Welcome Geology 101 Lecture on Volcanoes Chapter 4

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Welcome !Geology 101Lecture on
VolcanoesChapter 4

• Ken Sprenke
• .

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A volcano is an opening, or rupture, in a
planet's surface or crust, which allows hot,
molten rock, ash and gases to escape from below
the surface,
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An opening in the planets surface which allows
magma to escape from the subsurface.
Midocean ridges, hotspots, continental
rifts.
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Here, two lithospheric plates diverge from one
another. The cause of magma forming
beneath ocean ridges.
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• Explosive eruptions are favored by high gas
  content and high viscosity (andesitic to
  rhyolitic magmas).
• Expansion of gas bubbles is resisted by high
  viscosity of magma - results in building of
  pressure
• High pressure in gas bubbles causes the bubbles
  to burst when reaching the low pressure at the
  Earth's surface.

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This happens when an oceanic plate converges with
a continental plate. This lowers the
melting temperature of mantle rock beneath
subduction zones.
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Most volcanoes occur at convergent and divergent
plate margins.
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Some volcanoes such as Yellowstone and Hawaii are
caused by mantle hot spots.
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Millions of years and thousands of miles of
volcanism caused by the Hawaii hotspot.
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These are the likely cause of hotspots.
Examples are at Yellowstone and at the Big
Island of Hawaii. Geology word for magma once
it has escaped to the surface.
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Pocotatepete, an active stratovolcano in Mexico.
Its magma explodes when it reaches the surface.
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Bursting of bubbles fragments the magma into
pyroclasts and tephra (ash).
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Cloud of gas and tephra rises above volcano to
produce an eruption column that can rise up to 45
km into the atmosphere.
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Tephra that falls from the eruption column
produces a tephra fall deposit.
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If eruption column collapses a pyroclastic flow 
may occur, wherein gas and tephra rush down the
flanks of the volcano at high speed.  This is the
most dangerous type of volcanic eruption. 
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After the lateral blast on Mt St.Helens,
pyroclastic flows of hot gas and ash raced down
the mountain.
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Pyroclastic flows from Mt Vesuvius instantly
killed and entombed the citizens of Pompei and
Herculaneum in 79 AD.
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Lateral blasts and debris avalanches occur when
gas is released suddenly by a large landslide
taking out part of the volcano.
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Shield volcano.
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Mauna Loa A shield volcano.
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An extinct shield volcano
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Fissure Eruptions - An eruption that occurs along
a narrow crack or fissure in the Earth's surface.
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Most flows in the Columbia Plateau erupted from
fissures.
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Flow surfaces aa ---- rough pahoehoe --smooth
ropy texture
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Hexagonal columns form as lava shrinks. The
columns grow perpendicular to the cooling surface.
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Pressure ridges buckling of the lava surface due
to stress while the lava is cooling.
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Lava tube A tunnel below the solid surface of a
flow where lava continued to flow.
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Spatter cone small steep-sided cone formed from
molten material thrown into air that falls back
to surface and adheres together.
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200 m high cinder cone in California.
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Eldfell a 300 ft high cinder cone that appeared
suddenly in 1973.
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Volcanoes with gentle slopes formed of fluid
lavas. Cone-shaped hills resulting from
eruptions of scoria and pyroclastics. A
small cone formed by fluid lava and expanding
gas.
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Stratovolcano (composite volcano) - a volcano
built of interbedded lava flows and pyroclastic
material.
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Composite (strato-) volcano
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Composite volcano in the Phillipines.
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St.Helens before it blew up a stratovolcano..
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These are tall conical mountains composed of lava
flows and other ejecta in alternate layers.
Stratovolcanoes are also known by another
name.
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Mt Lassen--worlds largest lava dome.
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Some Volcanic Hazards Nuee ardente--dense cloud
of hot pyroclastic materials and gases. Lahar
Mudflow composed of volcanic materials
Volcanic gas emissions.
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Lahar from 1980 Mt St Helens Eruption.
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These are fast-moving, avalanche-like,
ground-hugging incandescent mixtures of hot
volcanic debris, ash, and gases that can travel
at speeds in excess of 150 km per hour.
These mudflows sweep down the steep sides of
composite volcanoes. After Pinotubo erupted,
this gas combined with water to form droplets of
sulfuric acid, blocking some of the sunlight from
reaching the Earth.
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Pillow lavas form most of the sea floor.
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Loihi--A seamount south of the Big Island and
soon to become the next Hawaiian Island.
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A common eruptive product of submarine volcanoes.
The geology word for extinct volcanoes
that rise at least 1000m above the sea floor.
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Strombolian eruptions are named because of
activity of Stromboli in Sicily. They are
characterized by huge clots of molten lava
bursting from the summit crater to form luminous
arcs through the sky.
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Vulcanian eruptions are characterised by a dense
cloud of ash-laden gas exploding from the crater
and rising high above the peak. Paricutin is a
classic example.
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Vulcanian eruption
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In a Peléan eruption or Nuée Ardente, a large
amount of gas, dust, ash, and lava fragments are
blown out of a central crater, fall back, and
form avalanches that move downslope at speeds as
great as 100 miles per hour.
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Phreatic eruptions (or steam-blast eruptions) are
driven by explosive expanding steam resulting
from cold ground or surface water coming into
contact with hot rock or magma.
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Plinian eruptions are usually the most powerful,
and involve the explosive ejection of relatively
viscous lava. Large plinian eruptions such as
during 18 May 1980 at Mount St. Helens or, more
recently, during 15 June 1991 at Pinatubo in the
Philippines can send ash and volcanic gas tens
of miles into the air. The resulting ash fallout
can affect large areas hundreds of miles
downwind.
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In 1779 Vesuvius, during a plinian eruption,
ejects a colossal lava fountain which reaches an
altitude of 4000m above the crater.
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Pompei was covered by several meters of ash 79 AD
during an earlier gigantic plinian eruption of
Vesuvius.
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Match the characteristics to the eruption
style(s) ___luminous arcs A.
Subglacial ___under ice B. Strombolian ___ash
cloud on top C. Pelean ___avalanches D.
Vulcanian ___blasted fragments E.
Hawaiian ___fissure lavas F. Phreatic ___huge
ash clouds G. Plinean ___pyroclastic
flows ___lava fountains ___lava clots down
slope ___lava streams ___Mt StHelens
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These lavas are highly fluid. These magmas are
very viscous These magmas are intermediate in
viscosity but explosive nonetheless. Smooth
ropy lava. Rough blocky lava.
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Yellowstone's youngest lava flow, the
70,000-year-old Pitchstone flow.
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In Yellowstone, he last one was 640,000 years
ago. In Yellowstone, the last one of these
was 70,000 years ago. Volcanoes identified as
being worthy of particular study in light of
their history of large, destructive eruptions and
proximity to populated areas.
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The most abundant volcanic gas. The second
and third most abundant volcanic gases.
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The most volcanically active world in the Solar
System. The planet with the largest
(probably extinct) volcanoes.
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Crater - a depression caused by explosive
ejection of magma or gas
A typical summit crater on a cinder cone.
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Caldera - a depression caused by collapse of a
volcano into the cavity once occupied by magma
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Formation of Crater Lake. Is Crater Lake a
crater? No! Crater Lake occupies a caldera.
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Volcanic feature formed by the collapse of land
following a volcanic eruption. A basin of a
roughly circular form within which occurs a vent
from which magma erupts as gases, lava, and
ejecta.
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Kimberlite Neck
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Volcanic neck
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Devils Tower--A volcanic neck
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Fissure Eruption
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The geological source of most of the worlds
commercial diamonds. Shiprock and Devils Tower
national monuments. A linear volcanic vent
through which lava erupts, usually a few meters
wide and may be many kilometers long.
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Magma properties affect volcanism
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By definition, all igneous rock is formed from
this. For a given rock, a rise in temperature
above this will cause melting. This type of
melting can occur when a decrease in pressure
lowers the solidus. Losing this can cause a
magma to solidify even if temperature and
pressure do not change.
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Silica has this effect on viscosity. Water
has this effect on viscosity. Temperature
has this effect on viscosity.
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In general, partial melting does this to the
density of the rock mass. A mass that rises
in the lithosphere because it is less dense than
the surrounding rock.
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Match left with right ___MtStHelens A.Composite
___Kilauea B. Shield ___Paricutin C. Cinder
Cone ___Wizard Island ___lahars ___lava
flows ___small, steep slopes ___extensive
area ___flows and pyroclasts
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Match left with right _____cinder
cone A.Explosive _____shield B.
Nonexplosive _____stratovolcano C. felsic
magma _____basalt plateau D. mafic
lava _____lava dome E. andesitic
magma _____tuff _____Hawaii _____Vesuvius _____Par
icutin _____Mt StHelens _____Iceland
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Most volcanoes occur at convergent and divergent
plate margins.
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Match left with plate boundary _____Mt Rainier A
Convergent _____Pinatubo B. Divergent _____Kilauea
C. Transform _____Surtsey D.
Intraplate _____Paricutin
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Next Class Ch 5 Sedimentary Rock. Project 2 due
930 am Weds Feb 4 Quiz 3 due Friday Feb 7 5
pm Quiz 4 due Friday Feb 13 5 pm