Volcanic Behavior is affected by:

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Chapter 5 Volcanoes

• Volcanic Behavior is affected by
• 1) Composition, 2) Temperature
• 3) Dissolved gases (Volatiles)
• All of which variously affect viscosity, i.e.,
  the thickness or resistance to flow of the
  lava.
• Gabbro/Basalt (Mafic) 50 silica
• Diorite/Andesite (Intermediate) 60 silica
• Granite/Rhyolite (Felsic) 70 silica
• Longer chains of silica tetrahedra greater
  viscosity.

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• Volatiles (Dissolved gases) tend to increase
  the fluidity of the magma. Are present in the
  lighter fractions of the magma, after the
  iron-rich dark minerals have dropped out. The
  gases, in sufficient quantities have the ability
  to drive the eruption.
• Gases in basaltic lavas cause the lava
  fountains shown in Figure 4.3, pg. 94. Because
  of the low silica content and high temperature,
  the fountains erupt freely, without major
  explosions.

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• In contrast, in high silica intermediate and
  rhyolitic lavas, the lower temperature and higher
  silica content causes plugs lava domes (p. 115)
  to develop in the volcanic neck (or vent),
  especially after the explosion.
• The result is The irresistible force (the
  gases) meets the immovable object (the
  solidified plug), pressure builds and eventually,
  the gases generally win.
• Basaltic magmas produce shield volcanoes and
  cinder cones. Intermediate magmas generally
  produce composite volcanoes. Felsic magmas
  produce caldera-type volcanoes.

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• See Figure 4.1 (pp. 92-93) Before and After
  pictures of Mt. St. Helens.

Other larger examples include El Chichon (Mexico)
Mt. Pinatubo (Phillipines).
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• The hot, fluid nature of basalt flows
  produce their own characteristic features.
• Basaltic flows ropy pahoehoe (Figure 4.5a)
  and rough, jagged aa (Figure 4.5b) flows.
  Pahoehoe forms when the plastic flow surface
  meets an obstruction.

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Pahoehoe texture
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Flow surface cools, the interior remains hot
and continues to flow, producing a lava tube.
Figure 4.6b shows an active lava tube in Hawaii.
Lava tubes allow the lavas to travel great
distances from their source.
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• Volcanic gases can include (as cited on page
  98) water vapor, carbon dioxide, nitrogen, sulfur
  dioxide, chlorine, hydrogen chloride, and argon.
• Ejecta volcanic bombs. From larger vents,
  bombs can be blown high enough to become
  streamlined before impact. From spatter vents,
  ejecta can include ribbons, and clots resembling
  cow dung.

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Other basaltic ejecta can include cinders and
scoria, usually erupted during the latter stages
of volcanic field activity.
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These cones are largely composed of cinder
material similar to that used in landscaping
called lava rock.
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Volcano Types Basaltic Shield volcanoes
low profile, basalt flows Ex. Aden Crater, New
Mexico
Cinder cones generally small, cinders scoria
piled around vent. Ex. Sunset Crater, AZ, West
Potrillo Mts., NM, Albuquerque Volcanoes, NM.
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Other types of basaltic eruptions and
materials include continental flood basalts, ex.,
Aden Afton Basalts (NM), Columbia River Basalts
(pp. 113-114)
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Flood basalts wide-spread without obvious
crater sources.
Flood basalts are fed by dikes, covered by the
eruptions.
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• Basalt ejecta from shield volcanoes spatter
  vents

Ribbons and clots from spatter vents
Streamlined volcanic bomb
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Composite (stratovolcanoes) large,
tall, composed of minor lava flows and
accumulations of unstable ash material from
explosive (pyro-clastic) eruptions. Examples
Mt. St. Helens, Mt. Fuji, Mt. Vesuvius, volcanoes
(p. 107, Figure 4.16). May be glacier-capped due
to size. Sudden melting of glaciers during
eruption can cause lahars volcanic mud flows
Mt. St. Helens, Nevado del Ruiz, Colombia
23,000 dead.
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Alternating viscous lava flows uncon-solidated
ash flows.
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• Composite volcanoes are generally associated
  with Island arc systems or Continental arc
  systems, i.e., inland from subduction zones.
  Volcanic materials are generally mafic to
  intermediate (more common).
• Aside from the lahars, the pyroclastic flows
  (Fig. 4.20) present a great danger near these
  volcanoes. Often called Nueé Ardente (glowing
  cloud) ash flows can travel 100 mph and have
  been traced 60 miles from eruptive center.

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Pompeii destruction (AD 79) pumice fall,
hot gases, and ash killed 16,000 people
(estimated). Nueé Ardente killed 28,000 in St.
Pierre, Martinique (Lesser Antilles), in 1902.
Krakatau explosion (1886) killed 36,000,
explosion was heard 3,000 miles away. Mt.
Tambora (1815) larger, caused N. American
famine in 1816. Typical Volcano Components
conduit or pipe brings lava to surface where it
is erupted through a vent. Crater is the
steep-walled depression at the summit of a
volcano. Calderas are large, circular
depressions, generally from post-eruption
collapse. Crater Lake and Hawaiian-type calderas
are described on pp. 111-112.
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• Crater sizes
• Santorini caldera (pg. 108) 5 mi. across.
• Crater Lake caldera (pg. 113) 6 mi. across.
• Yellowstone caldera (pg. 112) 43 mi. across.
• Yellowstone-Type Calderas - supervolcanoes
• Craters tens of miles across. Area swelled
  by rising pluton can cover 100 sq. mi..
  Histories can last millions of years.
• Last major Yellowstone eruption 630,000 years
  ago (estimated). 3 major episodes over last 2.1
  m.y..
• Long Valley caldera, CA and Valles caldera, NM
  are also youthful.
• Ash flows from calderas have been traced 100
  miles from their sources.

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• Because Intrusive Igneous Rocks represent
  solidified magma (below the surface), they are
  only exposed by
• Erosion,
• Faulting bringing the rocks to the surface,
• Both Erosion and Faulting
• Because of the slow cooling rate, intrusives have
  larger grains than extrusives.
• Intrusives may have served as conduits for
  extrusives to reach surface.

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• Intrusive igneous activity Internal
  processes, many related to overlying volcanic
  activity.
• Relationships to local geologic structures
  Discordant intrusions cut across local
  structures.
  Concordant intrusions are parallel to local
  (especially sedimentary) structures.
• Shapes tabular (dikes or sills), massive
  (batholiths, stocks, pegmatites), combination
  (laccoliths).
• Dikes discordant. Sills concordant. See p.
  151 for examples of both.

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Plutonic Igneous Rock terms
Laccolith
Volcanic neck
Dike
Sill
Batholith gt 40 mi2 Stock lt 40 mi2
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• Dikes tabular/ discordant

Dike extending from West Spanish Peak stock (see
p. 119).
Breccia dike in Castner Marble, Franklin Mts., El
Paso
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Laccoliths injected between layers, bulges
overlying rock, generally has a flat bottom.
Batholiths Igneous intrusions with an un-known
depth and at least 40 sq. mi. of surface
exposure. The Stone Mt. Granite would likely
qualify as a small batholith. Stocks Igneous
intrusions, smaller than batholiths, may be
small surface exposures of a deeper batholith.
Ground surface
Stock
Batholith
Xenolith
Deeper, larger Batholith
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• Examples of Laccoliths include La Sal Mts. and
  Henry Mts. in Utah

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• Another example of Laccoliths are the Sierra
  Blanca Peaks in West Texas. Erosion has revealed
  the flat bottom over limestone.

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• Batholiths rise as buoyant igneous bodies
  through the warmer, lower crust. In the cooler,
  brittle upper crust, stoping occurs, whereby the
  batholith fractures the overlying rock and loose
  chunks are incorporated into the pluton.

Stoped blocks that do not melt become xenoliths.
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Sierra Nevada Pluton - inland from
long-lasting, continental arc subduction zone.
Plutonic activity lasted 130 m.y..
Diagram shows an Island arc system, at the
convergence of two oceanic plates. Continental
arc occurs inland from oceanic/ continental
colli-sion subduction of oceanic plate (Figure
4.35, pg. 126).
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Below, light colored, pinkish rock is
granite pluton. Marble Diabase are stoped
xenoliths in the margins of the Precambrian Red
Bluff Granite.
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• Plate Tectonics Igneous Activity
• Convergent zone vulcanism (inland from subduction
  zones) - Circum-Pacific Ring of Fire volcanoes
  mainly composite volcanoes with explosive,
  volatile-rich magmas. Most of these erupt
  intermediate (andesitic) lavas and pyroclastics.
• North America Cascade Volcanoes, Aleutian
  Island Arc System
• South America - Andes Mts.
• Pacific Island Arcs Japan, Philipines,
  Indonesia.

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How convergent (subduction) zone may form
Rift zone dies Oceanic plate breaks loose
from continent due to sediment loading.
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• Oceanic/oceanic boundaries. When one plate
  sinks, gravity pulls it towards the mantle.
  Initial vulcanism is basaltic, derived from the
  melted plate .
• As the eruptions thicken the overlying plate
  margin, the increased thickness inhibits rise of
  basaltic magmas, magmatic differentiation
  (crystal settling) more silica-rich
  (intermediate to felsic), as arc matures.
  Friction from subduction contri-butes to heat and
  seawater in sediments contributes to lowered
  melting points, aiding partial melting and
  differentiation.

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• Continental Arc Volcanic systems Cascades,
  Andes, Mexico volcanoes, Sierra Nevada (old).
• When oceanic plate begins its subduction beneath
  continental plate, continental margin sediments,
  seawater, and partial melting of the oceanic
  crust triggers the rise of plutons. These
  plutons, may cause partial melting of the felsic
  continental crust. The mixing (and crustal
  thickness), result in intermediate to felsic
  magmas (diorites and granites).

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• Divergent Plate Boundaries Uprising mantle
  plumes (upwhellings) stretch and thin lower crust
  decompression melt, which is enhanced as magma
  nears the surface.
• Continental Rifts Continental crust thickness
  and silica content allows presence of both
  basaltic and intermediate magmas and lavas.
• Oceanic rift zones mafic basalts directly
  erupted from mantle-derived ultramafic plutons.

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• Intraplate Vulcanism (Oceanic plates)
  Unusually strong mantle plume breaks through
  plate. Examples Hawaii, Emperor Seamount Chain,
  Canary Islands.
• Intraplate Vulcanism (Continental plates) -
  mantle plume erupts within continental crust.
  Examples Yellowstone, Long Valley, CA, San
  Francisco Volcanic Field (Flagstaff, AZ and other
  N. AZ volcanics), Columbia River Basalts, Snake
  River Basalts, West Africa (Cameroon) and North
  Africa (Libya) volcanoes.
• Yellowstone, Long Valley, Valles rhyolitic
  (felsic) Others generally basaltic/intermediate
  http//volcano.und.nodak.edu/vwdocs/volc_images/n
  orth_america/north_america.html

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• Volcanoes and Climate
• Ash contributions to stratosphere are generally
  short-lived, ash eventually settles out.
• Sulfur dioxide (as aerosols) is more of a
  concern, is believed to reflect sunlight back
  into space.
• Mt. Pinatubo eruption in Philipines caused global
  cooling for about 2 years. Eruptions of Deccan
  Plateau flood basalts (Cretaceous Period) may
  have contributed to global cooling that stressed
  the dinosaurs. Eruptions of flood basalts in
  Siberia may have contributed to mass-extinctions
  at the end of the Permian Period (246 m.y.a).

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• Volcanic carbon dioxide caused increased
  Greenhouse Effect in the Cretaceous Period.
• This plays into the politically-driven modern
  hypothesis.
• http//www.john-daly.com/history.htm How the
  current hypothesis of carbon dioxide-caused
  global warming became a political animal.
• Many scientists believe that increased carbon
  dioxide is the effect of global warming, rather
  than the cause.

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• Current atmospheric carbon dioxide content is
  about 380 ppm 0.038
• Approximately 90 of the modern Greenhouse Effect
  is due to atmospheric water vapor and clouds.
• Estimates of Cretaceous atmospheric carbon
  dioxide range from 1100 ppm (.110) to 3600 ppm
  (.360). Evidence suggests that Cretaceous
  Period was very warm.
• When two things happen at the same time
  correlation, not causation.

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• When we find correlations in the past, e.g.,
  atmospheric carbon dioxide rises temperature
  rises, this doesnt tell you which one leads
  and which one follows. The computer models
  used by the IPCC supporters to support the CO2
  rise temperature rise may not include the
  effects of atmospheric water vapor and clouds.
• If temperature rises first oceans warm and lose
  dissolved CO2 (Coca Cola effect), increased
  biological activity (animals, bacteria, termites,
  etc.) increased CO2 .