VOLCANOES
COMPOSITION, TYPE OF DEPOSITS,
TYPE OF VOLCANOES AND
DISTRIBUTION
Volcano: A mound of material that is extruded to the
Earth’s surface from a vent that is connected to a
magma chamber via a feeder conduit.
Volcanoes are classified according to their form.
The form of a volcanoes depends on the type of
material that it is made up of.
The nature of the extruded material (and the volcano
itself) depends on the properties of the magma.
Magma: Molten rock within the Earth.
Magma is called lava when it reaches the surface.
The composition of magma determines the type of rock
that forms when it cools and its behavior during an
eruption.
Main controls on behavior:
chemical composition (largely silica dioxide - SiO2 content)
and
gas content (largely water vapor and CO2).
SiO2 content controls the viscosity of a magma.
Viscosity: a measure of how easily a fluid flows. Water has
a low viscosity, molasses has a much higher viscosity.
Viscosity, in turn, controls the amount of gas that can be
trapped in the magma.
The greater the viscosity the more gas in the magma.
There are three basic types of magma:
Basaltic Magma
Andesitic Magma
Rhyolitic Magma
The names are based on the rock type that forms when the
magma crystallizes.
Magma
Type
Chemical
Composition
Basaltic
45-55% SiO2;
High in Fe,
Mg, Ca; Low in
K, Na.
Temperature Viscosity
(degrees C)
1000 - 1200
Low
Gas
Content
Low
Andesitic 55-65% SiO2;
Intermediate
Fe, Mg, Ca,
Na, K
800-1000
Intermediate Intermediate
Rhyolitic 65-75% SiO2;
Low in Fe, Mg,
Ca; High in K,
Na
650-800
High
High
Overall, the behaviour of the magma determines the type
of volcano that develops:
Low SiO2 magmas, with little gas and low viscosity, flows
readily through their vents and across the land surface
when the lava escapes the vents.
High SiO2 magmas, gaseous and with high viscosity, tend
to plug their vents until the force of escaping magma blows
the vent clear; such magmas cause explosive volcanoes.
Types of volcanic deposits
(photos from USGS)
Volcanoes also vary in terms of the types of deposits that
they produce.
Lava: Hot (up to 1200 degrees C), fluid,molten rock
that flows along the land surface.
Lava can flow like viscous water, including forming lava
falls.
Pahoehoe: Lava with a ropelike surface texture due to
partial cooling as the lava flowed. Relatively hot, low
viscosity lava.
Pahoehoe
A thick deposit of pahoehoe lava
Aa: Blocky, rough lava flow. Due to high viscosity lava
that flowed pushing chunks of solid and semi-solid
blocks.
www.volcanovideo.com/Movies/p8vdclp.htm
Lava tube: A tube
formed by cooling and
solidifying of the lava
walls while fluid lava
continued to flow inside.
Pillows: A form of closed lava tube (with a bulbous
end) that forms when a lava flows into water (e.g., a
lake or ocean) and cools very rapidly.
http://oceanexplorer.noaa.gov/explorations/04fire/background/volcanism/media/pillow_lava_video.html
Pyroclastic material: Debris formed by a volcanic
explosion. Results when magma is very viscous.
Tephra: The general term for all pyroclastic material
that is ejected from a volcano. Different terms apply
according to the size of the tephra. (syn. Ejecta)
Ash: tephra that is finer than 2 mm in diameter.
Lapilli: from 2 mm to 64 mm in diameter.
Blocks: hard fragments greater
than 64 mm in diameter.
Bombs: soft, partially melted fragments greater than
64 mm in diameter.
Tuff: A deposit made up of ash.
Welded tuff: A deposit of pyroclastic material that was
laid down while still very hot and particles become
fused together.
Ash fall: Fallout of very fine ash from the air.
Volcanic ash fall during
mid-day with the
eruption of Mount
Pinatubo in the
Philippines.
Ash flow: Pyroclastic debris that flows downslope.
Lahar: A water saturated slurry of ash and other
volcanic debris that flows downslope.
Nuée Ardente (glowing cloud): A hot, gaseous cloud of
ash that flows down slope.
Flow speeds can reach 160
km/hr and temperatures can
exceed 600 degrees C.
http://volcano.und.nodak.edu/vwdocs/volc_images/img_mt_pelee.html
Classification of volcanoes
Volcanoes are classified according to their morphology.
The processes and deposits dictate the morphology of
volcanoes.
Three types of volcano:
Shield volcanoes: dominated by lava flows.
Muana Loa Volcano – the world’s largest volcano.
http://hvo.wr.usgs.gov/maunaloa/
Photograph by J.D. Griggs on January 10, 1985
Cinder cones: dominated by pyroclastics.
Forms an isolated conical mound of tephra.
Photograph by J.P. Lockwood on 1 December 1975
http://volcanoes.usgs.gov/Products/Pglossary/CinderCone.html
Stratovolcanoes: mixture of lavas and pyroclastics.
Syn. Composite volcanoes
Mount Mageik volcano, Alaska
Photograph by R. McGimsey on 15 July 1990
http://volcanoes.usgs.gov/Products/Pglossary/stratovolcano.html
Shield Volcanoes
Dominated by fluid, high temperature, low viscosity
basaltic magma.
Low, dome-shaped profile, like an inverted shield.
http://geoimages.berkeley.edu/GeoImages/Johnson/Landforms/Volcanism/ShieldVolcano.html
Typical slopes approximately 15 degrees.
Lava flows downslope, away from a central vent or a series
of vents.
Many shield volcanoes have a central caldera:
Calderas form after an
eruption when the surface
collapses.
Each caldera is located at
the site of a former
eruption.
USGS
Hawaiian Isands and Iceland are built from shield
volcanoes.
Mauna Loa is the largest volcano on Earth.
It makes up most of the
island of Hawaii.
The volcano rises 4,170 m above
sea level.
It covers an area of 5,271 km2.
Total volume of rock: 80,000 km3
Began to form 700,000 to 1,000,000 years ago when lava began to
flow to the sea floor.
Eruptions reached the surface 400,000 years ago.
Its great weight depresses the underlying crust by 8 km.
Low viscosity lava forms fountains of lava flowing from vents near
the volcano summit.
The lava flows easily down the
gentle slopes….reaching the ocean
during some eruptions.
Where the lava is relatively cool eruptions form small
cinder cones on the volcanoes surface.
Cinder Cones
Dominated by viscous, gaseous magmas
Relatively cool basaltic magmas or andesitic magmas
predominate.
Mount Edziza, British Columbia
Internally constructed entirely of layers of pyroclastic
deposits (blocks, bombs, lapilli).
Slopes are steep, at angle of repose.
Angle of repose: the natural maximum angle that a
pile of loose, unconsolidated material will form.
Typical angles: 30 to 40 degrees.
Range from several metres to over 300 m in height.
Commonly associated with old shield volcanoes with a
relatively cool, basaltic magma.
Paricutin volcano began to erupt in
a corn field in Mexico in 1943 and
continued until 1952.
The farmer had noticed a fissure
(vent) had opened in the field one
morning and from it was pouring
black ash.
In the first year the volcano grew to
336 m (almost 1 metre per day).
Rate of growth decreased steadily;
by 1952 the volcano was 424 m in
height.
Stratovolcanoes
Volcanoes that alternate
between periods of lava flows
(constructive phase) and
periods of explosive eruptions
(destructive phase).
Commonly called “composite
volcanoes” because they are
made up of both lava and
pyroclastic deposits.
Steep slopes, at angle of
repose or greater.
© Noemi Emmelheinz 2001
May lay dormant for thousands of years.
On average, andesitic magmas with a high gas content.
Actually, a mix of basaltic and rhyolitic magmas in many
cases.
Gases add great pressure when the feeder conduit
becomes plugged, contributing to the explosive power.
Can grow to thousands of metres high during
constructive lava flow phases.
The constructive phase often ends with a destructive phase
– an explosive eruption.
Mt. St. Helens Before
Mt. St. Helens After
Extensive ash falls and ash flows are commonly
produced during explosive phases.
After an eruption a large caldera remains.
Crater Lake is a caldera that remains following an
explosive eruption 7,700 years ago.
The eruption was 42 times more powerful than Mt. St.
Helens.
Styles of Volcanic Eruption
Eruption style is
determined by the
explosiveness and the
height of the column of
tephra.
USGS
Hawiian Eruptions
Eruptions dominated by
lava flows.
No significant ash column.
Any explosions are of a
small scale.
Strombolian eruptions
Intermittent explosion or fountaining of basaltic lava from
a single vent or crater.
Caused by the release of volcanic gases, and they normally
occur every few minutes.
Tephra ranges from
ash to bombs.
Vulcanian Eruptions
Periodic eruptions (decades apart) that are moderate
explosions.
Ejecta are hard (not melted).
Andesitic, gaseous magmas.
Pelean Eruptions
Similar to Vulcanian Eruptions
but include hot gas clouds
(Nuees Ardentes)
Plinian Eruptions
Very explosive volcanic
eruptions; long periods
between eruptions.
Involve a very viscous,
rhyolitic magma.
Form a plume of tephra that
extends upwards to the
stratosphere (>11 km high).
Dust can remain in the
stratosphere for years and
significantly cool the Earth.
Ultraplinian Eruptions
Very explosive eruptions that eject approximately 100 km3
or more of tephra and produce ash columns exceeding 25
km in height.
Phreatic Eruptions
Eruptions of tephra and large volumes of steam produced
when water makes contact with the magma.
Water flashes to steam and causes a very violent eruption.
The Distribution of volcanoes
The vast majority of volcanoes are located:
Parallel to oceanic trenches.
Along the oceanic ridge.
Over hot spots originating from the mantle.
Volcanoes along trenches
Examples: Japan, most Pacific Islands, Caribbean
Islands, west coast of North and South America.
2/3 of all volcanoes are along the Ring of Fire that
surrounds the Pacific Ocean.
Volcanoes result from magma rising off the melting
subducted plate.
The composition of the magma is andesitic (melted
basaltic crust plus sediment carried on the crust).
Magma is very gaseous,
particularly enriched with
water vapor.
Stratovoclanoes are
constructed from feeder
conduits extending to the
surface.
Granitic (rhyolitic) intrusions are also formed,
becoming trapped within the volcanic pile overlying
the region of subduction.
Potential for very explosive eruptions.
Mt. Fuji, Japan
A stratovolcano that has erupted 16 times since
781 AD.
The most recent eruption was in 1707-1708
0.8 cubic km of ash, blocks, and bombs were ejected
during that eruption.
(Greater than Mt. St.
Helens and there were
no fatalities).
Similar situation on the west coast of North and South
America.
Volcanoes formed by intrusion into the mountain
chains that result from compressive forces between
oceanic and continental crust.
Ojos del Salado, Chile – The world’s highest volcano.
Perched at 6,887 metres above sea level.
A stratovolcano that has not erupted in historic time.
Photo by Peter Francis
Volcanoes in Canada?
There are many inactive volcanoes in the Canadian Rocky Mountains.
None are erupting at the present time.
At least three have erupted over the past several hundred years.
For a catalogue of Canadian Volcanoes go to…..
http://gsc.nrcan.gc.ca/volcanoes/map/index_e.php
Oceanic Ridge Volcanoes
Most volcanic activity is under water.
Intrusion of material from the magma chamber creates
new oceanic crust as the sea floor spreads.
Basaltic pillow lavas
dominate the
submerged volcanoes.
Shield volcanoes occur where volcanic activity extends
to the surface (e.g., Iceland).
Iceland is growing by volcanic expansion of the ridge.
Hekla Volcano in the
distant background.
Hekla erupted four
times in the 20th
century, the last time in
1991.
Hekla covers about 80%
of Iceland and its volume
is approx. 12 cubic km.
Unlike the Hawaiian Island shield volcanoes most of Iceland’s lava
flows issue from linear fissures: fissure eruptions rather than vent
eruptions.
Vatnaolder Volcano forms a single peak with the classic shield form.
Cinder cones also form on the older portions of the island, away
from the most active area that runs along the middle of the oceanic
ridge.
Very small cinder cones, called spatter cones form where small
fountains of very fluid basaltic lava extrude material to the surface
for relatively short periods of time.
Some Icelandic volcanoes are buried beneath glaciers.
In 1973 the Eldfell volcano
erupted on the island of
Heimaey
Photos from the USGS.
Over three months the island grew substantially and the harbor was
greatly improved.
Volcanism associated with rifting
Volcanism Associated with subduction
Volcanoes and Hot Spots
Hot Spot: a point on the crust immediately above a hot
plume within the mantle.
Heat from the mantle (and some magma) rises to the hot
spot.
Rising mantle
material
termed a
mantle plume.
Hot spots can occur beneath oceanic or continental
crust.
Mechanism first proposed by J. Tuzo Wilson (a
Canadian geophysicist) to illustrate that plates actually
move.
The Hawaiian Islands
consist of eastern active
volcanic islands and
inactive volcanic
islands to the
northwest.
Further northwest of the islands are seamounts
(underwater mountains that are submerged islands).
http://www.biosbcc.net/ocean/marinesci/02ocean/hwgeo.htm
Just southeast of Hawaii is an undersea volcano known as
Loihi.
Until 1996 Loihi was thought to be
an inactive seamount.
It began erupting in 1996 and the eruptions
were preceded by a cluster of small
earthquakes indicating the movement of
magma.
The modern active island rests close to the hot spot
and its shield volcanoes are fed from the magma that
the hot spot generates.
http://www.biosbcc.net/ocean/marinesci/02ocean/hwgeo.htm
The Pacific plate is moving
towards the northwest.
The volcanic islands have
been successively “pushed
off” the hot spot by plate
movement.
As the crust moves it
ages, becomes cooler and
more dense, causing it to
subside.
The seamounts are old
islands that have subsided
to below sea level.
The seamounts represent even older islands that have
been pushed further from the hot spot.
Recent studies suggest that the Hawaiian Hot Spot has moved over
time.