, and
electrically conductive, especially when wet.
Lapilli - Rock fragments between 2 and 64 mm (0.08-2.5
in) in diameter that were ejected from a volcano during
an explosive eruption are called lapilli. Lapilli (singular:
lapillus) means "little stones" in Italian. Lapilli may
consist of many different types of tephra, including
scoria, pumice, and reticulite.
Accretionary Lapilli - Rounded tephra balls between 2 and 64 mm in diameter are called
accretionary lapilli if they consist of tiny ash particles. Volcanic ash sometimes form such
balls in an eruption column or cloud, owing to moisture or electrostatic forces. Lapilli
(singular: lapillus) means "little stones" in Italian.
Products of Volcanic Eruptions
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A volcanic block is a solid rock fragment greater than 64 mm in diameter that
was ejected from a volcano during an explosive eruption. Blocks commonly
consist of solidified pieces of old lava flows that were part of a volcano's
cone.
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Volcanic bombs are lava fragments that were ejected while viscous (partially
molten) and larger than 64 mm in diameter. Many acquire rounded
aerodynamic shapes during their travel through the air. Volcanic bombs
include breadcrust bombs, ribbon bombs, spindle bombs (with twisted ends),
spheroidal bombs, and "cow-dung" bombs.
Selection of Volcanic bombs
Classification of Volcanic Products
Lavas
Pyroclastic Rocks
New Terms
Calderas and Craters
A caldera is a large, usually circular depression at the summit of a volcano
formed when magma is withdrawn or erupted from a shallow underground
magma reservoir. The removal of large volumes of magma may result in loss of
structural support for the overlying rock, thereby leading to collapse of the
ground and formation of a large depression. Calderas are different from
craters, which are smaller, circular depressions created primarily by explosive
excavation of rock during eruptions.
Aniakchak Caldera formed during
an enormous explosive eruption
that expelled more than 50 km3
of magma about 3,450 years
ago. The caldera is 10 km in
diameter and 500-1,000 m deep.
Subsequent eruptions formed
domes, cinder cones, and
explosion pits on the caldera
floor.
‘A’a and pahoehoe lavas
A`a (pronounced "ah-ah") is a Hawaiian term for lava
flows that have a rough rubbly surface composed of
broken lava blocks called clinkers. The incredibly spiny
surface of a solidified `a`a flow makes walking very
difficult and slow. The clinkery surface actually covers
a massive dense core, which is the most active part of
the flow. As pasty lava in the core travels downslope,
the clinkers are carried along at the surface. At the
leading edge of an `a`a flow, however, these cooled
fragments tumble down the steep front and are buried
by the advancing flow. This produces a layer of lava
fragments both at the bottom and top of an `a`a flow.
Pahoehoe is a Hawaiian term for basaltic
lava that has a smooth, hummocky, or
ropy surface. A pahoehoe flow typically
advances as a series of small lobes and
toes that continually break out from a
cooled crust. The surface texture of
pahoehoe flows varies widely.
Pyroclastic Flows
A pyroclastic flow is a ground-hugging
avalanche of hot ash, pumice, rock
fragments, and volcanic gas that rushes
down the side of a volcano as fast as 100
km/hour or more. The temperature within
a pyroclastic flow may be greater than
500° C, sufficient to burn and carbonize
wood. Once deposited, the ash, pumice, and
rock fragments may deform (flatten) and
weld together because of the intense heat
and the weight of the overlying material.
Pyroclastic flow sweeps down the side of Mayon Volcano,
Philippines, during an explosive eruption on 15 September 1984.
Note the ground-hugging cloud of ash (lower left) that is
billowing from the pyroclastic flow and the eruption column
rising from the top of the volcano. Maximum height of the
eruption column was 15 km above sea level, and volcanic ash fell
within about 50 km toward the west. There were no casualties
from the 1984 eruption because more than 73,000 people
evacuated the danger zones as recommended by scientists of
the Philippine Institute of Volcanology and Seismology.
Effects of Pyroclastic Flows
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View north from the summit of Mount St.
Helens shows the pristine forest that
surrounded Spirit Lake (lower right) at
the base of the volcano before the 1980
eruption.
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View north from above the crater of
Mount St. Helens after the 18 May 1980
eruption at about the same elevation as
the former summit. The gray, ashcovered area surrounding Spirit Lake is
the former forest that was destroyed by
the eruption's enormous pyroclastic
surge, commonly known as the directed
blast or lateral blast. Note the increased
surface area of Spirit Lake compared to
the pre-eruption photograph. The lake's
elevation was raised by about 60 m to
1038 m when part of the eruption's
landslide swept into the lake (the
landslide began about 20 seconds before
the pyroclastic surge). Most of the lake's
surface is covered with tree trunks swept
into the lake by the surge.
Plinian Eruptions
Plinian eruptions are large explosive events that form enormous dark columns of
tephra and gas high into the stratosphere (>11 km). Such eruptions are named for
Pliny the Younger, who carefully described the disastrous eruption of Vesuvius in 79
A.D. This eruption generated a huge column of tephra into the sky, pyroclastic flows
and surges, and extensive ash fall. Many thousands of people evacuated areas around
the volcano, but about 2,000 were killed, including Pliny the Older.
Plinian Eruption Mt Pinatubo, Philippines
June 15, 1991
Some plinian eruptions inject such large
quantities of aerosols (small liquid
droplets) into the stratosphere that
surface temperatures on earth may
decrease slightly. The eruption of
Mount Pinatubo, Philippines, and the
1982 eruption of El Chichón, Mexico
caused temperatures worldwide to
decrease slightly. The massive 1815
eruption of Mount Tambora volcano,
Indonesia, is thought to have caused
the 1816 "Year without a Summer" in
the northeastern U.S., Canada, and
western Europe.
Phreatic Eruptions
Phreatic eruptions are steam-driven
explosions that occur when water beneath the
ground or on the surface is heated by magma,
lava, hot rocks, or new volcanic deposits (for
example, tephra and pyroclastic-flow
deposits). The intense heat of such material
(as high as 1,170° C for basaltic lava) may
cause water to boil and flash to steam,
thereby generating an explosion of steam,
water, ash, blocks, and bombs.
Phreatic eruption at the summit of Mount St. Helens,
Washington. Hundreds of these steam-driven explosive
eruptions occurred as magma steadily rose into the cone
and boiled groundwater. These phreatic eruptions
preceded the volcano's plinian eruption on 18 May 1980.
Strombolian Eruptions
Strombolian eruptions are characterized
by the intermittent explosion or
fountaining of basaltic lava from a single
vent or crater. Each episode is caused by
the release of volcanic gases, and they
typically occur every few minutes or so,
sometimes rhythmically and sometimes
irregularly. The lava fragments generally
consist of partially molten volcanic bombs
that become rounded as they fly through
the air.
The word strombolian is derived from the
volcano Stromboli, one of the Aeolian Islands
north of Sicily. Stromboli has been almost
continuously in eruption for at least the past
2,400 years.
Other volcanoes that often exhibit strombolian
activity include Etna (Italy), Pacaya
(Guatemala), and Erebus (Antarctica).
Vulcanian Eruptions
A vulcanian eruption is a type of explosive eruption that ejects new lava
fragments that do not take on a rounded shape during their flight through the
air. This may be because the lava is too viscous or already solidified. These
moderate-sized explosive eruptions commonly eject a large proportion of volcanic
ash and also breadcrust bombs and blocks. Andesitic and dacitic magmas are most
often associated with vulcanian eruptions, because their high viscosity
(resistance to flow) makes it difficult for the dissolved volcanic gases to escape
except under extreme pressure, which leads to explosive behavior.
Eruption column caused by a vulcaniantype explosive eruption on Oct 5 1998,
rises above Tavurvur Volcano in Rabaul
Caldera, Papua New Guinea.
Characterising Different Eruption Types
Structure of a Volcano
Generic Structure of a Volcano
• A volcanic vent is an opening exposed on the earth's surface where volcanic
material is emitted.
• All volcanoes contain a central vent underlying the summit crater of the volcano.
• The volcano's cone-shaped structure, or edifice, is built by the more-or-less
symmetrical accumulation of lava and/or pyroclastic material around this central
vent system.
• The central vent is connected at depth to a magma chamber, which is the main
storage area for the eruptive material.
• Because volcano flanks are inherently unstable, they often contain fractures
that descend downward toward the central vent, or toward a shallow-level
magma chamber.
• Such fractures may occasionally tap the magma source and act as conduits for
flank eruptions along the sides of the volcanic edifice.
• These eruptions can generate cone-shaped accumulations of volcanic material,
called parasitic cones.
• Fractures can also act as conduits for escaping volcanic gases, which are
released at the surface through vent openings called fumaroles.
Main Volcano Types
Although every volcano has a unique eruptive history, most can be grouped into
three main types based largely on their eruptive patterns and their general forms.
The form and composition of the three main volcano types can be summarized as
follows.
Scoria Cones (or Cinder Cones)
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Most common type of volcanic centre.
Small volcanic landforms built typically during subaerial strombolian eruptions of
basaltic and basaltic andesite magmas
Usually circular in plan view owing to formation from a point source
Elongate forms develop when eruptions continue along a large part of a fissure
which does not localise to a single point source vent
Usually have central bowl shaped craters
Basal diameter is up to 2.5 km and slopes of around 33°
Many layers in scoria cones are made up of scoria or cinder as well as mass-flow
deposits related to avalanching of material down the steep slopes but can also
include bombs of lava spatter
Often scoria cones have accompanying lava flows of fairly small volumes
Gas content of the magma associated with scoria cones increases towards the
end of the eruption and so the lava spatter ejected normally increases leaving a
collar of material on the cone.
Eruptions range in duration from a few days to a few years with 95 % of scoria
cone eruptions stopping within one year.
Scoria cones are very susceptible to weathering
Scoria Cones
This scoria cone on the flank of
Mount Etna is surrounded by a
younger basaltic lava flow.
This scoria cone (Pu`u ka Pele) was
erupted low on the southeast flank
of Mauna Kea Volcano. The cone is
95 m in height, and the diameter of
the crater at the top is 400 m.
Hualalai Volcano in background.
Scoria Cones – Additional Info
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Scoria cones usually erupt lava flows, either through a breach on one side of the
crater or from a vent located on a flank. Lava rarely issues from the top (except
as a fountain) because the loose, non cemented cinders are too weak to support
the pressure exerted by molten rock as it rises toward the surface through the
central vent.
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Perhaps the most famous scoria cone, Paricutin, grew out of a corn field in
Mexico in 1943 from a new vent. Eruptions continued for 9 years, built the cone
to a height of 424 meters, and produced lava flows that covered 25 km2.
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Scoria cones are commonly found on the flanks of shield volcanoes,
stratovolcanoes, and calderas. For example, geologists have identified nearly 100
scoria cones on the flanks of Mauna Kea, a shield volcano located on the Island
of Hawai`i.
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The Earth's most historically active scoria cone is Cerro Negro in Nicaragua. It
is part of a group of four young cinder cones NW of Las Pilas volcano. Since it
was born in 1850, it has erupted more than 20 times, most recently in 1992 and
1995.
Shield Volcanoes - Intro
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Basic characteristics of shield volcanoes
– Symmetrical and circular to elliptical in shape
– Convex-up piles of basaltic lava with slopes < 10°
– Built up by fluidal eruptions of basaltic lavas from central vents
and/or flank eruptions
– Shield basal diameter (Ws) varies between a few kilometers to over
100kms
– Shield heights (Hs) are on average 1/20 Ws
– Composed almost entirely of lava flows but also may contain
• < 1% pyroclastic deposits including scoria fall
• Deposits from phreatomagmatic and phreatic explosions
• Some oxidised soil horizons and epiclastic sediments
– Divided into two types
• Large or Hawaiian Shields
• Small or Icelandic Shields
Shield Volcanoes - Hawaiian
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Hawaiian Shield Volcanoes
– Summit calderas and major rift zones marked by spatter cones,
spatter ramparts, collapse craters (pit craters), scoria cones and
smaller superimposed monogenetic shields
– Shape usually controlled by eruptions from the rift zones
– Eruptions within the calderas occur slightly more frequently than on
the rifts but the eruptions from the lateral rifts that give the
shields their elongate form.
– Calderas range from 5 to 20kms in diameter
– Shields are built by lavas and minor pyroclastics as well as high level
intrusives which may be present in the summit caldera walls.
– Compositional differences occur as the shield volcano evolves
changing from tholeiitic to progressively more alkalic
– More explosive activity accompanies the eruptions of alkaline
magmas.
– Eruption frequency decreases with time
Hawaiian Volcanic Chain
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The two most active shields on Hawaii are
Kilauea and Mauna Loa.
Mauna Loa is the world’s largest active
volcano
– Rises nearly 9km from the pacific ocean
floor to its summit of 4169m above sea
level
– Total volume of 40,000km3
Combined growth rate of ~0.1 km3 per year
indicates both Kilauea and Mauna Loa could
have been built in less than 1 Ma
Large portion of the base of both volcanoes
made up of pillow lava formed by subaqueous
extrusions
Gravity sliding and slumping along normal
faults is common on the flanks and occurs in
response to oversteepening caused by
addition of lava flows and intrusion of magma
into the summit.
Mauna Loa
Snow-covered Moku`aweoweo Caldera atop Mauna Loa shield volcano
(Mauna Kea in background). The caldera is 3 x 5 km across, 183 m deep,
and is estimated to have collapsed between 600-750 years ago. Several
pit craters along the upper southwest rift zone of Mauna Loa (lower
right) also formed by collapse of the ground.
For more information on the
world’s largest volcano visit
http://hvo.wr.usgs.gov/maunaloa/
Shield Volcanoes - Icelandic
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Icelandic shield volcanoes
– Smaller – Ws < 15 km
– Symmetrical
– Almost entirely built up by effusive eruptions from a central summit
vent
– Summit crators usually < 1 km across and often have raised rims of
spatter
– Few radial fissures or lines of parasitic cones
– Generally composed of large numbers of thin pahoehoe flows
– Mostly monogenetic and usually constructed in less than 10 years.
Shield Volcanoes - Galapagos
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There is a third type of shield volcano known as the Galapagos type.
Very similar to Hawaiian shield volcanoes but the shape of the upper summit is
different
Gentle lower slopes that rise to steeper central slopes that flatten off around
spectacular summit calderas.
Usually more alkaline than Hawaiian volcanoes
Three-deminsional Space
Shuttle Image of the Alcedo
Shield Volcano, Galapagos -- The
near circular caldera of the
Alcedo shield volcano on the big
island of Isabela is a feature
common to many of the Galapagos
shield volcanoes. The image, taken
by the Space Shuttle Endeavor,
covers an area of about 75 km by
60 km. The oblique view was
constructed by overlaying a
Spaceborne Radar Image on a
digital elevation map. The vertical
scale is exaggerated by a factor
of 1.87.