Volcanoes and Volcanic Deposits 2

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Volcanoes and Volcanic Deposits 2

• IN THIS LECTURE
• Shield Volcanoes
• Stratovolcanoes
• Other Types of Volcanic Centres
• Flood Basalt Provinces
• Maar and Tuff Rings
• Intermediate-silicic centres
• Rhyolitic volcanoes
• Submarine spreading ridges and seamounts
• Intra- or subglacial volcanoes

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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.
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Stratovolcanoes

• Stratovolcanoes or composite volcanoes are the
  characteristic volcanic landform found at
  subducting plate margins
• They represent the most abundant large volcano on
  the Earths surface
• Stratovolcano morphology results from repeated
  eruptions of pyroclastics and relatively short
  lava flows from a central vent.
• Volcaniclastic deposits (pyroclastic and
  epiclastic) are usually very important
  volumetrically and can make up more than 70 of
  the volcanic succession the rest being lavas.
• At destructive plate margins, stratovolcanoes are
  built by eruptions of calc-alkaline magmas that
  are usually broadly andesitic or
  basaltic-andesite in composition.
• Alkaline magmas generate stratovolcanoes which
  are on average larger than their calc-alkaline
  counterparts.
• Average slopes on stratovolcanoes range from 15
  to 33.
• Most active stratovolcanoes are less than 100,000
  years old and have repose periods of up to 10,000
  years

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Stratovolcanoes

• Mount Mageik volcano viewed from the Valley of
  Ten Thousand Smokes, Katmai National Park and
  Preserve, Alaska. Mageik's broad summit consists
  of at least four separate structures built above
  different vents.
• Mount St. Helens is the youngest stratovolcano
  in the Cascades and the most active. Geologists
  have identified at least 35 layers of tephra
  erupted by the volcano in the past 3,500 years.
  This picture is prior to the 1980 eruption

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Stratovolcanoes

• Stratovolcanoes are composed of a wide variety of
  primary volcanic products
• Various lava types from basaltic through to
  rhyodacitic
• Pyroclastic flows
• Welded air-fall tuffs
• Ash deposits
• Ignimbrite deposits
• Pumice fall deposits
• This variety of volcanic products arises because
  the generation, evolution and type of magma
  erupted from these volcanoes is complex and could
  represent magma chambers on different levels with
  complex conduits between them and replenishment
  by different batches of primary basaltic magma
  rising through the system.
• The preservation of these primary volcanic
  products is complicated by the mass wastage and
  epiclastic processes that are common on the
  flanks of stratovolcanoes

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Stratovolcanoes
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Other Types of Volcanic Vents

• Lava and tephra can erupt from vents other than
  these three main volcano types. A fissure
  eruption, for example, can generate huge volumes
  of basalt lava that make up continental flood
  basalts
• Other types of volcanic edifices include
• Flood basalts
• Maars and tuff rings and cones
• Rhyolitic volcanoes
• Interediate or silicic multi-vent centres
• Inter- or glacial volcanoes

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Flood Basalts and their Source Vents

• The source vents to flood basalts are not central
  or point-source volcanoes
• They usually have high discharge rates up to 106
  m3 per second
• Flood basalts represent the largest single
  eruptive units known and usually have flowed
  great distances from their source.
• Flood Basalts built up by repeated eruptions
  forming a vast lava plateau which may cover areas
  gt 106 km with slopes generally less than 2-3
• Often closely associated with the initiation and
  early development of rifted margins
• Dominantly tholeiitic but alkali basalts are also
  common
• Many of the larger flows must have formed vast
  lava lakes that took many years to solidy as
  indicated by the well-developed massive columnar
  jointing preserved in many flood basalt provinces
• Columnar jointing is often two-tiered related to
  cooling fronts propagating inwards from both the
  top and bottom of the lava flow.

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Examples of Flood Basalts

• Mid-Miocene Columbia River Plateau or Basalts
• Occur in Washington, Oregon and Idaho
• Deposited within 2-3 Ma
• Cover 220,000 km2 and have an estimated volume of
  195,000 km3

• Mid-Tertiary Ethiopian-Yemen plateau
• Cretaceous Deccan Traps Northwestern India,
  500,000 km2 and volume of more than 1 million
  km3
• Cretaceous Parana-Etendeka province of southern
  Brazil-Uruguay-Namibia
• Jurassic Karoo in South Africa
• Jurassic Ferrar in South America

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Local Continental Flood Basalts
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Maars and Tuff Rings and Cones

• Volcanic craters that are usually monogenetic and
  produced by phreatomagmatic and phreatic
  eruptions
• Second only to scoria cones in abundance
• Maar is a general term for broad, low-rimmed
  volcanic craters that form when rising magma
  explosively interacts with ground water or
  surface-derived water below the original
  topographic surface and contain little or no
  juvenile magma
• Tuff rings have craters that lie on or above the
  pre-eruption surface and form when rising magma
  interacts explosively with abundant water close
  to or at the ground surface and contain a higher
  proportion of juvenile magma. Tuff rings are
  usually basaltic but more acidic one are also
  common
• Tuff cones differ from tuff rings by having
  smaller craters and larger height to width ratios
  and form in areas where surface water is located
  above the vent.
• Maars, tuff cones and tuff rings consist of
  pyroclastic deposits of stratified and
  cross-stratified ash.
• These types of volcanic centres often show a
  progression from phreatomagmatic to strombolian
  or hawaiian activity reflecting a decrease in the
  degree of magma-water interaction during
  eruption.
• Duration of eruptions is thought to be fairly
  short from a few days to a few weeks

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Maars and Tuff Rings and Cones
Distinguishing characteristics of maar-type
volcanoes
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Urinrek Maars, Alaska
Eruption column generated by phreatic and
magmatic explosions rises from the larger east
maar.

• Aerial view toward N of Ukinrek Maars, Alaska
  Lake Becharof at top of photo. Water partially
  fills the eastern maar and completely covers a
  lava dome that was erupted in the 100-m deep
  crater during a 10-day eruption in 1977. Maar is
  about 300 m in diameter.

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Rhyolitic Volcanoes

• Rhyolitic volcanic centres are some of the
  largest volcanic landforms on the Earths surface
• Usually polygenetic, multivent centres
• Usually consist of multiple eruption points or
  volcanoes
• Usually found in extensional tectonic regimes
  such as rifts, grabens and marginal basins.
• Typically lack a topographically impressive cone
  cf stratovolcanoes
• Sometimes form large broad volcano-tectonic
  depressions called inverse volcanoes of which
  Lake Taupo in NZ is the type example
• Typically consist of a collection of low
  rhyolitic hills composed of rhyolite domes,
  coulees and pumice cones, rising from gently
  sloping ignimbrite sheets which may contain more
  than one ignimbrite sheet
• Largest rhyolitic caldera known to exist is Lake
  Toba in Sumatra which has rim dimensions of 100 x
  35 km.
• Eruption rates are typically very low on the
  order of thousands of years and the period of
  repose may be quite long as much as one million
  years indicating that some rhyolitic volcanoes
  may have quite long lifespans.
• Lake Taupo has been active for 0.6 Ma, while
  Yellowstone has been active for 2 Ma.

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Rhyolitic Volcanoes

• Ignimbrite forming eruptions are generally
  associated with major structural changes to the
  volcano
• Caldera collapse occurs during or after the
  eruption, around a circular ring fracture formed
  above the drained or draining magma chamber.
• Later volcanic activity is concentrated in this
  ring fracture.
• Explosive phases precede the eruption of
  rhyolite domes and flows but rhyolite lavas do
  not travel far from the vent.
• Rhyolitic volcanoes are thought to go through an
  evolutionary cycle with the following seven
  stages
• Regional tumescence and generation of ring
  fractures
• Ignimbrite eruptions
• Caldera collapse
• Pre-resurgence volcanism and intra-caldera
  sedimentation
• Resurgent doming
• Major ring-fracture volcanism and
• Terminal fumarolic and hot spring activity.

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Large Volume Rhyolite Lavas??

• Felsic magmas will either (1) erupt explosively
  to produce extensive deposits of tephra, or (2)
  nonexplosively to produce degassed, viscous lava
  (domes, coulees, or obsidian flows) which advance
  only short distances from their vents. There has
  been a significant amount of controversy,
  therefore, over rare rhyolite lavas that appear
  to occur as large-volume flows (10-100 cubic
  kilometers).
• Most such flows occur near continental hotspots.
  The best known examples are those associated with
  (1) the Yellowstone hotspot track near the
  Idaho-Oregon border, and (2) the Ethiopian
  hotspot in northeastern Africa. These
  large-volume felsic volcanic rocks have outcrop,
  hand specimen, and thin section characteristics
  typical of lava flows. However, many
  volcanologists suspect that they are not lava
  flows at all, but rather rheomorphic ignimbrites.
  These are densely welded pyroclastic flows of
  pumice and ash, which were thick and hot enough
  to flow downslope and obliterate primary
  pyroclastic structures. They suggest that the
  original pumice and ash fragments have been
  streaked out like toffee strands so that the
  pyroclastic nature of the flow becomes
  unrecognizable.
• Is this the case with the large rhyolite lavas
  of the Lebombo Monocline?

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Intermediate-silicic multi-vent centres

• These types of volcanic centres are similar to
  Rhyolitic volcanoes but have lavas that are
  andesitic to dacitic in composition and often
  alkaline.
• Normally involve a caldera and caldera collapse
  processes after explosive eruption activity
• Often surrounded by large ignimbrite sheets
  similar to rhyolitic volcanoes

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Intra- or subglacial volcanoes

• The type locality for these eruptions is Iceland.
• Compositionally all lavas types may occur
  including basaltic, andesitic, dacitic and
  rhyolitic.
• Typically form steep sided ridges called Tindas
  or steep circular table mountains called tuyas
• Basaltic subglacial volcanoes consist principally
  of masses of pillow lavas, palagonitised
  hyaloclastite breccias and sideromelane
  fragments.
• Silicic eruptions beneath ice are likely to
  initially be explosive similar to subaerial
  silicic eruptions.