Volcanoes and Volcanic Deposits

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Volcanoes and Volcanic Deposits

• IN THIS LECTURE
• Monogenetic and Polygenetic volcanoes
• Shield Volcanoes
• Flood Basalts
• Scoria Cones
• Maars and Tuff Cones and Rings

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Monogenetic versus polygenetic

• 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

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Volcanoes and Plate Tectonics
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Characteristics of Volcanic rocks
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Products of Volcanic Eruptions

• 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.

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Products of Volcanic Eruptions

• 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.
• 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 (lt4mm) 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
• 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.

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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.
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Products of Volcanic Eruptions

• 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.
• 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
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Classification of Volcanic Products
Lavas
Pyroclastic Rocks
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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.
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Aa and pahoehoe lavas

• Aa (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 aa
  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 aa 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 aa 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.
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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.
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Effects of Pyroclastic Flows

• 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.
• 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, ash-covered 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.

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Plinian Eruptions

• Plinian eruptions are large explosive events
  that form enormous dark columns of tephra and gas
  high into the stratosphere (gt11 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.
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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.
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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).
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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 vulcanian-type
explosive eruption on Oct 5 1998, rises above
Tavurvur Volcano in Rabaul Caldera, Papua New
Guinea.
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Characterising Different Eruption Types
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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.

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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.

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Scoria Cones (or Cinder Cones)

• 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

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Scoria Cones

• This scoria cone on the flank of Mount Etna is
  surrounded by a younger basaltic lava flow.
• This scoria cone (Puu 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.

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Scoria Cones Additional Info

• 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.
• 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.
• 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 Hawaii.
• 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.

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Shield Volcanoes - Intro

• Basic characteristics of shield volcanoes
• Symmetrical and circular to elliptical in shape
• Convex-up piles of basaltic lava with slopes lt
  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
• lt 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

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Shield Volcanoes - Hawaiian

• 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

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Hawaiian Volcanic Chain

• The two most active shields on Hawaii are Kilauea
  and Mauna Loa.
• Mauna Loa is the worlds 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.

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Mauna Loa

• Snow-covered Mokuaweoweo 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 worlds largest
volcano visit http//hvo.wr.usgs.gov/maunaloa/
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Shield Volcanoes - Icelandic

• Icelandic shield volcanoes
• Smaller Ws lt 15 km
• Symmetrical
• Almost entirely built up by effusive eruptions
  from a central summit vent
• Summit crators usually lt 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.

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Shield Volcanoes - Galapagos

• 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.