Download Volcanoes Part I: classification, deposits, and their distribution

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Part I
Composition
Types of deposits
Types of volcanoes
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
Basaltic
Chemical
Composition
45-55% SiO2;
High in Fe,
Mg, Ca; Low in
K, Na.
Andesitic 55-65% SiO2;
Intermediate
Fe, Mg, Ca,
Na, K
Rhyolitic 65-75% SiO2;
Low in Fe, Mg,
Ca; High in K,
Na
Temperature Viscosity
(degrees C)
1000 - 1200
Low
Gas
Content
Low
800-1000
Intermediate Intermediate
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.
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
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.
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.
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.
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.
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.